linux-kernel - Re: [PATCH 1/2] Allow a kthread to declare that it calls task_work

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Date:   Tue, 5 Dec 2023 12:29:43 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     NeilBrown <neilb@...e.de>
Cc:     Dave Chinner <david@...morbit.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, Oleg Nesterov <oleg@...hat.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jeff Layton <jlayton@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nfs@...r.kernel.org
Subject: Re: [PATCH 1/2] Allow a kthread to declare that it calls
 task_work_run()

On Tue, Dec 05, 2023 at 07:48:20PM +1100, NeilBrown wrote:
> 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.
> > > 
> > > 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.
> > > 
> > > 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.
> > 
> > We fixed that specific XFS problem in 5.9.
> > 
> > https://lore.kernel.org/linux-xfs/20200622081605.1818434-1-david@fromorbit.com/
> 
> Good to know - thanks.
> 
> > 
> > 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.
> 
> > 
> > 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. 
> > 
> > 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.

I'm still confused about this memory exhaustion claim?
If this is a filesystem problem it's pretty annoying that we have to
work around it by exposing task work to random modules.