Re: [PATCH v4 17/22] lib/stackdepot: allow users to evict stack traces

[Marco Elver]

On Thu, 4 Jan 2024 at 11:18, Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> On Thu, Jan 04, 2024 at 10:25:40AM +0100, Marco Elver wrote:
> > I think a boolean makes the interface more confusing for everyone
> > else. At that point stack_depot_put merely decrements the refcount and
> > becomes a wrapper around refcount_dec, right?
>
> Thanks Marco for the feedback.
>
> Fair enough.
>
> > I think you want to expose the stack_record struct anyway for your
> > series, so why not simply avoid calling stack_depot_put and decrement
> > the refcount with your own helper (there needs to be a new stackdepot
> > function to return a stack_record under the pool_rwlock held as
> > reader).
>
> Yeah, that was something I was experimenting with my last version.
> See [0], I moved the "stack_record" struct into the header so page_owner
> can make sense of it. I guess that's fine right?

Not exposing the internals would be better, but at this point I think
your usecase looks like it's doing something that is somewhat out of
the bounds of the stackdepot API. I also don't see that it makes sense
to add more helpers to the stackdepot API to deal with such special
cases.

As such, I'd reason that it's ok to expose the struct for this special usecase.

> If so, I'd do as you mentioned, just decrementing it with my own helper
> so no calls to stack_depot_put will be needed.
>
> Regarding the locking, I yet have to check the patch that implements
> the read/write lock, but given that page_owner won't be evicting
> anything, do I still have to fiddle with the locks?

You need to grab the lock as a reader to fetch the stack_record and
return it. All that should be hidden behind whatever function you'll
introduce to return the stack_record (stack_depot_find()?).

> > Also, you need to ensure noone else calls stack_depot_put on the stack
> > traces you want to keep. If there is a risk someone else may call
> > stack_depot_put on them, it obviously won't work (I think the only
> > option then is to introduce a way to pin stacks).
>
> Well, since page_owner won't call stack_depot_put, I don't see
> how someone else would be able to interfere there, so I think
> I am safe there.
>
> [0] https://patchwork.kernel.org/project/linux-mm/patch/20231120084300.4368-3-osalvador@xxxxxxx/
>
> --
> Oscar Salvador
> SUSE Labs