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

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jun 28, 2023 at 1:41 AM Christian Brauner <brauner@kernel.org> wrote:
>
> On Wed, Jun 28, 2023 at 12:46:43AM -0700, Suren Baghdasaryan wrote:
> > On Wed, Jun 28, 2023 at 12:26 AM Christian Brauner <brauner@kernel.org> 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@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
>
> I don't know if this talks about kernfs_ops (likely) or talks about
> f_ops but fyi for f_ops this isn't a problem. See fops_get() in
> do_dentry_open() which takes a reference on the mode that provides the
> fops. And debugfs - a little more elaborately - handles this as well.
>
> > > > > 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?
>
> Imho, if there's still someone with an fd referencing that file then
> there's still a user. That's unlink() while holding an fd in a nutshell.
>
> But generically, afaui what you seem to want is:
>
> (1) a way to shutdown functionality provided by a kernfs node on removal
>     of that node
> (2) a way to release the resources of a kernfs node
>
> So (2) is seemingly what kernfs_ops->release() is about but it's also
> used for (1). So while I initially thought about ->drain() or whatever
> it seems what you really want is for struct kernfs_ops to gain an
> unlink()/remove()/rmdir() method(s). The method can be implemented by
> interested callers and should be called when the kernfs node is removed.
>
> And that's when you can shutdown any functionality without freeing the
> resources.
>
> Imho, if you add struct gimmegimme_ops with same names as f_op you
> really to mirror them as close as possible otherwise you're asking for
> problems.

Thanks for the feedback!
To summarize my understanding of your proposal, you suggest adding new
kernfs_ops for the case you marked (1) and change ->release() to do
only (2). Please correct me if I misunderstood. Greg, Tejun, WDYT?