This walks back some of the attempt at strict object namespacing in
struct lfs3_cfg:
- cfg.file_cache_size -> cfg.fcache_size
- filecfg.cache_size -> filecfg.fcache_size
- filecfg.cache_buffer -> filecfg.fcache_buffer
- cfg.gbmap_re_thresh -> cfg.regbmap_thresh
Motivation:
- cfg.regbmap_thresh now matches cfg.gc_regbmap_thresh, instead of using
awkwardly different namespacing patterns.
- Giving fcache a more unique name is useful for discussion. Having
pcache, rcache, and then file_cache was a bit awkward.
Hopefully it's also more clear that cfg.fcache_size and
filecfg.fcache_size are related.
- Config in struct lfs3_cfg is named a bit more consistently, well, if
you ignore gc_*_* options.
- Less typing.
Though this gets into pretty subjective naming territory. May revert
this if the new terms are uncomfortable after use.
Note: This affects the blocking lfs3_alloc_repopgbmap as well as
incremental gc/traversal repopulations. Now all repop attempts return
LFS3_ERR_NOSPC when we don't have space for the gbmap, motivation below.
This reverts the previous LFS3_t_NOSPC soft error, in which traversals
were allowed to continue some gc/traversal work when encountering
LFS3_ERR_NOSPC. This results in a simpler implementation and fewer error
cases to worry about.
Observation/motivation:
- The main motivation is noticing that when we're in low-space
conditions, we just start spamming gbmap repops even if they all fail.
That's really not great! We might as well just mark the flash as dead
if we're going to start spamming erases!
At least with an error the user can call rmgbmap to try to make
progress.
- If we're in a low-space condition, something else will probably return
LFS3_ERR_NOSPC anyways. Might as well report this early and simplify
our system.
- It's a simpler model, and littlefs3 is already much more complicated
than littlefs2. Maybe we should lean more towards a simpler system
at the cost of some niche optimizations.
---
This had the side-effect of causing more lfs3_alloc_ckpoints to return
errors during testing, which revealed a bug in our uz/uzd_fuzz tests:
- We weren't flushing after writes to the opened RDWR files, which could
cause delayed errors to occur during the later read checks in the
test.
Fortunately LFS3_O_FLUSH provides a quick and easy fix!
Note we _don't_ adopt this in all uz/uzd_fuzz tests, only those that
error. It's good to test both with and without LFS3_O_FLUSH to test
that read-flushing also works under stress.
Saves a bit of code:
code stack ctx
before: 37260 2352 688
after: 37220 (-0.1%) 2352 (+0.0%) 688 (+0.0%)
code stack ctx
gbmap before: 40220 2368 856
gbmap after: 40184 (-0.1%) 2368 (+0.0%) 856 (+0.0%)
In lfs3_fs_grow, we need to update any gbmaps to match the new disk
size. The actual patch to the gbmap is easy, but it does get a bit
delicate since we need to feed the gbmap with an allocator in the new
disk size.
Fortunately, the opportunistism of the gbmap allocator avoids any
catch-22 issues, as long as we make sure to not trigger any gbmap
rebuilds.
Adds a bit of code, but not much:
code stack ctx
before: 37168 2352 684
after: 37168 (+0.0%) 2352 (+0.0%) 684 (+0.0%)
code stack ctx
gbmap before: 39000 2456 800
gbmap after: 39116 (+0.3%) 2456 (+0.0%) 800 (+0.0%)
Having gbmap/bmap used in different places for the same thing was
confusing. Preferring gbmap as it is consistent with other gstate (grm
queue, gcksums), even if it is a bit noisy.
It's interesting to note what didn't change:
- The BM* range tags: LFS3_TAG_BMFREE, etc. These already differs from
the GBMAP* prefix enough, and adopting GBM* would risk confusion for
actual gstate.
- The gbmap revdbg string: "bb~r". We don't have enough characters for
anything else!
- dbgbmap.py/dbgbmapsvg.py. These aren't actually related to the gbmap,
so the name difference is a good thing.
The main change is error propagation in lfs3_alloc_ckpoint. Since
lfs3_alloc_ckpoint writes to disk during bmap rebuilds, it can now fail
in all sorts of ways. Fortunately lfs3_alloc_ckpoint should only ever be
called by write operations, where these errors are be expected.
With bmap rebuild errors now reported correctly, this unblocks most of
the remaining test TODOs:
- Passing test_badblocks
- Passing test_ck
- Passing test_trvs
With this, LFS3_YES_BMAP is now passing all but two tests, which are
still ifndef-disabled as a temporary measure:
- test_btree - We make some low-level assumptions about the lookahead
allocator when testing btrees. It's probably not worth trying to get
this passing with the bmap allocator.
- test_grow - This one does need fixing! We currently don't update
on-disk bmaps correctly when growing the filesystem.
Code changes minimal:
code stack ctx
before: 36912 2368 684
after: 36912 (+0.0%) 2368 (+0.0%) 684 (+0.0%)
code stack ctx
bmap before: 38456 2400 812
bmap after: 38512 (+0.1%) 2400 (+0.0%) 812 (+0.0%)
There's really no reason to immediately commit the bmap to disk, at
least no until the first mdir commit, when we need to at least discard
the previous bmap state.
We already do all the gstate handling in lfs3_mdir_commit anyways, and
piggybacking on mdir commit lets us get rid of the annoying extra mdir
param in lfs3_alloc_ckpoint.
This does mean a slightly higher risk of needing to re-rebuild the bmap
after a powerloss, but in theory only if the user does something weird
like writing to a file and never calling sync. Most on-disk operations
terminate in an mdir commit as that's how any state change becomes
atomically visibile in littlefs.
Saves a nice bit of stack:
code stack ctx
before: 36920 2368 684
after: 36920 (+0.0%) 2368 (+0.0%) 684 (+0.0%)
code stack ctx
bmap before: 38552 2472 812
bmap after: 38464 (-0.2%) 2400 (-2.9%) 812 (+0.0%)
At least at a proof-of-concept level, there's still a lot of cleanup
needed.
To make things work, lfs3_alloc_ckpoint now takes an mdir, which
provides the target for gbmap gstate updates.
When the bmap is close to empty (configurable via bmap_scan_thresh), we
opportunistically rebuild it during lfs3_alloc_ckpoints. The nice thing
about lfs3_alloc_ckpoint is we know the state of all in-flight blocks,
so rebuilding the bmap just requires traversing the filesystem + in-RAM
state.
We might still fall back to the lookahead buffer, but in theory a well
tuned bmap_scan_thresh can prevent this from becoming a bottleneck (at
the cost of more frequent bmap rebuilds).
---
This is also probably a good time to resume measuring code/ram costs,
though it's worth repeating the above note about the bmap work still
needing cleanup:
code stack ctx
before: 36840 2368 684
after: 36920 (+0.2%) 2368 (+0.0%) 684 (+0.0%)
Haha, no, the bmap isn't basically free, it's just an opt-in features.
