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

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Jun 27, 2023 at 6:54 PM Tejun Heo <tj@kernel.org> wrote:
>
> Hello,
>
> On Tue, Jun 27, 2023 at 02:58:08PM -0700, Suren Baghdasaryan wrote:
> > Ok in kernfs_generic_poll() we are using kernfs_open_node.poll
> > waitqueue head for polling and kernfs_open_node is freed from inside
> > kernfs_unlink_open_file() which is called from kernfs_fop_release().
> > So, it is destroyed only when the last fput() is done, unlike the
> > ops->release() operation which we are using for destroying PSI
> > trigger's waitqueue. So, it seems we still need an operation which
> > would indicate that the file is truly going away.
>
> If we want to stay consistent with how kernfs behaves w.r.t. severing, the
> right thing to do would be preventing any future polling at severing and
> waking up everyone currently waiting, which sounds fine from cgroup behavior
> POV too.

That's actually what we are currently doing for PSI triggers.
->release() is handled by cgroup_pressure_release() which signals the
waiters, waits for RCU grace period to pass (per
https://elixir.bootlin.com/linux/latest/source/include/linux/wait.h#L258)
and then releases all the trigger resources including the waitqueue
head. However as reported in
https://lore.kernel.org/all/20230613062306.101831-1-lujialin4@huawei.com
this does not save us from the synchronous polling case:

                                                  do_select
                                                      vfs_poll
cgroup_pressure_release
    psi_trigger_destroy
        wake_up_pollfree(&t->event_wait) -> unblocks vfs_poll
        synchronize_rcu()
        kfree(t) -> frees waitqueue head
                                                     poll_freewait()
-> uses waitqueue head


This happens because we release the resources associated with the file
while there are still file users (the file's refcount is non-zero).
And that happens because kernfs can call ->release() before the last
fput().

>
> Now, the challenge is designing an interface which is difficult to make
> mistake with. IOW, it'd be great if kernfs wraps poll call so that severing
> is implemented without kernfs users doing anything, or at least make it
> pretty obvious what the correct usage pattern is.
>
> > Christian's suggestion to rename current ops->release() operation into
> > ops->drain() (or ops->flush() per Matthew's request) and introduce a
> > "new" ops->release() which is called only when the last fput() is done
> > seems sane to me. Would everyone be happy with that approach?
>
> I'm not sure I'd go there. The contract is that once ->release() is called,
> the code backing that file can go away (e.g. rmmod'd). It really should
> behave just like the last put from kernfs users' POV.

I 100% agree with the above statement.

> For this specific fix,
> it's safe because we know the ops is always built into the kernel and won't
> go away but it'd be really bad if the interface says "this is a normal thing
> to do". We'd be calling into rmmod'd text pages in no time.
>
> So, I mean, even for temporary fix, we have to make it abundantly clear that
> this is not for usual usage and can only be used if the code backing the ops
> is built into the kernel and so on.

I think the root cause of this problem is that ->release() in kernfs
does not adhere to the common rule that ->release() is called only
when the file is going away and has no users left. Am I wrong?
Thanks,
Suren.

>
> Thanks.
>
> --
> tejun