gbmap: Renamed gbmap_scan_thresh -> gbmap_rebuild_thresh

I think a good rule of thumb is if you refer to some variable/config/
field with a different name in comments/writing/etc more often than not,
you should just rename the variable/config/field to match.

So yeah, gbmap_rebuild_thresh controls when the gbmap is rebuilt.

Also touched up the doc comment a bit.

lfs3.c

@@ -10973,7 +10973,7 @@ static inline int lfs3_alloc_ckpoint(lfs3_t *lfs3) {
     // do we need to rebuild the gbmap?
     if (lfs3_f_isgbmap(lfs3->flags)
             && lfs3->lookahead.gbmapped < lfs3_min(
-                lfs3->cfg->gbmap_scan_thresh,
+                lfs3->cfg->gbmap_rebuild_thresh,
                 lfs3->block_count)) {
         int err = lfs3_alloc_rebuildgbmap(lfs3);
         if (err) {

lfs3.h

@@ -546,15 +546,16 @@ struct lfs3_cfg {
     lfs3_size_t crystal_thresh;
     #endif
 
-    // Threshold for when to rebuild block-map information. littlefs
+    // Threshold for when to rebuild the global on-disk block-map (gbmap).
-    // will attempt to rebuild the block-map when fewer than this many
+    // littlefs will attempt to rebuild the gbmap when fewer than this
-    // blocks are known. Larger values rebuild the block-map more
+    // many blocks are known. Larger values rebuild the gbmap more
     // frequently, reducing the chance of falling back to a slower
-    // allocator at a performance cost.
+    // allocator at the cost of amortized allocator throughput.
     //
-    // 0 only rebuilds the block-map when empty.
+    // 0 only rebuilds the gbmap when empty, but note rebuilding the
+    // gbmap may require allocating blocks.
     #ifdef LFS3_GBMAP
-    lfs3_block_t gbmap_scan_thresh;
+    lfs3_block_t gbmap_rebuild_thresh;
     #endif
 };
 

runners/bench_runner.h

@@ -120,7 +120,7 @@ void bench_permutation(size_t i, uint32_t *buffer, size_t size);
     BENCH_DEFINE(INLINE_SIZE,           BLOCK_SIZE/4                        ) \
     BENCH_DEFINE(FRAGMENT_SIZE,         LFS3_MIN(BLOCK_SIZE/8, 512)         ) \
     BENCH_DEFINE(CRYSTAL_THRESH,        BLOCK_SIZE/8                        ) \
-    BENCH_DEFINE(GBMAP_SCAN_THRESH,  BLOCK_COUNT/4                          ) \
+    BENCH_DEFINE(GBMAP_REBUILD_THRESH,  BLOCK_COUNT/4                       ) \
     BENCH_DEFINE(ERASE_VALUE,           0xff                                ) \
     BENCH_DEFINE(ERASE_CYCLES,          0                                   ) \
     BENCH_DEFINE(BADBLOCK_BEHAVIOR,     LFS3_EMUBD_BADBLOCK_PROGERROR       ) \
@@ -154,7 +154,7 @@ void bench_permutation(size_t i, uint32_t *buffer, size_t size);
 
 #ifdef LFS3_GBMAP
 #define BENCH_GBMAP_CFG \
-    .gbmap_scan_thresh   = GBMAP_SCAN_THRESH,
+    .gbmap_rebuild_thresh   = GBMAP_REBUILD_THRESH,
 #else
 #define BENCH_GBMAP_CFG
 #endif

runners/test_runner.h

@@ -111,7 +111,7 @@ void test_permutation(size_t i, uint32_t *buffer, size_t size);
     TEST_DEFINE(INLINE_SIZE,            BLOCK_SIZE/4                         ) \
     TEST_DEFINE(FRAGMENT_SIZE,          LFS3_MIN(BLOCK_SIZE/8, 512)          ) \
     TEST_DEFINE(CRYSTAL_THRESH,         BLOCK_SIZE/8                         ) \
-    TEST_DEFINE(GBMAP_SCAN_THRESH,  BLOCK_COUNT/4                           ) \
+    TEST_DEFINE(GBMAP_REBUILD_THRESH,   BLOCK_COUNT/4                        ) \
     TEST_DEFINE(ERASE_VALUE,            0xff                                 ) \
     TEST_DEFINE(ERASE_CYCLES,           0                                    ) \
     TEST_DEFINE(BADBLOCK_BEHAVIOR,      LFS3_EMUBD_BADBLOCK_PROGERROR        ) \
@@ -145,7 +145,7 @@ void test_permutation(size_t i, uint32_t *buffer, size_t size);
 
 #ifdef LFS3_GBMAP
 #define TEST_GBMAP_CFG \
-    .gbmap_scan_thresh   = GBMAP_SCAN_THRESH,
+    .gbmap_rebuild_thresh   = GBMAP_REBUILD_THRESH,
 #else
 #define TEST_GBMAP_CFG
 #endif