With -DLFS3_YES_BMAP=1:
code stack ctx
no bmap: 36920 2368 684
yes bmap: 38552 (+4.4%) 2472 (+4.4%) 812 (+18.7%)
This doesn't fully replace the lookahead buffer, but at least augments
it with known bmap state when available.
To be honest, this is a minimal effort hack to try to get something
benchmarkable without dealing with all the catch-22 issues that a
self-support bmap allocator would encounter (allocating blocks for the
bmap requires a bmap, oh no).
Though now that I'm writing this, maybe this is a reasonable long-term
solution? Having the lookahead buffer to fall back on solves a lot of
problems, and, realistically, it's unlikely to be a performance
bottleneck unless the user has extreme write requests (>available
storage?).
---
Also tweaked field naming to be consistent between the bmap and
lookahead buffer.
This is an indulgence to simplify the upcoming auxiliary btree work.
Brings back the previously-reverted per-btree leaf caches, where each
lfs3_btree_t keeps track of two rbyds: The root and the most recently
accessed leaf.
At the surface level, this optimizes repeated access to the same btree
leaf. A common pattern for a number of littlefs's operations that has
proven tricky to manually optimize:
- Btree iteration
- Pokes for our crystalization heuristic
- Checksum collision resolution for dids and (FUTURE) ddkeys
- Related rattrs attached to a single bid
But the real motivation is to drop lfs3_btree_*lookupleaf and simplify
the internal APIs. If repeated lfs3_btree_lookup*s are already
efficient, there's no reason for extra leaf-level APIs, and in theory
any logic that interacts with btrees will be simpler.
---
This comes at a cost (humorously about the same amount as the
tag-returning refactor, if you ignore the extra 28 bytes of ctx).
Unsurprisingly, increasing the size of lfs3_btree_t has the biggest
impact on stack and ctx:
code stack ctx
before: 36084 2336 656
after: 36784 (+1.9%) 2400 (+2.7%) 684 (+4.3%)
Also note from the previous commit messages: Btree leaf caching has
resulted in surprisingly little performance improvement for our current
benchmarks + implementation. It turns out if you're dominated by write
cost, optimizing btree lookups -- which already skip rbyd fetches, has
barely noticeable impact.
---
A note on reverting!
Eventually (after the auxiliary btree work) it will probably make sense
to revert this -- or at least provide a non-leaf-caching build for
code/RAM sensitive users.
I don't think this should be reverted as-is. Instead, I think we should
allow the option to just disable the leaf cache, while keeping the
simpler internal API. This would give us the best of all three worlds:
- A small code/RAM option
- Optimal btree iteration/nearby-lookup performance
- Simpler internal APIs
The only reason this isn't already implemented is because I want to
avoid fragmenting the codebase further while we're still in development
mode.
- test_traversal -> test_trvs
- lfs3_traversal_t -> lfs3_trv_t
- lfs3_btraversal_t -> lfs3_btrv_t
- t -> trv
- bt -> btrv
- lfs3_traversal_* -> lfs3_trv_*
- lfs3_btraversal_* -> lfs3_btrv_*
The traversal type is becoming one of the more fundamental types in
littlefs, and if DIR and REG both get shortened names, it makes sense
for TRV to have one as well.
This also removes the temptation to use t for traversals, which is
probably an even worse name.
---
Note that lfs3_btree_traverse, lfs3_mtree_traverse, etc, remain
unaffected. This may change in the future, but it's interesting to note
that verbs seem to need much less typing than nouns.
- lfs3_btree_lookupleaf
- lfs3_btree_lookupnext
- lfs3_btree_lookup
- lfs3_btree_traverse
- NOT lfs3_btree_namelookup
Looks like we're starting to claw back stack usage a bit. This makes
sense as the btree logic involves the most layers -- with out-pointers
it needs more temporary copies to inspect tags along the way:
code stack ctx
before: 36576 2376 656
after: 36520 (-0.2%) 2352 (-1.0%) 656 (+0.0%)
See the relevant commit for why. These just added surprisingly little
performance benefit for the code/stack cost.
Maybe in a future performance-preferring littlefs driver.
The idea here, is we give each lfsr_btree_t an optional leaf rbyd, in
addition to the root rbyd. This leaf rbyd acts as a cache for the most
recent leaf, allowing nearby btree lookups to skip the full btree walk.
Unfortunately, this failed on pretty much every measurable metric...
---
The motivation for this is that we often do a bunch of nearby btree
lookups:
- Btree iteration via lfsr_btree_lookupnext is a bit naive, walking from
the root every step.
- Our crystallization algorithm requires a bunch of nearby lookups to
figure out our crystallization heuristic. Currently at most 4, when
you need to lookup both crystal neighbors and then _also_ both
fragment neighbors for coalescing.
- Checksum collision resolution for dids and (FUTURE) ddkeys can require
an unbounded number of sequential lookups.
Though to be fair, this is an exceptional case if our checksum is any
good.
- Bids with multiple rattrs require nearby lookups to resolve.
Though currently this can be explicitly avoided via
lfsr_btree_lookupleaf + lfsr_rbyd_lookup.
The theory was that cases like these could explicitly keep track of the
leaf rbyd to avoid full btree walks, but in practice this never really
worked out. Tracking if we're still in the relevant leaf rbyd just adds
too much logic/code cost.
But if this leaf tracking logic was implemented once in the btree
layer...
The other theoretical benefit was being able to move more rbyds off the
stack. Sure our btrees take up more RAM, but if that results in stack
savings, that may be a win.
Oh, and this would let our btree API and rbyd API converge without
performance concerns. Internal users could in theory call
lfsr_btree_lookupnext + lfsr_btree_lookup with the same performance as
explicitly tracking the rbyd.
---
But this was a complete failure!
First the good news: There was a modest speedup of around ~2x to linear
reads.
And that's the good news.
Now the bad news:
1. There was no noticeable performance gain in any other benchmarks.
To be fair, we're at the early stages of benchmarking, so the
benchmarks may not be the most thorough, but thinking about it, there
are some explanations:
- In any benchmark that writes, fetch + erase + prog dominates. Being
able to skip fetches during lookups makes our btree lookups
surprisingly cheap!
- Any random read heavy benchmark is likely thrashing this cache,
which is to be expected.
- For small 1-block btrees, the leaf cache is useless because the
entire btree is cache in the root rbyd.
And keep in mind, our blocks are BIG. "Small" here could be on
the order of ~128KiB-1MiB for NAND flash.
- For the mtree, fetched mdirs actually already act as a sort of leaf
cache.
The extra btree leaf cache isn't doing _nothing_, but each layer of
the mtree has diminishing returns due to btree's ridiculous
branching factor.
- For file btrees, we're explicitly caching the leaf fragments/
blocks, so the extra btree leaf cache has diminishing returns for
the same reason.
2. Code cost was bad, stack cost was worse:
code stack ctx
before: 37172 2288 636
after: 38068 (+2.4%) 2416 (+5.6%) 664 (+4.4%)
Tracking the leaf required more code, that's expected. And, to be
fair, the current code has had a lot more time to congeal.
