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From:   Andrey Konovalov <andreyknvl@gmail.com>
Date:   Tue, 31 Jan 2023 19:59:55 +0100
Message-ID: <CA+fCnZfjbHaS9So6gO_3ZkgLazJXYAtw-PNV5C0xhAjzVE3p-Q@mail.gmail.com>
Subject: Re: [PATCH 01/18] lib/stackdepot: fix setting next_slab_inited in init_stack_slab
To:     Alexander Potapenko <glider@google.com>
Cc:     andrey.konovalov@linux.dev, Marco Elver <elver@google.com>,
        Vlastimil Babka <vbabka@suse.cz>, kasan-dev@googlegroups.com,
        Evgenii Stepanov <eugenis@google.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrey Konovalov <andreyknvl@google.com>
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On Tue, Jan 31, 2023 at 10:30 AM Alexander Potapenko <glider@google.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.