Re: loopback-mounted ext4 sees hole-filling (on rawhide, but not F15)

[Eric Sandeen <sandeen@redhat.com>]

On 4/19/11 11:16 AM, Jim Meyering wrote:
> When I run the following script on rawhide (2.6.39-0.rc3.git2.0.fc16.x86_64)
> it shows that a file with holes sometimes ends up with some non-NUL bytes
> where the holes should be.
> 
> For best results, run it in an empty directory, preferably on a tmpfs FS.
> I've run it on ext4, too.
> 
> It creates a 4MB ext4 file system, mounts it, and writes two files,
> one with holes, the other identical, but with NULs instead,
> and then runs sync, compares the two files, and unmounts
> and removes the temporary file and the mount point directory.
> 
> The key is that it unlinks the backing file after mounting
> and before writing the files.

I've found that disabling mblk_io_submit by adding ",nomblk_io_submit"
to the mount options in the testcase fixes this corruption case.

(6fd7a46781999c32f423025767e43b349b967d57 ext4: enable mblk_io_submit by default
turned it back on by default after the last corruption was found & fixed)

-Eric

> =====================================
> #!/bin/sh
> set -e
> dd if=/dev/zero of=blob bs=4k count=1000 >/dev/null 2>&1
> mkdir mnt
> mkfs -q -t ext4 -F blob
> mount -oloop blob mnt
> 
> # Removing the backing file is key.
> rm -f blob
> cwd=$PWD
> cd mnt
> 
> # Create a reference file.  Just like the following one,
> # but with explicit NULs in place of holes.
> perl -e '$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 || { cmp -l ref j1|head -20; fail=1; }
> 
> cd /; umount "$cwd/mnt"
> rm -rf "$cwd/blob" "$cwd/mnt"
> 
> exit $fail
> =====================================
> 
> Interestingly, if you remove the "sync", it exits 0 every time.
> 
> Beware that sometimes you (accidentally?) get
> all NUL blocks, so the script does occasionally exit 0.
> To demonstrate, I turned off the cmp -l ...|head bit and ran it
> for a while, printing only exit status:
> 
>     $ while :; do /t/loop-bug; printf $?; done
>     110101111111111111111111111111101111110111111111111111101111111110101111101^C
>     [Exit 130 (INT)]
> 
> Jim
> 
> P.S., this all started because I accidentally removed a backing
> file, and that made some in-progress FIEMAP-related tests fail.
> 
> PPS, here's sample output, when it exits nonzero:
> 
>   # bash /t/bug-demo                                                    :
>        9   0  41
>       17   0 240
>       18   0 216
>       19   0 125
>       20   0   1
>       29   0   3
>       30   0   3
>       31   0   3
>       32   0   3
>       33   0 142
>       34   0  40
>       35   0 155
>       36   0 156
>       37   0 164
>       39   0 142
>       41   0  41
>       49   0 146
>       50   0 141
>       51   0 151
>       52   0 154
> --
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