What wasn't expected was the stack cost.
Unfortunately these caches didn't really take any rbyds off the stack
hot-path:
- We _can_ get rid of the rbyd in lfsr_btree_lookup/namelookup, but
we were already hacking our way around the critical one in
lfsr_mtree_lookup/namelookup by reusing the mdir's rbyd!
- We can't even abuse the leaf rbyd in the commit logic, since the
target btree can end up iterated/traversed by lfs_alloc.
That was a fun bug.
And the addition of a second rbyd to lfsr_btree_t increases both ctx
and stack anywhere btrees are allocated.
Maybe this will make more sense when we add the auxiliary btrees, or
after more benchmarking, but for now the theoretical performance
improvements just aren't worth it.
Will probably revert this, but I wanted to commit it in case the idea is
worth resurrecting in the future, if in the future nearby btree lookups
are a bigger penalty than they are now.
So now calling lfsr_file_sync on zombied files is a noop:
// create a file
lfsr_file_t a;
lfsr_file_open(&lfs, &a, "a",
LFS_O_RDWR | LFS_O_CREAT | LFS_O_EXCL) => 0;
// remove, creating a zombie
lfsr_remove(&lfs, "a") => 0;
// sync, this is now a noop (previously LFS_ERR_NOENT)
lfsr_file_sync(&lfs, &a) => 0;
// close is also a noop
lfsr_file_close(&lfs, &a) => 0;
I've been on the fence on this for a while, on one hand erroring
provides more information to the user, on the other hand a noop is less
surprising if the user comes from other systems.
Ended up making this a noop. I figured minimizing surprises is good API
design, and the user can always use lfsr_stat to check if the file still
exists.
This also matches POSIX, and, perhaps more importantly, the current
version of littlefs.
---
Note that lfsr_file_resync still errors with LFS_ERR_NOENT. It's hard to
argue the file "matches the state of disk" otherwise.
Code changes minimal:
code stack ctx
before: 35784 2440 640
after: 35780 (-0.0%) 2440 (+0.0%) 640 (+0.0%)
Seems like a better name now that LFS_TYPE_STICKYNOTE is its own file
type.
Though this does contain some tests that I think don't even use
stickynotes...
This adds the LFS_TYPE_STICKYNOTE type, allowing users to interact with
stickynotes as long as they aren't orphaned.
This hopefully solves the long-standing mess that was the LFS_O_EXCL
API.
---
As for what I mean by orphaned vs non-orphaned stickynotes:
Non-orphaned stickynotes represent files that have been "created" (via
LFS_O_CREAT), but not "committed" (via sync/close). You can still close
and convert the stickynote to a reg file, so these aren't orphans. These
are also called "uncreated" files in some parts of the codebase:
- open+O_CREAT -> non-orphaned stickynote (uncreated file)
Orphaned stickynotes are possible by either removing an open file, or
desyncing a file before sync/close. These are still invisible to the
user and will be eventually cleaned up after the last file handle is
closed:
- open+remove -> orphaned stickynote (zombied file)
- open+O_CREAT+desync+close -> orphaned stickynote (orphaned file)
Desynced files are a bit special. Even though they technically aren't
orphaned, they also behave like orphaned file handles:
- open+O_CREAT+close -> orphaned stickynote (desynced file)
The idea is this mimics the state of files post-close, and allows for
some tricks like using a desync file as a temporary file with no
observable effects on the filesystem.
---
The motivation for this comes from staring at the LFS_O_EXCL API for too
long and realizing the problem is that littlefs's API contradicts itself
when it comes to whether or not uncreated files exist.
This solution is to consistently treat uncreated files as though they
exist (the alternative would make LFS_O_EXCL pretty much useless), but I
really didn't want to do this as having what appears to be normal files
disappear after powerloss risks confusion.
The compromise here is to give these files a special type, repurposing
the internal LFS_TAG_STICKYNOTE, which hopefully hints to the user these
won't behave like normal files.
If the user is more interested in POSIX compatibility, they can always
map these to either LFS_TYPE_REG or LFS_ERR_NOENT, whichever they think
is the least confusing.
As a quirk of littlefs's API, stickynotes should never actually contain
any data, and will always have size 0.
However they can have custom attributes assigned now (which is I guess
ok? also TODO should probably test this).
---
The implementation right now is a bit naive, I mostly just wanted to get
the tests working again in this new model. It may be possible to claw
back some of this code cost:
code stack ctx
before: 35740 2440 640
after: 35952 (+0.6%) 2440 (+0.0%) 640 (+0.0%)
This was a surprising side-effect the script rework: Realizing the
internal btree/rbyd lookup APIs were awkwardly inconsistent and could be
improved with a couple tweaks:
- Adopted lookupleaf name for functions that return leaf rbyds/mdirs.
There's an argument this should be called lookupnextleaf, since it
returns the next bid, unlike lookup, but I'm going to ignore that
argument because:
1. A non-next lookupleaf doesn't really make sense for trees where
you don't have to fetch the leaf (the mtree)
2. It would be a bit too verbose
- Adopted commitleaf name for functions that accept leaf rbyds.
This makes the lfsr_bshrub_commit -> lfsr_btree_commit__ mess a bit
more readable.
- Strictly limited lookup and lookupnext to return rattrs, even in
complex trees like the mtree.
Most use cases will probably stick to the lookupleaf variants, but at
least the behavior will be consistent.
- Strictly limited lookup to expect a known bid/rid.
This only really matters for lfsr_btree/bshrub_lookup, which as a
quirk of their implementation _can_ lookup both bid + rattr at the
same time. But I don't think we'll need this functionality, and
limited the behavior may allow for future optimizations.
Note there is no lfsr_file_lookup. File btrees currently only ever
have a single leaf rattr, so this API doesn't really make sense.
Internal API changes:
- lfsr_btree_lookupnext_ -> lfsr_btree_lookupleaf
- lfsr_btree_lookupnext -> lfsr_btree_lookupnext
- lfsr_btree_lookup -> lfsr_btree_lookup
- added lfsr_btree_namelookupleaf
- lfsr_btree_namelookup -> lfsr_btree_namelookup
- lfsr_btree_commit__ -> lfsr_btree_commit_
- lfsr_btree_commit_ -> lfsr_btree_commitleaf
- lfsr_btree_commit -> lfsr_btree_commit
- added lfsr_bshrub_lookupleaf
- lfsr_bshrub_lookupnext -> lfsr_bshrub_lookupnext
- lfsr_bshrub_lookup -> lfsr_bshrub_lookup
- lfsr_bshrub_commit_ -> lfsr_bshrub_commitleaf
- lfsr_bshrub_commit -> lfsr_bshrub_commit
- lfsr_mtree_lookup -> lfsr_mtree_lookupleaf
- added lfsr_mtree_lookupnext
- added lfsr_mtree_lookup
- added lfsr_mtree_namelookupleaf
- lfsr_mtree_namelookup -> lfsr_mtree_namelookup
- added lfsr_file_lookupleaf
- lfsr_file_lookupnext -> lfsr_file_lookupnext
- added lfsr_file_commitleaf
- lfsr_file_commit -> lfsr_file_commit
Also added lookupnext to Mdir/Mtree in the dbg scripts.
