linux-ext4 - Re: fiemap bugs on sparse files.

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Message-ID: <AANLkTikMhTM6c1=ihMj0XOZD0VYVLoHOeoGV4_7qvN=b@mail.gmail.com>
Date:	Wed, 23 Feb 2011 16:59:24 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: fiemap bugs on sparse files.

On Wed, Feb 23, 2011 at 5:18 AM, Chris Mason <chris.mason@...cle.com> wrote:
> [ 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 >
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.

>
> 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().
>
> 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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-- 
Best Wishes
Yongqiang Yang
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