linux-kernel - Re: [PATCH 1/2] kernfs: add kernfs_ops.free operation to free resources tied to the file

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Date:   Tue, 27 Jun 2023 10:36:21 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Tejun Heo <tj@...nel.org>, gregkh@...uxfoundation.org,
        peterz@...radead.org, lujialin4@...wei.com,
        lizefan.x@...edance.com, hannes@...xchg.org, mingo@...hat.com,
        ebiggers@...nel.org, oleg@...hat.com, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 1/2] kernfs: add kernfs_ops.free operation to free
 resources tied to the file

On Tue, Jun 27, 2023 at 10:30 AM Christian Brauner <brauner@...nel.org> wrote:
>
> On Tue, Jun 27, 2023 at 10:09:27AM -0700, Suren Baghdasaryan wrote:
> > On Tue, Jun 27, 2023 at 1:24 AM Christian Brauner <brauner@...nel.org> wrote:
> > >
> > > On Mon, Jun 26, 2023 at 10:31:49AM -1000, Tejun Heo wrote:
> > > > On Mon, Jun 26, 2023 at 01:17:12PM -0700, Suren Baghdasaryan wrote:
> > > > > diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
> > > > > index 73f5c120def8..a7e404ff31bb 100644
> > > > > --- a/include/linux/kernfs.h
> > > > > +++ b/include/linux/kernfs.h
> > > > > @@ -273,6 +273,11 @@ struct kernfs_ops {
> > > > >      */
> > > > >     int (*open)(struct kernfs_open_file *of);
> > > > >     void (*release)(struct kernfs_open_file *of);
> > > > > +   /*
> > > > > +    * Free resources tied to the lifecycle of the file, like a
> > > > > +    * waitqueue used for polling.
> > > > > +    */
> > > > > +   void (*free)(struct kernfs_open_file *of);
> > > >
> > > > I think this can use a bit more commenting - ie. explain that release may be
> > > > called earlier than the actual freeing of the file and how that can lead to
> > > > problems. Othre than that, looks fine to me.
> > >
> > > It seems the more natural thing to do would be to introduce a ->drain()
> > > operation and order it before ->release(), no?
> >
> > I assume you mean we should add a ->drain() operation and call it when
> > kernfs_drain_open_files()  causes kernfs_release_file()? That would
> > work but if any existing release() handler counts on the current
> > behavior (release() being called while draining) then we should find
> > and fix these. Hopefully they don't really depend on the current
> > behavior but I dunno.
>
> Before I wrote that I did a naive
>
>         > git grep -A 20 kernfs_ops | grep \\.release
>         kernel/cgroup/cgroup.c- .release                = cgroup_file_release,
>         kernel/cgroup/cgroup.c- .release                = cgroup_file_release,
>
> which only gave cgroup_release_file(). Might be I'm missing some convoluted
> callchains though or macro magic...
>
> ->release() was added in
>
>     commit 0e67db2f9fe91937e798e3d7d22c50a8438187e1
>     kernfs: add kernfs_ops->open/release() callbacks
>
>     Add ->open/release() methods to kernfs_ops.  ->open() is called when
>     the file is opened and ->release() when the file is either released or
>     severed.  These callbacks can be used, for example, to manage
>     persistent caching objects over multiple seq_file iterations.
>
>     Signed-off-by: Tejun Heo <tj@...nel.org>
>     Acked-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>     Acked-by: Acked-by: Zefan Li <lizefan@...wei.com>
>
>
> which mentions "either releases or severed" which imho already points to
> separate methods.

Interesting. I guess we can add op->drain() and make all existing
handlers of ops->release() to handle ops->drain() with the same
handler. That should keep them happy and for my case I'll be releasing
resources only inside ops->release(). Does that sound good?

>
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