ext4: 1k-blocksize loopback corruption without removing backing file

ext4: 1k-blocksize loopback corruption without removing backing file

[Jim Meyering]

Much like this report,

    loopback-mounted ext4 sees hole-filling (on rawhide, but not F15)
    http://thread.gmane.org/gmane.comp.file-systems.ext4/24454

but not requiring that one remove the backing file, this script shows the
same sort of corruption with a loopback-ext4-in-loopback-ext4 file system:
[Note these FS are small enough that they get a blocksize of 1024 by default.
 When I specify -b 2048 or 4096, the problem goes away. ]

=============================================
#!/bin/sh
set -e
dd if=/dev/zero of=blob bs=9k count=1000 >/dev/null 2>&1
mkdir mnt
mkfs -q -t ext4 -F blob
mount -oloop blob mnt

c1=$PWD
cd mnt

dd if=/dev/zero of=blob bs=4k count=1000 >/dev/null 2>&1
mkdir m2
mkfs -q -t ext4 -F blob
mount -oloop blob m2

c2=$PWD
cd m2

# Create a reference file.  Just like the following one,
# but with explicit NULs in place of holes.
perl -e '$n=1024; for (1..71) { print "\0"x$n, chr($_)x$n };' \
  -e 'close *STDOUT or die "$!"' > ref

# Seek 1KB, write 1KB of data, seek 1KB, write 1KB of data, etc....
perl -e '$n = 1 * 1024; *F = *STDOUT;' \
  -e 'for (1..71) { sysseek (*F, $n, 1)' \
  -e '&& syswrite (*F, chr($_)x$n) or die "$!"}' > j1

# filefrag -vs j1

sync
cmp -s ref j1 && fail=0 || fail=1

cd /
umount "$c2/m2" "$c1/mnt"
rm -rf "$c1/blob" "$c1/mnt"

exit $fail
=============================================

This shows that it fails most of the time for me in tmpfs on a rawhide
guest (2.6.39-0.rc3.git2.0.fc16.x86_64) running on an F15 host.  YMWV.

    $ while :; do ./ext4-bug; printf $?; done
    111111110111111110111111011111111110101111111111
    [Exit 130 (INT)]

Re: ext4: 1k-blocksize loopback corruption without removing backing file

[Curt Wohlgemuth]

As Eric mentions, disabling mblk_io_submit makes this work.

I've reproduced this on my system, and I'm testing a fix for it which
I'll try to post tomorrow.  It's limited to block size < page size
partitions.

Curt

On Thu, Apr 21, 2011 at 11:27 AM, Jim Meyering <jim@meyering.net> wrote:
> Much like this report,
>
>    loopback-mounted ext4 sees hole-filling (on rawhide, but not F15)
>    http://thread.gmane.org/gmane.comp.file-systems.ext4/24454
>
> but not requiring that one remove the backing file, this script shows the
> same sort of corruption with a loopback-ext4-in-loopback-ext4 file system:
> [Note these FS are small enough that they get a blocksize of 1024 by default.
>  When I specify -b 2048 or 4096, the problem goes away. ]
>
> =============================================
> #!/bin/sh
> set -e
> dd if=/dev/zero of=blob bs=9k count=1000 >/dev/null 2>&1
> mkdir mnt
> mkfs -q -t ext4 -F blob
> mount -oloop blob mnt
>
> c1=$PWD
> cd mnt
>
> dd if=/dev/zero of=blob bs=4k count=1000 >/dev/null 2>&1
> mkdir m2
> mkfs -q -t ext4 -F blob
> mount -oloop blob m2
>
> c2=$PWD
> cd m2
>
> # Create a reference file.  Just like the following one,
> # but with explicit NULs in place of holes.
> perl -e '$n=1024; for (1..71) { print "\0"x$n, chr($_)x$n };' \
>  -e 'close *STDOUT or die "$!"' > ref
>
> # Seek 1KB, write 1KB of data, seek 1KB, write 1KB of data, etc....
> perl -e '$n = 1 * 1024; *F = *STDOUT;' \
>  -e 'for (1..71) { sysseek (*F, $n, 1)' \
>  -e '&& syswrite (*F, chr($_)x$n) or die "$!"}' > j1
>
> # filefrag -vs j1
>
> sync
> cmp -s ref j1 && fail=0 || fail=1
>
> cd /
> umount "$c2/m2" "$c1/mnt"
> rm -rf "$c1/blob" "$c1/mnt"
>
> exit $fail
> =============================================
>
> This shows that it fails most of the time for me in tmpfs on a rawhide
> guest (2.6.39-0.rc3.git2.0.fc16.x86_64) running on an F15 host.  YMWV.
>
>    $ while :; do ./ext4-bug; printf $?; done
>    111111110111111110111111011111111110101111111111
>    [Exit 130 (INT)]
> --
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