linux-kernel - Re: [PATCH 01/18] lib/stackdepot: fix setting next_slab_inited in init_stack

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Date:   Wed, 1 Feb 2023 12:51:45 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Andrey Konovalov <andreyknvl@...il.com>
Cc:     andrey.konovalov@...ux.dev, Marco Elver <elver@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, kasan-dev@...glegroups.com,
        Evgenii Stepanov <eugenis@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH 01/18] lib/stackdepot: fix setting next_slab_inited in init_stack_slab

On Tue, Jan 31, 2023 at 8:00 PM Andrey Konovalov <andreyknvl@...il.com> wrote:
>
> On Tue, Jan 31, 2023 at 10:30 AM Alexander Potapenko <glider@...gle.com> wrote:
> >
> > Wait, I think there's a problem here.
> >
> > > diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> > > index 79e894cf8406..0eed9bbcf23e 100644
> > > --- a/lib/stackdepot.c
> > > +++ b/lib/stackdepot.c
> > > @@ -105,12 +105,13 @@ static bool init_stack_slab(void **prealloc)
> > >                 if (depot_index + 1 < STACK_ALLOC_MAX_SLABS) {
> > If we get to this branch, but the condition is false, this means that:
> >  - next_slab_inited == 0
> >  - depot_index == STACK_ALLOC_MAX_SLABS+1
> >  - stack_slabs[depot_index] != NULL.
> >
> > So stack_slabs[] is at full capacity, but upon leaving
> > init_stack_slab() we'll always keep next_slab_inited==0.
> >
> > Now every time __stack_depot_save() is called for a known stack trace,
> > it will preallocate 1<<STACK_ALLOC_ORDER pages (because
> > next_slab_inited==0), then find the stack trace id in the hash, then
> > pass the preallocated pages to init_stack_slab(), which will not
> > change the value of next_slab_inited.
> > Then the preallocated pages will be freed, and next time
> > __stack_depot_save() is called they'll be allocated again.
>
> Ah, right, missed that.
>
> What do you think about renaming next_slab_inited to
> next_slab_required and inverting the used values (0/1 -> 1/0)? This
> would make this part of code less confusing.

"Required" as in "requires a preallocated buffer, but does not have one yet"?
Yes, that's probably better.
(In any case we'll need to add a comment to that variable explaining
the circumstances under which one or another value is possible).