Unfortunately this did add both code and stack, but only because of the
optional mdir returns in the mtree lookups:
code stack ctx
before: 35520 2440 636
after: 35548 (+0.1%) 2472 (+1.3%) 636 (+0.0%)
And the related config options:
- cfg->file_buffer_size -> cfg->file_cache_size
- file->cfg->buffer_size -> file->cfg->cache_size
- file->cfg->buffer -> file->cfg->cache_buffer
The original motivation to rename this to file->buffer was to better
align with what other filesystems call this, but I think this is a case
where internal consistency is more important than external consistency.
file->cache better matches lfs->pcache and lfs->rcache, and makes it
easier to read code involving both file->cache and other user-provided
buffers.
Keeping the upstream name also helps with continuity.
Mainly just for self-documentation reasons.
This may also make it easier to add LFSR_RATTR_BUF-specific asserts/
tweaks/etc, and helps future refactoring.
But functionally LFSR_RATTR_BUF is equivalent to LFSR_RATTR for now.
No code changes.
This finishes the eager -> lazy attr encoding rework.
Which makes it a good time to look at the total savings from adopting
lazy attr encoding, though there's still a bit of tinkering to do (eager
branches, cksum tags, etc):
code stack ctx
before lazy-attrs: 36280 2576 636
after lazy-attrs: 35592 (-1.9%) 2472 (-4.0%) 636 (+0.0%)
A ~free 688 byte savings in code and 104 bytes in stack is not bad.
This fully adopts LFSR_RATTR__ and friends:
- LFSR_RATTR -> LFSR_RATTR__ or LFSR_RATTR_DATA__
- LFSR_RATTR_BUF -> LFSR_RATTR__
- LFSR_RATTR_CAT -> LFSR_RATTR_CAT__
- LFSR_RATTR_NOOP -> LFSR_RATTR_NOOP__
- LFSR_RATTR_NAME -> LFSR_RATTR_NAME__
Note the new LFSR_RATTR__ macro also lets us a drop the special rattr
macros, at the cost of a bit less type safety:
- LFSR_RATTR_RATTRS -> LFSR_RATTR__
- LFSR_RATTR_MOVE -> LFSR_RATTR__
- LFSR_RATTR_GRM -> LFSR_RATTR__ (we weren't using this?)
- LFSR_RATTR_SHRUBCOMMIT -> LFSR_RATTR__
Curiously, this ended up adding ~88 bytes to lfsr_file_carve:
function (0 added, 0 removed) osize nsize dsize
lfsr_file_carve 1228 1316 +88 (+7.2%)
lfsr_mdir_commit 2144 2152 +8 (+0.4%)
lfsr_mdir_commit__ 1192 1188 -4 (-0.3%)
lfsr_file_truncate 184 182 -2 (-1.1%)
lfsr_mount 98 96 -2 (-2.0%)
TOTAL 35508 35596 +88 (+0.2%)
I'm really not sure why, all I can think of is maybe the change from a
forced-inline function to a macro added a bunch of compiler noise?
Still, 80 bytes is not worth two competing LFSR_RATTR APIs. Though
it may be worth looking into this in the future.
Total code changes:
code stack ctx
before: 35508 2472 636
after: 35596 (+0.2%) 2472 (+0.0%) 636 (+0.0%)
This is the correct name for our rbyd attr type, even if it requires a
bit more typing.
lfsr_attr_t would be a better name, but that conflicts with our
user-facing attrs.
While they are a bit more annoying to call, init functions give the
compiler a chance to deduplicate common struct initialization logic. So
we should probably prefer init functions for any structs larger than a
couple words.
The cost of each init is small, but it really adds up!
code stack ctx
before: 38036 2608 752
after: 37844 (-0.5%) 2608 (+0.0%) 752 (+0.0%)
We already have lfsr_cat_t so...
lfsr_rattr_t is a pretty fundamental type for littlefs, unfortunately
the name "rattr" is a mouthful. Shortening this to just "rat" hopefully
makes things easier to read at the cost of it being a bit less clear
what lfsr_rat_t actually is.
Though it's possible I've been staring at the dwarf spec (DW_AT_*) for
too long...
To avoid the obvious conflict with lfs_attr. Unlike lfsr_rattr_t,
lfs_attr is user facing, so it gets priority.
This name may change in the future if something better comes up, but in
the meantime we need to change the name to _something_.
Is this the reason Linux/BSD/etc call these xattrs?
(Note littlefs's attrs are much more limited than xattrs. We should
_not_ call these xattrs in case we want to add true xattrs in the
future.)
These are our current set of general-purpose high-level tests that can
be turned to when needing to test a wide range of filesystem operations.
They were getting a bit hard to keep track of without a consistent
prefix, especially since no individual test suite can actually use all
of them at the same time.
Now, finding these tests is as simple as: ./scripts/test.py -L *_spam_*
I also renamed a couple because their names were starting to get
ridiculous. I mean just look at
test_badblocks_alternating_spam_orphanzombiedir_fuzz...
- *_spam_orphanzombie_fuzz -> *_spam_oz_fuzz
- *_spam_orphanzombiedir_fuzz -> *_spam_ozd_fuzz
- *_spam_file_pl_fuzz -> *_spam_f_pl_fuzz
- *_spam_filedir_pl_fuzz -> *_spam_fd_pl_fuzz
Here are all of the current spam tests and contexts we use them in:
traversal badblocks relocations
| gc ck grow | powerloss exhaustion
dir_many y y y y y y
dir_fuzz y y y y y y y
file_many y y y y y y
file_fuzz y y y y y y y
fwrite_fuzz y y y y y
oz_fuzz y y y y y y y
ozd_fuzz y y y y y y y
f_pl_fuzz y y y
fd_pl_fuzz y y y
Instead of testing every block (which test_badblocks_every already
does) with a single random bit-error, the new test_ck_spam tests
continuously throw bit-errors at the filesystem until it fails.
This should reveal much more interesting failures than flipping a single
bit in the entire device, while also taking less testing time. And we
still have test_badblocks_every to make sure no specific problem blocks
(except the mrootanchor) are missed.
This makes test_ck_spam more similar to test_exhaustion than
test_badblocks_every.
All this being said, these tests are still sort of in stasis until
rollback protection gets sorted out. So we're not actually testing
anything interesting yet...
I've also reverted the test_badblocks -> test_ck dependency, since we
want to keep the longer-running tests near the end of the queue.
These are basically the same as our test_badblock tests, except we
accept LFS_ERR_CORRUPT. This lets us test more checking modes that may
not enable recovery (ckreads, ckfetches, etc).
Well, in theory, at least. The lack of rollback protection gets in the
way of both ckreads and ckfetches, so we're currently only testing
ckprogs, which isn't much of an improvement. At least this gets the
scaffolding in place...
