fiemap bugs on sparse files.

[ resend, sorry if this is a dup ]

Hi everyone,

We've had reports on btrfs that cp is giving us files full of zeros instead of actually copying them.  It was tracked down to a bug with the btrfs fiemap implementation where it was returning holes for delalloc ranges.

Newer versions of cp are trusting fiemap to tell it where the holes are, which does seem like a pretty neat trick.

I decided to give xfs and ext4 a shot with a few tests cases too, xfs passed with all the ones btrfs was getting wrong, and ext4 got the basic delalloc case right.

# mkfs.ext4 /dev/xxx
# mount /dev/xxx /mnt
# dd if=/dev/zero of=/mnt/foo bs=1M count=1
# fiemap-test foo
ext:   0 logical: [       0..     255] phys:        0..     255 flags: 0x007 tot: 256

Horray!  But once we throw a hole in, things go bad:

# mkfs.ext4 /dev/xxx
# mount /dev/xxx /mnt
# dd if=/dev/zero of=/mnt/foo bs=1M count=1 seek=1
# fiemap-test foo
< no output >

We've got a delalloc extent after the hole and ext4 fiemap didn't find it.  If I run sync to kick the delalloc out:

# sync
# fiemap-test foo
ext:   0 logical: [     256..     511] phys:    34048..   34303 flags: 0x001 tot: 256

fiemap-test is sitting in my /usr/local/bin, and I have no idea how it got there.  It's full of pretty comments so I know it isn't mine, but you can grab it here:

http://oss.oracle.com/~mason/fiemap-test.c

xfsqa has a fiemap program too.

-chris
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Quoted 22 lines · Chris Mason

[ resend, sorry if this is a dup ]

Hi everyone,

We've had reports on btrfs that cp is giving us files full of zeros instead of actually copying them.  It was tracked down to a bug with the btrfs fiemap implementation where it was returning holes for delalloc ranges.

Newer versions of cp are trusting fiemap to tell it where the holes are, which does seem like a pretty neat trick.

I decided to give xfs and ext4 a shot with a few tests cases too, xfs passed with all the ones btrfs was getting wrong, and ext4 got the basic delalloc case right.

# mkfs.ext4 /dev/xxx
# mount /dev/xxx /mnt
# dd if=/dev/zero of=/mnt/foo bs=1M count=1
# fiemap-test foo
ext:   0 logical: [       0..     255] phys:        0..     255 flags: 0x007 tot: 256

Quoted 7 lines

Horray!  But once we throw a hole in, things go bad:

# mkfs.ext4 /dev/xxx
# mount /dev/xxx /mnt
# dd if=/dev/zero of=/mnt/foo bs=1M count=1 seek=1
# fiemap-test foo
< no output >

Actually, there is no extent in extent tree now, so ext4_ext_walk_space() will pass ext4_ext_fiemap_cb() a variable of struct ext4_ext_cache with the requested length. But in ext4_ext_fiemap_cb() just the paging contains start block is got via find_get_page(), if find_get_page() return null, ext4_ext_fiemap_cb()  thinks the whole request range is empty and it returns request range.

In 1st case, find_get_page() will succeed.

It seems that we should get no. of pages in page cache if find_get_page() fails, and correct the range to be returned.

Right?

If right I will send a patch.

Quoted 7 lines

We've got a delalloc extent after the hole and ext4 fiemap didn't find it.  If I run sync to kick the delalloc out:

# sync
# fiemap-test foo
ext:   0 logical: [     256..     511] phys:    34048..   34303 flags: 0x001 tot: 256

Now, there is a extent in extent tree at least. and ext4_ext_walk_space() will regulate the request range to be passed to ext4_ext_fiemap_cb().

Quoted 7 lines

fiemap-test is sitting in my /usr/local/bin, and I have no idea how it got there.  It's full of pretty comments so I know it isn't mine, but you can grab it here:

http://oss.oracle.com/~mason/fiemap-test.c

Quoted 8 lines

xfsqa has a fiemap program too.

-chris
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--
Best Wishes
Yongqiang Yang

Quoted 38 lines · Yongqiang Yang

Quoted 22 lines · Chris Mason

[ resend, sorry if this is a dup ]

Hi everyone,

We've had reports on btrfs that cp is giving us files full of zeros instead of actually copying them. It was tracked down to a bug with the btrfs fiemap implementation where it was returning holes for delalloc ranges.

Newer versions of cp are trusting fiemap to tell it where the holes are, which does seem like a pretty neat trick.

I decided to give xfs and ext4 a shot with a few tests cases too, xfs passed with all the ones btrfs was getting wrong, and ext4 got the basic delalloc case right.

# mkfs.ext4 /dev/xxx
# mount /dev/xxx /mnt
# dd if=/dev/zero of=/mnt/foo bs=1M count=1
# fiemap-test foo
ext:   0 logical: [       0..     255] phys:        0..     255 flags: 0x007 tot: 256

Quoted 7 lines

Horray! But once we throw a hole in, things go bad:

# mkfs.ext4 /dev/xxx
# mount /dev/xxx /mnt
# dd if=/dev/zero of=/mnt/foo bs=1M count=1 seek=1
# fiemap-test foo
< no output >

Actually, there is no extent in extent tree now, so ext4_ext_walk_space() will pass ext4_ext_fiemap_cb() a variable of struct ext4_ext_cache with the requested length. But in ext4_ext_fiemap_cb() just the paging contains start block is got via find_get_page(), if find_get_page() return null, ext4_ext_fiemap_cb() thinks the whole request range is empty and it returns request range.

Quoted 4 lines

In 1st case, find_get_page() will succeed.

It seems that we should get no. of pages in page cache if find_get_page() fails, and correct the range to be returned.

We can call find_get_pages() with nr_pages=1 instead. And we can regulate the range with page->index if it is not the the paging contains start block.

Quoted 4 lines

Right?

If right I will send a patch.