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[RFC,04/17] ext4: fix incorrect block bitmap free clusters update on metadata overlap

Message ID 20260508121539.4174601-5-libaokun@linux.alibaba.com
State Changes Requested
Headers show
Series ext4/lib-crc: LBS performance part 1 - incremental CRC32c for bitmap checksums | expand

Commit Message

Baokun Li May 8, 2026, 12:15 p.m. UTC 
In ext4_mb_mark_diskspace_used(), when the allocator detects that the
allocated blocks overlap with filesystem metadata, it enters an error
recovery path that marks these blocks as used in the bitmap via
ext4_mb_mark_context() with flags=0.

Without EXT4_MB_BITMAP_MARKED_CHECK, ext4_mb_mark_context() assumes all
bits in the range will be flipped, so it sets changed=len unconditionally.
However, in a corrupted filesystem, some of these metadata blocks may
already be marked as used (bit=1) in the bitmap. Since mb_set_bits() is
idempotent (sets bits to 1 regardless of current state), bits that are
already set won't actually change, but the free clusters count is still
decremented by the full range length, leading to an inaccurate free
clusters count.

Fix this by passing EXT4_MB_BITMAP_MARKED_CHECK, which correctly counts
only the bits that actually changed state.

Fixes: 2f94711b098b ("ext4: call ext4_mb_mark_context in ext4_mb_mark_diskspace_used")
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/ext4/mballoc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
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Patch

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index ed1bd00e11cd..ff2023c9f52c 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4228,7 +4228,7 @@  ext4_mb_mark_diskspace_used(struct ext4_allocation_context *ac, handle_t *handle
 					   ac->ac_b_ex.fe_group,
 					   ac->ac_b_ex.fe_start,
 					   ac->ac_b_ex.fe_len,
-					   0, NULL);
+					   EXT4_MB_BITMAP_MARKED_CHECK, NULL);
 		if (!err)
 			err = -EFSCORRUPTED;
 		return err;