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

[Christian Brauner]

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@xxxxxxxxxx> 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@xxxxxxxxxx>
    Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
    Acked-by: Acked-by: Zefan Li <lizefan@xxxxxxxxxx>


which mentions "either releases or severed" which imho already points to
separate methods.