[LTP] [PATCH RFC] sync_file_range02: remove the toplimit of write back

[Sumit Garg]

+ Cyril (who originally proposed upper bound check)

On Wed, 18 Dec 2019 at 14:55, Jan Stancek <jstancek@redhat.com> wrote:
>
>
> ----- Original Message -----
> > " The test's assumptions are fundamentally false; it thinks it can look
> > at IO counters (tst_dev_bytes_written) for a disk before and after a
> > system call, and attribute all of the IO seen to the system call that
> > was made - this isn't necessarily correct. Other processes may generate
> > IO in the background.
>
> We create our own block device, so there shouldn't be other processes
> writing to it.
>
> > ext4 defers a lot of IO on a freshly made filesystem to the kernel -
> > for example it will initialize the journal and inode tables after the
> > mount
>
> Journal was my guess as well.

To avoid similar scenarios, I suggested to add a "sync()" call just
prior to test here [1]. And I couldn't reproduce the failure in
1000-times run with 4.19 kernel.

Also, I think having a "sync()" call becomes more important in case we
remove the upper bound otherwise test might not provide reliable
results.

[1] http://lists.linux.it/pipermail/ltp/2019-October/014157.html

-Sumit

>
> > Let's remove the toplimit of write back, and think as long as we synced
> > at least the expected amount, the test passes. The +10% limit seems
> > arbitrary.
>
> I think this is reasonable approach until we find better way
> to measure what was synced.
>
> Acked-by: Jan Stancek <jstancek@redhat.com>
>
>
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