From: Qu Wenruo wqu@suse.com
[ Upstream commit c28ea613fafad910d08f67efe76ae552b1434e44 ]
[BUG] When running fstresss, we can hit strange data csum mismatch where the on-disk data is in fact correct (passes scrub).
With some extra debug info added, we have the following traces:
0482us: btrfs_do_readpage: root=5 ino=284 offset=393216, submit force=0 pgoff=0 iosize=8192 0494us: btrfs_do_readpage: root=5 ino=284 offset=401408, submit force=0 pgoff=8192 iosize=4096 0498us: btrfs_submit_data_bio: root=5 ino=284 bio first bvec=393216 len=8192 0591us: btrfs_do_readpage: root=5 ino=284 offset=405504, submit force=0 pgoff=12288 iosize=36864 0594us: btrfs_submit_data_bio: root=5 ino=284 bio first bvec=401408 len=4096 0863us: btrfs_submit_data_bio: root=5 ino=284 bio first bvec=405504 len=36864 0933us: btrfs_verify_data_csum: root=5 ino=284 offset=393216 len=8192 0967us: btrfs_do_readpage: root=5 ino=284 offset=442368, skip beyond isize pgoff=49152 iosize=16384 1047us: btrfs_verify_data_csum: root=5 ino=284 offset=401408 len=4096 1163us: btrfs_verify_data_csum: root=5 ino=284 offset=405504 len=36864 1290us: check_data_csum: !!! root=5 ino=284 offset=438272 pg_off=45056 !!! 7387us: end_bio_extent_readpage: root=5 ino=284 before pending_read_bios=0
[CAUSE] Normally we expect all submitted bio reads to only touch the range we specified, and under subpage context, it means we should only touch the range specified in each bvec.
But in data read path, inside end_bio_extent_readpage(), we have page zeroing which only takes regular page size into consideration.
This means for subpage if we have an inode whose content looks like below:
0 16K 32K 48K 64K |///////| |///////| |
|//| = data needs to be read from disk | | = hole
And i_size is 64K initially.
Then the following race can happen:
T1 | T2 --------------------------------+-------------------------------- btrfs_do_readpage() | |- isize = 64K; | | At this time, the isize is | | 64K | | | |- submit_extent_page() | | submit previous assembled bio| | assemble bio for [0, 16K) | | | |- submit_extent_page() | submit read bio for [0, 16K) | assemble read bio for | [32K, 48K) | | | btrfs_setsize() | |- i_size_write(, 16K); | Now i_size is only 16K end_io() for [0K, 16K) | |- end_bio_extent_readpage() | |- btrfs_verify_data_csum() | | No csum error | |- i_size = 16K; | |- zero_user_segment(16K, | PAGE_SIZE); | !!! We zeroed range | !!! [32K, 48K) | | end_io for [32K, 48K) | |- end_bio_extent_readpage() | |- btrfs_verify_data_csum() | ! CSUM MISMATCH ! | ! As the range is zeroed now !
[FIX] To fix the problem, make end_bio_extent_readpage() to only zero the range of bvec.
The bug only affects subpage read-write support, as for full read-only mount we can't change i_size thus won't hit the race condition.
Signed-off-by: Qu Wenruo wqu@suse.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Sasha Levin sashal@kernel.org --- fs/btrfs/extent_io.c | 21 ++++++++++++++++----- 1 file changed, 16 insertions(+), 5 deletions(-)
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c index c9cee458e001..ff1a0f97ba84 100644 --- a/fs/btrfs/extent_io.c +++ b/fs/btrfs/extent_io.c @@ -2972,12 +2972,23 @@ static void end_bio_extent_readpage(struct bio *bio) if (likely(uptodate)) { loff_t i_size = i_size_read(inode); pgoff_t end_index = i_size >> PAGE_SHIFT; - unsigned off;
- /* Zero out the end if this page straddles i_size */ - off = offset_in_page(i_size); - if (page->index == end_index && off) - zero_user_segment(page, off, PAGE_SIZE); + /* + * Zero out the remaining part if this range straddles + * i_size. + * + * Here we should only zero the range inside the bvec, + * not touch anything else. + * + * NOTE: i_size is exclusive while end is inclusive. + */ + if (page->index == end_index && i_size <= end) { + u32 zero_start = max(offset_in_page(i_size), + offset_in_page(end)); + + zero_user_segment(page, zero_start, + offset_in_page(end) + 1); + } } ASSERT(bio_offset + len > bio_offset); bio_offset += len;