Btrfs: fiemap: pass correct bytenr when fm_extent_count is zero

[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>

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index af2f040..8e4a7cd 100644 (file)
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -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