[PATCH AUTOSEL 5.14 193/252] btrfs: grab correct extent map for subpage compressed extent read

Sasha Levin
2021-09-09T12:01:04Z

From: Qu Wenruo <wqu@suse.com>

[ Upstream commit 557023ea9f06baf2659b232b08b8e8711f7001a6 ]

[BUG]
When subpage compressed read write support is enabled, btrfs/038 always
fails with EIO.

A simplified script can easily trigger the problem:

mkfs.btrfs -f -s 4k $dev
mount $dev $mnt -o compress=lzo

xfs_io -f -c "truncate 118811" $mnt/foo
xfs_io -c "pwrite -S 0x0d -b 39987 92267 39987" $mnt/foo > /dev/null

sync
btrfs subvolume snapshot -r $mnt $mnt/mysnap1

xfs_io -c "pwrite -S 0x3e -b 80000 200000 80000" $mnt/foo > /dev/null
sync

xfs_io -c "pwrite -S 0xdc -b 10000 250000 10000" $mnt/foo > /dev/null
xfs_io -c "pwrite -S 0xff -b 10000 300000 10000" $mnt/foo > /dev/null

sync
btrfs subvolume snapshot -r $mnt $mnt/mysnap2

cat $mnt/mysnap2/foo
# Above cat will fail due to EIO

[CAUSE]
The problem is in btrfs_submit_compressed_read().

When it tries to grab the extent map of the read range, it uses the
following call:

em = lookup_extent_mapping(em_tree,
page_offset(bio_first_page_all(bio)),
fs_info->sectorsize);

The problem is in the page_offset(bio_first_page_all(bio)) part.

The offending inode has the following file extent layout

item 10 key (257 EXTENT_DATA 131072) itemoff 15639 itemsize 53
generation 8 type 1 (regular)
extent data disk byte 13680640 nr 4096
extent data offset 0 nr 4096 ram 4096
extent compression 0 (none)
item 11 key (257 EXTENT_DATA 135168) itemoff 15586 itemsize 53
generation 8 type 1 (regular)
extent data disk byte 0 nr 0
item 12 key (257 EXTENT_DATA 196608) itemoff 15533 itemsize 53
generation 8 type 1 (regular)
extent data disk byte 13676544 nr 4096
extent data offset 0 nr 53248 ram 86016
extent compression 2 (lzo)

And the bio passed in has the following parameters:

page_offset(bio_first_page_all(bio)) = 131072
bio_first_bvec_all(bio)->bv_offset = 65536

If we use page_offset(bio_first_page_all(bio) without adding bv_offset,
we will get an extent map for file offset 131072, not 196608.

This means we read uncompressed data from disk, and later decompression
will definitely fail.

[FIX]
Take bv_offset into consideration when trying to grab an extent map.

And add an ASSERT() to ensure we're really getting a compressed extent.

Thankfully this won't affect anything but subpage, thus we only need to
ensure this patch get merged before we enabled basic subpage support.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/compression.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 30d82cdf128c..ad700cf287c9 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -673,6 +673,7 @@ blk_status_t btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
struct page *page;
struct bio *comp_bio;
u64 cur_disk_byte = bio->bi_iter.bi_sector << 9;
+ u64 file_offset;
u64 em_len;
u64 em_start;
struct extent_map *em;
@@ -682,15 +683,17 @@ blk_status_t btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,

em_tree = &BTRFS_I(inode)->extent_tree;

+ file_offset = bio_first_bvec_all(bio)->bv_offset +
+ page_offset(bio_first_page_all(bio));
+
/* we need the actual starting offset of this extent in the file */
read_lock(&em_tree->lock);
- em = lookup_extent_mapping(em_tree,
- page_offset(bio_first_page_all(bio)),
- fs_info->sectorsize);
+ em = lookup_extent_mapping(em_tree, file_offset, fs_info->sectorsize);
read_unlock(&em_tree->lock);
if (!em)
return BLK_STS_IOERR;

+ ASSERT(em->compress_type != BTRFS_COMPRESS_NONE);
compressed_len = em->block_len;
cb = kmalloc(compressed_bio_size(fs_info, compressed_len), GFP_NOFS);
if (!cb)
--
2.30.2