[Fsync issue in btrfs]
In addition to the above, I would like to bring to your notice that :
After doing a fallocate or fallocate zero_range with keep size option,
a fsync() operation would have no effect at all. If we crash after the
fsync, on recovery the blocks allocated due to the fallocate call are
lost. This aligns with a patch work here[1] for a similar issue with
punch_hole. A simple test scenario that reproduces this bug is :

1. write (0-16K) to a file foo
2. sync()
3. fallocate keep_size (16K - 20K)
4. fsync(foo)
5. Crash

On recovery, all blocks allocated in step 3 are lost (this is the true
even when fallocate is replaced by zero_range operation supported in
kernel 4.16 )
Could you explain why a fsync() of the file would still not persist
the metadata(block count in this case), across power failures?

[1] https://patchwork.kernel.org/patch/5830801/

Jayashree Mohan

On Wed, Feb 21, 2018 at 8:23 PM, Jayashree Mohan
<jayashree2...@gmail.com> wrote:
> Hi,
> On btrfs (as of kernel 4.15), say we fallocate a file with keep_size
> option, followed by fdatasync() or fsync(). If we now crash, on
> recovery we see a wrong block count and all the blocks allocated
> beyond the eof are lost. This bug was reported(xfstest generic/468)
> and patched on ext4[1], and a variant of this, that did not recover
> the correct file size was patched in f2fs[2]. I am wondering why this
> is still not fixed in btrfs. You can reproduce this bug on btrfs using
> a tool called CrashMonkey that we are building at UT Austin, which is
> a test harness for filesystem crash consistency checks[3]
> To reproduce the bug, simply run :
>  ./c_harness -f /dev/sda -d /dev/cow_ram0 -t btrfs -e 102400  -v
> tests/generic_468.so
> Is there a reason why this is not yet patched in btrfs? I don't see
> why even after a fsync(), losing the blocks allocated beyond the eof
> are acceptable.
> [1] https://patchwork.kernel.org/patch/10120293/
> [2] https://sourceforge.net/p/linux-f2fs/mailman/message/36104201/
> [3] https://github.com/utsaslab/crashmonkey
> Thanks,
> Jayashree Mohan
> 2nd Year PhD in Computer Science
> University of Texas at Austin.
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