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

[Suren Baghdasaryan]

On Wed, Jun 28, 2023 at 12:26 AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> On Tue, Jun 27, 2023 at 08:09:46PM -0700, Suren Baghdasaryan wrote:
> > On Tue, Jun 27, 2023 at 6:54 PM Tejun Heo <tj@xxxxxxxxxx> 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@xxxxxxxxxx
> > 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?
>
> So imho, ultimately this all comes down to rmdir() having special
> semantics in kernfs. On any regular filesystem an rmdir() on a directory
> which is still referenced by a struct file doesn't trigger an
> f_op->release() operation. It's just that directory is unlinked and
> you get some sort of errno like ENOENT when you try to create new files
> in there or whatever. The actual f_op->release) however is triggered
> on last fput().
>
> But in essence, kernfs treats an rmdir() operation as being equivalent
> to a final fput() such that it somehow magically kills all file
> references. And that's just wrong and not supported.

Thanks for the explanation, Christian!
If kernfs is special and needs different rules for calling
f_op->release() then fine, but I need an operation which tells me
there are no users of the file so that I can free the resources.
What's the best way to do that?

>
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