This also inverts the test_ck -> test_badblocks dependency. Now that
these both have exhaustive tests, we might as well limit test_badblocks
to simple erroring erases/progs and let test_ck check the ck checks.
So just like ckreads, ckprogs is now opt-in, requiring both 1. defining
LFS_CKPROGS at compile-time, and 2. passing the LFS_M_CKPROGS flag
during lfsr_mount.
_Unlike_ ckreads, ckprogs is actually a very lightweight feature. So the
difference between compiling with/without ckprogs is really quite small:
code stack
before: 36480 2680
yes-ckprogs: 36480 (+0.0%) 2680 (+0.0%)
no-ckprogs: 36428 (-0.1%) 2680 (+0.0%)
It's almost not worth putting behind an ifdef if not for consistency
with ckreads.
This is mainly to solve the weird check-hole where passing CKPROGS/
CKREADS as mount flags has no effect on lfsr_format (I mean, it'd be a
bit silly if it did somehow):
LFS_F_RDWR 0 // Format the filesystem as read and write
LFS_F_CKPROGS 0x00000010 // Check progs by reading back progged data
LFS_F_CKREADS 0x00000020 // Check reads via parity bits/checksums
This makes lfsr_format a more cumbersome interface, but I don't know if
this is necessarily a bad thing. There's always risk of data loss when
calling lfsr_format, so maybe it should be a pain to call.
At the very least, format flags may be useful in the future for
enabling/disabling format-time things such as the planned block-map,
parity-tree, etc. Though it's unclear if such significant settings
should be format flags or somehow encoded as fields in our config
struct.
---
The LFS_F_* format flags of course ended up conflicting with our
internal LFS_F_* flags, so I renamed most of the internal flags to match
the closest flag set they participate in:
- LFS_F_TYPE -> LFS_O_TYPE
- LFS_F_UNFLUSH -> LFS_O_UNFLUSH
- LFS_F_UNSYNC -> LFS_O_UNSYNC
- LFS_F_ORPHAN -> LFS_O_ORPHAN
- LFS_F_ZOMBIE -> LFS_O_ZOMBIE
- LFS_F_ORPHANS -> LFS_I_ORPHANS
- LFS_F_UNCOMPACTED -> LFS_I_UNCOMPACTED
- LFS_F_TSTATE -> LFS_T_TSTATE
- LFS_F_BTYPE -> LFS_T_BTYPE
- LFS_F_DIRTY -> LFS_T_DIRTY
- LFS_F_MUTATED -> LFS_T_MUTATED
This may make it a bit less clear which flags are a part of the public
API, vs intended only for internal use, but at the very least our asserts
in format/mount/open/etc should catch most of these mistakes.
---
Code cost ended up being pretty minimal. Actually negative. This is the
second time we're _adding_ a feature that somehow saves code, though the
reality for this one is we're really just pushing constants up into the
user's stack frame. Still, it's a good indication the cost of format
flags is small:
code stack
before: 36452 2680
after: 36448 (-0.0%) 2680 (+0.0%)
Metastability is a rather nasty error condition where successive reads
to a memory location may return different values, either due to bus
issues or a failed prog. It's a tricky error condition to detect, and
one that ckreads was, in theory, supposed to help with.
To help test metastability (and other single-bit errors), emubd gained
several new features:
- LFS_EMUBD_BADBLOCK_PROGFLIP - Prog flips a bit
- LFS_EMUBD_BADBLOCK_READFLIP - Read flips a bit sometimes
- LFS_EMUBD_POWERLOSS_METASTABLE - Reads may flip a bit
These only affect a single bit in a given block, but by randomizing
which bit during every erase (and exhaustive bit testing in test_ck) we
should still see some fairly interesting bit-error patterns over time.
It's a bit difficult to test with more than a single bit error because
you can quickly find checksum/parity collisions when fuzz testing. But
there may be other interesting error patterns to look at in the future?
Also the erase_cycles implementation got a bit of a rework since it was
lopsided previously (progs/reads would always error before erases). And
since I was messing with emubd's internals I added lfs_emubd_markbad/
markgood and a few other convenience functions that seem useful:
- lfs_emubd_seed - Manually set the prng, needed in test_ck actually
- lfs_emubd_markbad - Mark block as bad, same as wear=-1
- lfs_emubd_markgood - Mark block as good, same as wear=0
- lfs_emubd_badbit - Get which big failed
- lfs_emubd_setbadbit - Set which bit will fail
- lfs_emubd_randomizebadbit - Randomize bad bit on erase
- lfs_emubd_markbadbit - Mark bit as bad, same as setbadbit+markbad
---
The intention of this new metastability emulation was to extend test_ck
to test ckreads/ckprogs. This went... interestingly.
The good news, the new emulation and tests worked quite well. They were
able to quite quickly show that ckreads is fundamentally not able to
detect all single-bit errors in our current design.
The problem boils down to the fact that the location of our parity bits
depends on the tag's leb128-encoded size. If a bit flip changes this
size field, we end up with a new parity bit, which 50/50 may or may not
detect the error.
For example, one bit flip:
40 0c 00 12 80 0d ff ff
'----.----' ^--------------------.
'- altble 0xc w0 -18 parity=1
40 0c 80 12 80 0d ff ff
'-------.-------' ^----------------------.
'- altble 0xc w2304 -1664 parity=1
This doesn't make ckreads _completely_ useless, just mostly useless. We
can still use it to check parity bits, but without a systematic proof.
But there's enough problems with ckreads: performance, RAM, code, etc,
that I think it may just be an interesting proof-of-concept and not
something users should actually use. Checking reads in the bd-layer
solves all of these problems...
---
At the very least ckprogs gets better testing, thanks to new tests in
test_ck and the addition of LFS_EMUBD_BADBLOCK_PROGFLIP in
test_badblocks.
The extra testing also found a ckprog/ckread hole in that we don't
ckprog/ckread during lfsr_format! I fixed this by making lfsr_format
always use ckprogs/ckreads if available, but maybe lfsr_format should
take its own set of flags?
Funnily enough this had no impact on code size since it probably just
changed the constant in a constant pool:
code stack
before: 37872 3048
after: 37872 (+0.0%) 3048 (+0.0%)
The main reason for this change is to allow keeping track of existing
known-free blocks while trying to find more free blocks. This makes it
so failed filesystem traversals don't result in negative progress, which
is nice.
This was difficult in the previous lookahead scheme, since we we'd need
to shift the lookahead buffer to keep off=0 rooted at the first bit.
Shifting bytes is relatively easily with memmove, but it gets tricky
when shifting bits:
lookahead before: ???? ???? ???? ??00 1101 0101 00?? ????
^ ^
off off+size
shift: 0011 0101 0100 ???? ???? ???? ???? ????
^ ^
off off+size
traverse: 0011 0101 0100 0000 0000 0000 1100 0000
^ ^
off off+size
Instead, we now just let the lookahead buffer wrap around. No shifting
required:
lookahead before: ???? ???? ???? ??00 1101 0101 00?? ????
