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

[Suren Baghdasaryan]

On Tue, Jun 27, 2023 at 11:42 AM Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> Hello, Christian.
>
> On Tue, Jun 27, 2023 at 07:30:26PM +0200, Christian Brauner wrote:
> ...
> > ->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.
>
> This is because kernfs has revoking operation which doesn't exist for other
> filesystems. Other filesystem implemenations can't just say "I'm done. Bye!"
> and go away. Even if the underlying filesystem has completely failed, the
> code still has to remain attached and keep aborting operations.
>
> However, kernfs serves as the midlayer to a lot of device drivers and other
> internal subsystems and it'd be really inconvenient for each of them to have
> to implement "I want to go away but I gotta wait out this user who's holding
> onto my tuning knob file". So, kernfs exposes a revoke or severing semantics
> something that's exposing interface through kernfs wants to stop doing so.
>
> If you look at it from file operation implementation POV, this seems exactly
> like ->release. All open files are shutdown and there won't be any future
> operations. After all, revoke is forced closing of all fd's. So, for most
> users, treating severing just like ->release is the right thing to do.
>
> The PSI file which caused this is a special case because it attaches
> something to its kernfs file which outlives the severing operation bypassing
> kernfs infra. A more complete way to fix this would be supporting the
> required behavior from kernfs side, so that the PSI file operates on kernfs
> interface which knows the severing event and detaches properly. That said,
> currently, this is very much an one-off.
>
> Suren, if you're interested, it might make sense to pipe poll through kernfs
> properly so that it has its kernfs operation and kernfs can sever it. That
> said, as this is a fix for something which is currently causing crashes,
> it'd be better to merge this simpler fix first no matter what.

I'm happy to implement the right fix if you go into more details.
AFAIKT kernfs_ops already has poll() operation, we are hooking
cgroup_file_poll() to it and using kernfs_generic_poll(). I thought
this is the right way to pipe poll through kernfs but if that's
incorrect, please let me know. I'm happy to fix that.
Thanks,
Suren.

>
> Thanks.

>
> --
> tejun