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

[Marco Elver]

On Thu, 4 Jan 2024 at 09:52, Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> On Mon, Nov 20, 2023 at 06:47:15PM +0100, andrey.konovalov@xxxxxxxxx wrote:
> > From: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> >
> > Add stack_depot_put, a function that decrements the reference counter
> > on a stack record and removes it from the stack depot once the counter
> > reaches 0.
> >
> > Internally, when removing a stack record, the function unlinks it from
> > the hash table bucket and returns to the freelist.
> >
> > With this change, the users of stack depot can call stack_depot_put
> > when keeping a stack trace in the stack depot is not needed anymore.
> > This allows avoiding polluting the stack depot with irrelevant stack
> > traces and thus have more space to store the relevant ones before the
> > stack depot reaches its capacity.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
>
> I yet have to review the final bits of this series, but I'd like to
> comment on something below
>
>
> > +void stack_depot_put(depot_stack_handle_t handle)
> > +{
> > +     struct stack_record *stack;
> > +     unsigned long flags;
> > +
> > +     if (!handle || stack_depot_disabled)
> > +             return;
> > +
> > +     write_lock_irqsave(&pool_rwlock, flags);
> > +
> > +     stack = depot_fetch_stack(handle);
> > +     if (WARN_ON(!stack))
> > +             goto out;
> > +
> > +     if (refcount_dec_and_test(&stack->count)) {
> > +             /* Unlink stack from the hash table. */
> > +             list_del(&stack->list);
> > +
> > +             /* Free stack. */
> > +             depot_free_stack(stack);
>
> It would be great if stack_depot_put would also accept a boolean,
> which would determine whether we want to erase the stack or not.

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?

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).

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).


> For the feature I'm working on page_ower [1], I need to keep track
> of how many times we allocated/freed from a certain path, which may
> expose a potential leak, and I was using the refcount to do that,
> but I don't want the record to be erased, because this new
> functionality won't be exclusive with the existing one.
>
> e.g:  you can check /sys/kernel/debug/page_owner AND
> /sys/kernel/debug/page_owner_stacks
>
> So, while the new functionaliy won't care if a record has been erased,
> the old one will, so information will be lost.
>
> [1] https://patchwork.kernel.org/project/linux-mm/cover/20231120084300.4368-1-osalvador@xxxxxxx/
>
>
>
> --
> Oscar Salvador
> SUSE Labs