^ ^
off off+size
traverse: 0000 0000 1100 0000 1101 0101 0000 0000
^
off
^
off+size
This gets a bit confusing with the lookahead window also wrapping around
disk, but the math works out with enough modulos (if modulos are too
expensive, we should eventually be able to optimize these into simple
bit masks via compile-time config).
In the future, if we move away from the const config struct, it would
also be nice to try to reducing the number of modulos by storing the
lookahead buffer size in bits instead of bytes...
Note that if the lookahead buffer is larger than disk, the lookahead
window will sort of travel around the underlying buffer. This isn't
inherently a problem, but it did cause some bugs.
To avoid similar bit-related problems with zeroing, lfs_alloc_inc now
also zeros bits as we allocate/skip them, so bits should always be zero
when we start a lookahead traversal. Though note we still need to
manually memset the buffer when discarding lookahead state in init/grow.
---
The end result is surprisingly a net savings in terms of code size. I
guess mainly due to dropping all the lfs_alloc_shift calls:
code stack
before: 36472 2680
after: 36412 (-0.2%) 2680 (+0.0%)
This has been a long-time coming, mount flags are just too useful for
configuring a filesystem at runtime.
Currently this is limited to LFS_M_RDONLY and LFS_M_CKPROGS, but there
are a few more planned in the future:
LFS_M_RDWR = 0x0000, // Mount the filesystem as read and write
LFS_M_RDONLY = 0x0001, // Mount the filesystem as readonly
LFS_M_STRICT* = 0x0002, // Error if on-disk config does not match
LFS_M_FORCE* = 0x0004, // Ignore compat flags, mount readonly
LFS_M_FORCEWITHRECKLESSABANDON*
= 0x0008, // Ignore compat flags, mount read write
LFS_M_CKPROGS = 0x0010, // Check progs by reading back progged data
LFS_M_CKREADS* = 0x0020, // Check reads via checksums
* Hypothetical
As a convenience, we also return mount flags in the struct lfs_fsinfo's
flags field as their relevant LFS_I_* variants. Though only to match
statvfs, and only because it's cheap, littlefs's API is low-level and we
should expect users to know what flags they passed to lfsr_mount.
As for the new mount flags:
- LFS_M_RDONLY - For consistency with existing APIs, this just asserts
on write operations, which makes it a bit useless... But the info flag
LFS_I_RDONLY may be useful for falling back to a readonly mode if
we encounter on-disk compat issues.
At least if implement the theoretical LFS_UNTRUSTED_USER mode
LFS_M_RDONLY could become a runtime error.
- LFS_M_RDWR - This really just exists to compliment LFS_M_RDONLY and to
match LFS_O_RDONLY/LFS_O_RDWR. It's just an alias for 0, and I don't
think there will ever be a reason to make it non-0 (but I can always
be wrong!).
- LFS_M_CKPROGS - This replaces the check_progs config option and avoids
using a full byte to store a bool.
We should probably also have a compile-time option to compile this out
(LFS_NO_CKPROGS?), but that's a future thing to do.
This ended up adding a surprising bit of code, considering we're just
moving flags around, and noise in lfs_alloc added a bit of stack again:
code stack
before: 35880 2672
after: 35932 (+0.1%) 2680 (+0.3%)
This just provides a simple, easy-to-call, wrapper over the new
traversal API:
int lfsr_fs_gc(lfs_t *lfs, uint32_t flags);
The main difference from its previous incarnation, is that lfsr_fs_gc
now takes a flags argument to indicate exactly what gc operations to
perform. This gives the user more control, and may also make the API
more robust towards adding new features:
LFS_GC_MTREEONLY = 0x0010, // Only traverse the mtree
LFS_GC_MKCONSISTENT = 0x0020, // Make the filesystem consistent
LFS_GC_LOOKAHEAD = 0x0040, // Populate lookahead buffer
LFS_GC_COMPACT = 0x0080, // Compact metadata logs
LFS_GC_CKMETA = 0x0100, // Check metadata checksums
LFS_GC_CKDATA = 0x0200, // Check metadata + data checksums
LFS_GC_REPAIRMETA+ = 0x0400, // Repair metadata blocks
LFS_GC_REPAIRDATA+ = 0x0800, // Repair metadata + data blocks
+ Planned
Alternatively, gc_flags could have been added as a config option. But
making gc_flags a function argument matches other flag APIs (open
mainly), and is slightly more flexible in that it allows a system to do
different gc operations in different system states (though this could
also be accomplished with the hypothetical lfsr_fs_gccfg, which would
probably be good to add anyways).
Worst case, defining a system-wide define that you always pass to
lfsr_fs_gc accomplishes roughly the same thing.
---
This adds a bit more code, mainly to check if we actually need to
traverse, and to make sure traversals accomplish all of the requested
work.
code stack
before: 35448 2680
after: 35708 (+0.7%) 2672 (-0.3%)
Curiously it also saved a bit of stack, which is a bit silly given this
commit is purely code addition. Apparently something in lfs_alloc and
lfsr_fs_gc is shared, getting uninlined, and messing with the stack
measurement. lfs_alloc is quite sensitive to stack changes after all.
There can always be more tests, but I think these give a nice set of
coverage over corner-cases in our traversal clobbering scheme.
These did find a couple bugs:
- If we clobber an inlined mroot, we need to adjust the mid by two
mdirs, but only if there is no mtree/mdirs.
To avoid this and other mid-related headaches, we just provide the new
mid in lfsr_mdir_commit, since we always know it here.
- lfsr_mdir_commit compares mdirs by mptr, which means we need to
clobber traversal's mdir's mptrs or else lfsr_mdir_commit will clobber
already-clobbered traversals.
There may be a better way to solve this, but it will probably get into
the weeds with how lfsr_mdir_commit relies on mids vs mptrs...
Code changes:
code stack
before: 34570 2624
after: 34566 (-0.0%) 2624 (+0.0%)
Now that the dust has settled and we sort of know what the traversal
implementation will look like, we can look at the before and after to
get a rough idea of how much the traversal API actually costs:
code stack
no-traversal (before): 33886 2560
yes-traversal (after): 34566 (+2.0%) 2624 (+2.5%)
Note this still includes the annoying lfsr_btree_traverse inlining stack
cost, which isn't really the traversal API's fault and may be avoidable
in the future.
We don't actually need these, all we need are utils defined for the
largest integer size we operate on, currently uint32_t.
Counterintuitively this should make it easier to adopt different integer
widths in the future.
Or maybe this will bite us when lfs_off_t >> lfs_size_t? Oh well, if
that's the case we can fix it then.
No code changes:
code stack
before: 33886 2560
after: 33886 (+0.0%) 2560 (+0.0%)
Well this turned into a never-ending can of worms...
I guess the good news is our newly added lfsr_grow_incr_* tests are
_very_ good at finding post-error-resume bugs.
