linux-kernel - Re: [PATCH v2 3/3] fs: Use delayed shrinker unregistration

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Message-ID: <20230608163622.GA1435580@mit.edu>
Date:   Thu, 8 Jun 2023 12:36:22 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Dave Chinner <david@...morbit.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Kirill Tkhai <tkhai@...ru>, akpm@...ux-foundation.org,
        vbabka@...e.cz, viro@...iv.linux.org.uk, brauner@...nel.org,
        djwong@...nel.org, hughd@...gle.com, paulmck@...nel.org,
        muchun.song@...ux.dev, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, zhengqi.arch@...edance.com
Subject: Re: [PATCH v2 3/3] fs: Use delayed shrinker unregistration

On Tue, Jun 06, 2023 at 11:24:32AM +1000, Dave Chinner wrote:
> On Mon, Jun 05, 2023 at 05:38:27PM -0700, Roman Gushchin wrote:
> > Hm, it makes the API more complex and easier to mess with. Like what will happen
> > if the second part is never called? Or it's called without the first part being
> > called first?
> 
> Bad things.
> 
> Also, it doesn't fix the three other unregister_shrinker() calls in
> the XFS unmount path, nor the three in the ext4/mbcache/jbd2 unmount
> path.
> 
> Those are just some of the unregister_shrinker() calls that have
> dynamic contexts that would also need this same fix; I haven't
> audited the 3 dozen other unregister_shrinker() calls around the
> kernel to determine if any of them need similar treatment, too.
> 
> IOWs, this patchset is purely a band-aid to fix the reported
> regression, not an actual fix for the underlying problems caused by
> moving the shrinker infrastructure to SRCU protection.  This is why
> I really want the SRCU changeover reverted.

There's been so much traffic on linux-fsdevel so I missed this thread
until Darrick pointed it out to me today.  (Thanks, Darrick!)

>From his description, and my quick read-through of this thread....
I'm worried.

Given that we're at -rc5 now, and the file system folks didn't get
consulted until fairly late in the progress, and the fact that this
may cause use-after-free problems that could lead to security issues,
perhaps we shoould consider reverting the SRCU changeover now, and try
again for the next merge window?

Thanks!!

						- Ted