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From: "NeilBrown" <neilb@suse.de>
To: "Dave Chinner" <david@fromorbit.com>
Cc: "Al Viro" <viro@zeniv.linux.org.uk>,
 "Christian Brauner" <brauner@kernel.org>, "Jens Axboe" <axboe@kernel.dk>,
 "Oleg Nesterov" <oleg@redhat.com>, "Chuck Lever" <chuck.lever@oracle.com>,
 "Jeff Layton" <jlayton@kernel.org>, "Ingo Molnar" <mingo@redhat.com>,
 "Peter Zijlstra" <peterz@infradead.org>, "Juri Lelli" <juri.lelli@redhat.com>,
 "Vincent Guittot" <vincent.guittot@linaro.org>, linux-fsdevel@vger.kernel.org,
 linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org
Subject:
 Re: [PATCH 1/2] Allow a kthread to declare that it calls task_work_run()
In-reply-to: <ZW7GKku/F2QK9MrC@dread.disaster.area>
References: <20231204014042.6754-1-neilb@suse.de>,
 <20231204014042.6754-2-neilb@suse.de>, <ZW7GKku/F2QK9MrC@dread.disaster.area>
Date: Tue, 05 Dec 2023 19:48:20 +1100
Message-id: <170176610023.7109.11175368186869568821@noble.neil.brown.name>

On Tue, 05 Dec 2023, Dave Chinner wrote:
> On Mon, Dec 04, 2023 at 12:36:41PM +1100, NeilBrown wrote:
> > User-space processes always call task_work_run() as needed when
> > returning from a system call.  Kernel-threads generally do not.
> > Because of this some work that is best run in the task_works context
> > (guaranteed that no locks are held) cannot be queued to task_works from
> > kernel threads and so are queued to a (single) work_time to be managed
> > on a work queue.
> >=20
> > This means that any cost for doing the work is not imposed on the kernel
> > thread, and importantly excessive amounts of work cannot apply
> > back-pressure to reduce the amount of new work queued.
> >=20
> > I have evidence from a customer site when nfsd (which runs as kernel
> > threads) is being asked to modify many millions of files which causes
> > sufficient memory pressure that some cache (in XFS I think) gets cleaned
> > earlier than would be ideal.  When __dput (from the workqueue) calls
> > __dentry_kill, xfs_fs_destroy_inode() needs to synchronously read back
> > previously cached info from storage.
>=20
> We fixed that specific XFS problem in 5.9.
>=20
> https://lore.kernel.org/linux-xfs/20200622081605.1818434-1-david@fromorbit.=
com/

Good to know - thanks.

>=20
> Can you reproduce these issues on a current TOT kernel?

I haven't tried.  I don't know if I know enough details of the work load
to attempt it.

>=20
> If not, there's no bugs to fix in the upstream kernel. If you can,
> then we've got more XFS issues to work through and fix.=20
>=20
> Fundamentally, though, we should not be papering over an XFS issue
> by changing how core task_work infrastructure is used. So let's deal
> with the XFS issue first....

I disagree.  This customer experience has demonstrated both a bug in XFS
and bug in the interaction between fput, task_work, and nfsd.

If a bug in a filesystem that only causes a modest performance impact
when used through the syscall API can bring the system to its knees
through memory exhaustion when used by nfsd, then that is a robustness
issue for nfsd.

I want to fix that robustness issue so that unusual behaviour in
filesystems does not cause out-of-proportion bad behaviour in nfsd.

I highlighted this in the cover letter to the first version of my patch:

https://lore.kernel.org/all/170112272125.7109.6245462722883333440@noble.neil.=
brown.name/

  While this might point to a problem with the filesystem not handling the
  final close efficiently, such problems should only hurt throughput, not
  lead to memory exhaustion.

Thanks,
NeilBrown


>=20
> -Dave.
> --=20
> Dave Chinner
> david@fromorbit.com
>=20