Implementation-wise, this was fairly straightforward thanks to prior
work by BrianPugh, kaetemi, and myself:
1. Made block_count pseudo-optional by adding lfs.block_count so we can
mutate it based on what we find on-disk.
This was done a bit different from the previous implementation,
instead of setting block_count=0 to read the block_count from disk,
we allow any block_count <= the configured block_count.
This matches how we handle name_limit/file_limit/etc, and allows
users to mount a filesystem with unknown block_count while asserting
an upper bound.
2. Added lfsr_fs_grow, which can grow the filesystem.
The is basically the same as the previous implementation except we're
a bit more careful with the lookahead buffer.
I thought the previous impl might have been broken w.r.t. lookahead
buffer, but fortunately it's only broken in a way that makes us think
newly available blocks are temporarily in-use. Which is a bit funny.
One interesting thing that came out with more aggressive tests is
that it's possible to get locked-up in lfsr_fs_preparemutation trying
to clean up grms/orphans before we change the filesystem size.
Fortunately it turns out we don't _really_ need to call
lfsr_fs_preparemutation here. This gets a bit delicate, but means we
should always be able to grow a full filesystem.
To test this I've added both the simple grow/error tests from the
previous version, as well as a set of fuzz tests (a la test_relocations
and friends) that incrementally grow the filesystem when encountering
LFS_ERR_NOSPC. These have a surprising amount coverage, testing
lfsr_fs_grow, lfsr_fs_stat, lfsr_fs_size, and resuming operations after
encountering an error.
Which also means they found bugs:
- lfs_alloc_setinuse was not broken before, because lookahead.start was
always a multiple of lookahead_size. But now with lfs_alloc_discard,
this invariant may not be true.
I've just changed all lookahead.start updates to mod block_count. This
adds a bit of code, but is much easier to reason about.
While fixing this, I also added an assert to never allocate blocks
{0,1} in lfs_alloc. This is a good assert to have, but did require
some tweaks to test_btree to avoid these blocks.
- We were incorrectly patching grms in lfsr_mdir_commit when mdelta=0.
Funnily enough we also proceed to ignore the patched grm most of the
time when mdelta=0, so this went unnoticed.
- It turns out we're completely ignoring rid=-1 attrs if we split the
mroot. Not sure how this was missed. It's a bit important.
Note this is still broken. Fixing this requires some rather invasive
changes to lfsr_mdir_commit's internal logic that should probably be
in another commit...
Note again fwrite_fuzz is omitted. Currently the state of data in opened
files is undefined after a failed write, so this wouldn't really be
testing anything interesting...
More features = more code, and all of this bug fixing meant several
things contributed to code/stack changes in this commit:
code stack
before: 33654 2592
+variable block_count: 33646 (-0.0%) 2584 (+0.0%)
+lfsr_fs_grow: 33818 (+0.5%) 2584 (-0.3%)
+lookahead-start-fix: 33842 (+0.6%) 2584 (-0.3%)
+grm-patch-fix (after): 33850 (+0.6%) 2584 (-0.3%)
Wild that variable block_count actually saves code/stack. I guess the
indirect lfs->cfg->block_count load can get costly...
It's counter-intuitive, but no top-level API should return
LFS_ERR_CORRUPT. Instead, if we can't make progress because of a corrupt
block, we should return LFS_ERR_NOSPC. This makes it easier for users to
write code that is well behaved even when a device is end-of-life.
It's up to our mroot extension algorithm to make sure this case can't be
reached in normal operation unless the device is _actually_ at
end-of-life.
Because mroot extension is a bit of a special case, we weren't
converting these corrupt errors to nospc errors consistently. This is
fixed now, along with a couple more hopefully-useful logging statements.
Found while playing around with test_exhaustion + block_recycles=-1.
This should assert on bad wear-leveling, but LFS_ERR_CORRUPT was
unexpected. Added an explicit test because this is an easy thing to let
split through:
- test_badblocks_mrootanchor_wear
Code changes were surprisingly minimal, I wonder if constants are being
swapped out somewhere low-level?
code stack
before: 33766 2600
after: 33770 (+0.0%) 2600 (+0.0%)
The main test additions are the test_powerloss tests, intended to be
high-level tests over difficult/weird powerloss environments (such as
out-of-order writes!):
- test_powerloss_dir_many - 2242 pls
- test_powerloss_file_many - 8856 pls
- test_powerloss_file_pl_fuzz - 384508 pls
- test_powerloss_filedir_pl_fuzz - 268339 pls
But there was also a bunch of other test movement in the late-stage/
high-level tests. I'm trying to keep the core of these tests somewhat
consistent so we have a nice template to extend for future testing, in
case we want to test other environmentalish concerns, but not all of
these tests make sense in all of these contexts:
badblocks powerloss relocations exhaustion
dir_many y y y
dir_fuzz y y y
file_many y y y
file_fuzz y y y
fwrite_fuzz y y
orphanzombie_fuzz y y y
orphanzombiedir_fuzz y y y
file_pl_fuzz y y
filedir_pl_fuzz y y
Why not:
- dir/file_many+exhaustion? - Needs to be unbounded
- dir/file_fuzz+powerloss? - Takes O(n^2)
- fwrite_fuzz+powerloss? - Takes O(n^2)
- fwrite_fuzz+relocations? - Doesn't really test anything
- orphanzombie*_fuzz+powerloss? - Powerloss kills zombies
- file*_pl_fuzz+badblocks? - PL + Badblocks currently incompactible
- file*_pl_fuzz+exhaustion? - PL + Badblocks currently incompactible
---
Of course, in order to actually get out-of-order write testing working,
we need to implement out-of-order write syncing.
Fortunately this was a simple exercise in placing lfsr_bd_sync calls
before any mdir commits where we may have unsynced data:
- in lfsr_file_sync, to sync any pending file data
- in lfsr_mdir_commit, to sync any mroot/mtree changes
We also call lfsr_bd_sync _after_ mdir commits in case users expect to
sequence any filesystem-external operations such as network, UI, etc. In
theory this could be optional, but no users have really requested it
yet, so leave that for a potential future improvement:
- in lfsr_mdir_commit
- in lfsr_formatinited (really just because we don't go through
lfsr_mdir_commit)
Note that lfsr_rbyd_commit has been relaxed in the scheme. It only
flushes caches, and does _not_ call lfsr_bd_sync. This is useful for
allowing multiple B-tree nodes to be written out-of-order, also long as
the whole thing is synchronized before any mdir commit.
All of these lfsr_bd_sync calls add a bit of code, but not really an
amount to care about:
code stack
before: 33678 2600
after: 33766 (+0.3%) 2600 (+0.0%)
While exploring the test_badblocks ERASENOOP failure more, I realized
the problem is that we are nesting crc32cs.
To be clear, using crc32cs to validate progs in general is not an issue,
that is perfectly fine on paper. The issue is that we were using crc32cs
to validate progs _that contain crc32cs_.
