linux-kernel - Re: [PATCH 05/15] stackdepot: use fixed-sized slots for stack records

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Date:   Wed, 30 Aug 2023 10:21:25 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     andrey.konovalov@...ux.dev
Cc:     Marco Elver <elver@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...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 05/15] stackdepot: use fixed-sized slots for stack records

On Tue, Aug 29, 2023 at 7:11 PM <andrey.konovalov@...ux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@...gle.com>
>
> Instead of storing stack records in stack depot pools one right after
> another, use 32-frame-sized slots.

I am slightly concerned about the KMSAN use case here, which defines
KMSAN_STACK_DEPTH to 64.
I don't have a comprehensive stack depth breakdown, but a quick poking
around syzkaller.appspot.com shows several cases where the stacks are
actually longer than 32 frames.
Can you add a config parameter for the stack depth instead of
mandating 32 frames everywhere?

As a side note, kmsan_internal_chain_origin()
(https://elixir.bootlin.com/linux/latest/source/mm/kmsan/core.c#L214)
creates small 3-frame records in the stack depot to link two stacks
together, which will add unnecessary stackdepot pressure.
But this can be fixed by storing both the new stack trace and the link
to the old stack trace in the same record.