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

Suren Baghdasaryan
2023-06-27T17:36:21Z

On Tue, Jun 27, 2023 at 10:30 AM Christian Brauner <brauner@kernel.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@kernel.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-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Acked-by: Greg Kroah-Hartman <gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org>
> Acked-by: Acked-by: Zefan Li <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
>
>
> 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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