Looking at the collision, we can see the fully expanded lleb128s we use
for our cksum tags:
00 00 00 ff b0 02 00 87 80 80 00 3e c0 7f 7e => bdfa9b10
ab 77 de c2 b0 03 00 87 80 80 00 3e 38 d5 22 => bdfa9b10
'-.-' ^ '----.----' '----.----'
'----|------|-----------|-- cksum tag
'------|-----------|-- cksum weight (0)
'-----------|-- cksum size + padding
'-- cksum crc32c
So we ended up perfectly aligning the cksum's crc32c with our cache
line. Lucky us.
Unfortunately funny math makes it so that whenever a crc32c contains a
crc32c, the inner crc32c sort of cancels itself out from the outer
crc32c. So these two messages end up mathematically equivalent, even
though they contain different data:
crc(m) = m(x) x^|P|-1 mod P
crc(m ++ crc(m)) = (m(x) x^|P|-1 + (m(x) x^|P|-1 mod P)) x^|P|-1 mod P
crc(m ++ crc(m)) = (m(x) x^|P|-1 + m(x) x^|P|-1) x^|P|-1 mod P
crc(m ++ crc(m)) = 0 x^|P|-1 mod P
crc(m ++ crc(m)) = 0
So using a crc32c to check progs is not fit for purpose.
This leaves us with a couple options:
1. Use a different checksum, or do something like rearranging bytes to
avoid this cancelling out issue. Unfortunately this gets tricky since
crc32cs are linear, simply using an xor mask won't work...
2. Don't check progs at such a low-level, but at a high-level using the
rbyd/data block crc32cs. Since this would mean only one crc32c, this
would avoid nesting issues. Unfortunately this would probably come
with quite a high code cost to try to keep track of both the
before+after rbyd cksums everywhere...
3. Just read back the data into the rcache to compare at the byte-level,
which would mean clobbering our rcache when prog checking is enabled.
This commit goes with option 3., which is probably the simplest. It also
removes any question of crc32c collision, which could be a real nuisance
when debugging low-level block device operations, a use case where prog
checking will hopefully be quite valuable.
Clobbering the rcache also has the advantage of reverting the prog
>= read requirement, which is nice for flexibility. Though this needs to
be tested.
---
There was a bit of a hiccup, and that was how prog checking interacts
with lfsr_bd_cpy. lfsr_bd_cpy used the rcache to hold data being copied
to/from disk, but this data needs to be checked, and prog checking would
clobber the rcache. Problems! I guess this is one footgun of the
internal lfsr_bd_readnext API...
The solution is to instead turn this around and use the pcache to hold
any copied data, since this would not be clobbered when prog checking.
This has some other knock-on effects, mainly that we can't take
advantage of read hints in lfsr_bd_cpy, but has the added advantage of
potentially not clobbering the rcache at all when no checking progs.
Code changes were fairly minimal:
code stack
before: 33718 2608
after: 33690 (-0.1%) 2608 (+0.0%)
The initial goal was the simplify these layers. Keyword being initial.
Unfortunately these layers are both complex and subtle, so the goal
shifted more to be rigorous and reliable.
This mainly meant rearranging our prog/read loops to follow a consistent
style, with higher-priority buffers being sorted out before flushing
things. This gets a bit tricky with wanting to support both cache
bypassing and buffer-lending prognext/readnext, but with some redundant
prognext/readnext calls it's doable.
We also now aggressively discard rcaches on pcache conflicts. This
change does rely on the prog >= read assumption. Discarding rcaches
means we should no longer have overlapping caches, so hopefully no more
zombie rcache issues.
Our bypassing heuristic was also tweaked a bit. Now, in addition to
alignment, >= read/prog_size, and >= hint requirements, we also require
operations to be >= r/pcache_size. This should improve cache usage when
r/pcache_size >> read/prog_size, since we were too eager to bypass
before.
Long story short, this ended up being more just things shifting around
than a significant simplification of the bd layers. At least we ended up
with a nice bit of stack savings:
code stack
before: 33682 2640
after: 33718 (+0.1%) 2608 (-1.2%)
Also, test_badblocks with LFS_EMUBD_BADBLOCK_ERASENOOP is now failing. I
was worried the amount of fuzz testing we do would eventually end up
with a naturally occuring crc32c collision, and sure enough we did! Yayy
yyyy...
00 00 00 ff b0 02 00 87 80 80 00 3e c0 7f 7e => bdfa9b10
ab 77 de c2 b0 03 00 87 80 80 00 3e 38 d5 22 => bdfa9b10
Need to think about what to do with this... For now I've just commented
out the problematic test.
This configuration option enables the previous behavior of reading back
every prog to check that the data was written correctly.
Unfortunately, this brings a bit of baggage, thanks to our cache
interactions being more complicated now:
- We really want to reuse the rcache for prog validation, despite the
cache performance implications. Unfortunately, we simply can't, thanks
to the new bd utility functions tying up the rcache. lfsr_bd_cpy, for
example, does not expect rcache to be invalidated between a read and
prog, and if it is, things break (I may or may not have found this by
experience).
These bd utilities are valuable, so we really need some other way to
validate our progs.
- Since we can't rely on the rcache, this leaves checksumming as the
only option for validating progs. Checksumming isn't perfect, as there
is a decent chance of false negatives, but to be honest it's probably
good enough for anything that's not malicious.
- This also adds the new constraint that we need to be able to read back
any prog into the pcache, which implies read_size <= prog_size. This
constraint didn't exist when we could clobber our rcache, but this is
not worth throwing away the new bd utilities. Not to mention
clobbering our rcache could hurt cache performance.
Why not make read_size <= prog_size conditional on check_progs?
The main reason is convenience. One very compelling use case for
check_progs is to help debug unknown filesystem/integration failures,
buf if you can't enable check_progs without changing the filesystem
configuration, you can't really rely on check_progs for debugging.
This helps future proof what we expect from block devices, in case
future error detection/correction mechanisms can benefit from our
prog_size always being readable.
Code changes were not that significant, however there was a surprising
stack cost. This seems to be because lfsr_bd_read__ can now be called
from multiple places, causing it to no longer be inlined in
lfsr_bd_read_, costing a bit of stack for the additional function call:
before: 33566 2624
after: 33682 (+0.3%) 2640 (+0.6%)
This acts as a marker to indicate a fuzz test. It should reference a
define, usually SEED, that can be randomized to get interesting test
permutations.
This is currently unused, but could lead to some interesting uses such
as time-based fuzz testing. It's also just useful for inspecting the
tests (make test-list).
We've been wasting a lot of test cycles thanks to REMOUNT. Using a test
define for this effectively duplicates the test, when we really just
want to run more post-test code without additional mutation.
The main reason for REMOUNT has been to save typing, which, well, is not
a bad reason, these tests involve a lot of typing...
But this is probably a hammer/nail situation. If we replace these with a
small post-test loop, we can save quite a bit of time:
make test -j before: 5791.9s
make test -j after: 5123.8s (-11.5%)
Some tests still use a REMOUNT define, but these should be limited to
cases where remount actually changes the test's behavior.
Christopher Haster