Re: [PATCH 0/2]: CEPHFS: allow for races between direct and buffered writes

[NeilBrown <neilb@suse.com>]

[-- Attachment #1: Type: text/plain, Size: 1385 bytes --]

On Wed, Aug 31 2016, Yan, Zheng wrote:

> On Wed, Aug 31, 2016 at 10:56 AM, NeilBrown <neilb@suse.com> wrote:
>>
>> Hi,
>>  it is quite possible for O_DIRECT and buffered writes to a file to
>>  race.
>>  The xfstests test suite has a test - generic/036 - which tests this
>>  case.
>>
>>  Unlike most filesystems, cephfs does not hold inode_lock() across
>>  direct writes.  This means that buffer pages can become dirty while
>>  direct writes are happening.  This confused ceph a little.
>
> how about make ceph_write_iter() hold inode_lock() for direct/sync write?

You could probably do that.  You could even use inode_lock_shared() to
allow multiple writers to perform O_DIRECT writes to the same file in
parallel.

But I don't know why ceph did this differently from every other
filesystem, and the git commit message doesn't shine any light on that
question.  I didn't want to propose a change that I didn't understand
the consequences of.

If you did extend the locking over ceph_direct_read_write(), it would
probably make sense to remove the truncate_inode_pages_range() call as
it should be redundant.

Thanks,
NeilBrown


>
> Regards
> Yan, Zheng
>>
>>  The following two patches allow ceph to handle this possibility a
>>  little more cleanly.  The more important patch removes a WARN_ON() for
>>  a circumstance which can easily be triggered.
>>
>> Thanks,
>> NeilBrown

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 818 bytes --]