3.16.70-rc1 review patch. If anyone has any objections, please let me know.
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From: Filipe Manana fdmanana@suse.com
commit 8e928218780e2f1cf2f5891c7575e8f0b284fcce upstream.
In the past we had data corruption when reading compressed extents that are shared within the same file and they are consecutive, this got fixed by commit 005efedf2c7d0 ("Btrfs: fix read corruption of compressed and shared extents") and by commit 808f80b46790f ("Btrfs: update fix for read corruption of compressed and shared extents"). However there was a case that was missing in those fixes, which is when the shared and compressed extents are referenced with a non-zero offset. The following shell script creates a reproducer for this issue:
#!/bin/bash
mkfs.btrfs -f /dev/sdc &> /dev/null mount -o compress /dev/sdc /mnt/sdc
# Create a file with 3 consecutive compressed extents, each has an # uncompressed size of 128Kb and a compressed size of 4Kb. for ((i = 1; i <= 3; i++)); do head -c 4096 /dev/zero for ((j = 1; j <= 31; j++)); do head -c 4096 /dev/zero | tr '\0' "\377" done done > /mnt/sdc/foobar sync
echo "Digest after file creation: $(md5sum /mnt/sdc/foobar)"
# Clone the first extent into offsets 128K and 256K. xfs_io -c "reflink /mnt/sdc/foobar 0 128K 128K" /mnt/sdc/foobar xfs_io -c "reflink /mnt/sdc/foobar 0 256K 128K" /mnt/sdc/foobar sync
echo "Digest after cloning: $(md5sum /mnt/sdc/foobar)"
# Punch holes into the regions that are already full of zeroes. xfs_io -c "fpunch 0 4K" /mnt/sdc/foobar xfs_io -c "fpunch 128K 4K" /mnt/sdc/foobar xfs_io -c "fpunch 256K 4K" /mnt/sdc/foobar sync
echo "Digest after hole punching: $(md5sum /mnt/sdc/foobar)"
echo "Dropping page cache..." sysctl -q vm.drop_caches=1 echo "Digest after hole punching: $(md5sum /mnt/sdc/foobar)"
umount /dev/sdc
When running the script we get the following output:
Digest after file creation: 5a0888d80d7ab1fd31c229f83a3bbcc8 /mnt/sdc/foobar linked 131072/131072 bytes at offset 131072 128 KiB, 1 ops; 0.0033 sec (36.960 MiB/sec and 295.6830 ops/sec) linked 131072/131072 bytes at offset 262144 128 KiB, 1 ops; 0.0015 sec (78.567 MiB/sec and 628.5355 ops/sec) Digest after cloning: 5a0888d80d7ab1fd31c229f83a3bbcc8 /mnt/sdc/foobar Digest after hole punching: 5a0888d80d7ab1fd31c229f83a3bbcc8 /mnt/sdc/foobar Dropping page cache... Digest after hole punching: fba694ae8664ed0c2e9ff8937e7f1484 /mnt/sdc/foobar
This happens because after reading all the pages of the extent in the range from 128K to 256K for example, we read the hole at offset 256K and then when reading the page at offset 260K we don't submit the existing bio, which is responsible for filling all the page in the range 128K to 256K only, therefore adding the pages from range 260K to 384K to the existing bio and submitting it after iterating over the entire range. Once the bio completes, the uncompressed data fills only the pages in the range 128K to 256K because there's no more data read from disk, leaving the pages in the range 260K to 384K unfilled. It is just a slightly different variant of what was solved by commit 005efedf2c7d0 ("Btrfs: fix read corruption of compressed and shared extents").
Fix this by forcing a bio submit, during readpages(), whenever we find a compressed extent map for a page that is different from the extent map for the previous page or has a different starting offset (in case it's the same compressed extent), instead of the extent map's original start offset.
A test case for fstests follows soon.
Reported-by: Zygo Blaxell ce3g8jdj@umail.furryterror.org Fixes: 808f80b46790f ("Btrfs: update fix for read corruption of compressed and shared extents") Fixes: 005efedf2c7d0 ("Btrfs: fix read corruption of compressed and shared extents") Tested-by: Zygo Blaxell ce3g8jdj@umail.furryterror.org Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Ben Hutchings ben@decadent.org.uk --- fs/btrfs/extent_io.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/fs/btrfs/extent_io.c +++ b/fs/btrfs/extent_io.c @@ -2936,11 +2936,11 @@ static int __do_readpage(struct extent_i */ if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags) && prev_em_start && *prev_em_start != (u64)-1 && - *prev_em_start != em->orig_start) + *prev_em_start != em->start) force_bio_submit = true;
if (prev_em_start) - *prev_em_start = em->orig_start; + *prev_em_start = em->start;
free_extent_map(em); em = NULL;