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Btrfs: fiemap: pass correct bytenr when fm_extent_count is zero

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[BUG]
fm_mapped_extents is not correct when fm_extent_count is 0
Like:
   # mount /dev/vdb5 /mnt/btrfs
   # dd if=/dev/zero bs=16K count=4 oflag=dsync of=/mnt/btrfs/file
   # xfs_io -c "fiemap -v" /mnt/btrfs/file
   /mnt/btrfs/file:
   EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
     0: [0..127]:        25088..25215       128   0x1

When user space wants to get the number of file extents,
set fm_extent_count to 0 to run fiemap and then read fm_mapped_extents.

In the above example, fiemap will return with fm_mapped_extents set to 4,
but it should be 1 since there's only one entry in the output.

[REASON]
The problem seems to be that disko is only set if
fieinfo->fi_extents_max is set. And this member is initialized, in the
generic ioctl_fiemap function, to the value of used-passed
fm_extent_count. So when the user passes 0 then fi_extent_max is also
set to zero and this causes btrfs to not initialize disko at all.
Eventually this leads emit_fiemap_extent being called with a bogus
'phys' argument preventing proper fiemap entries merging.

[FIX]
Move the disko initialization earlier in extent_fiemap making it
independent of user-passed arguments, allowing emit_fiemap_extent to
properly handle consecutive extent entries.

Signed-off-by: Robbie Ko <robbieko@synology.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Robbie Ko 2018-05-07 16:42:04 +08:00 committed by David Sterba
parent 3ca57bd620
commit 9d311e11fc
1 changed files with 1 additions and 3 deletions
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4
fs/btrfs/extent_io.c
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@ -4545,7 +4545,7 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
offset_in_extent = em_start - em->start;
em_end = extent_map_end(em);
em_len = em_end - em_start;
disko = 0;
disko = em->block_start + offset_in_extent;
flags = 0;
/*
@ -4568,8 +4568,6 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
u64 bytenr = em->block_start -
(em->start - em->orig_start);
disko = em->block_start + offset_in_extent;
/*
* As btrfs supports shared space, this information
* can be exported to userspace tools via