linux-kernel - Re: [PATCH] stackdepot: Make max number of pools boot-time configurable

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Message-ID: <CAENh_SSDNyGcuE=yL9GEt9gTWY+2BepVj1s+x60CthD7-HGp2g@mail.gmail.com>
Date: Fri, 18 Jul 2025 16:39:01 +0100
From: Matt Fleming <matt@...dmodwrite.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, kernel-team@...udflare.com, 
	Marco Elver <elver@...gle.com>, Alexander Potapenko <glider@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Oscar Salvador <osalvador@...e.de>, Vlastimil Babka <vbabka@...e.cz>, 
	Matt Fleming <mfleming@...udflare.com>
Subject: Re: [PATCH] stackdepot: Make max number of pools boot-time configurable

On Tue, Jul 15, 2025 at 12:38 AM Andrew Morton
<akpm@...ux-foundation.org> wrote:
>
>
> Geeze that was all quite the mouthful.  Can't we just do this?
>
> --- a/lib/stackdepot.c~a
> +++ a/lib/stackdepot.c
> @@ -36,13 +36,11 @@
>  #include <linux/memblock.h>
>  #include <linux/kasan-enabled.h>
>
> -#define DEPOT_POOLS_CAP 8192
> -/* The pool_index is offset by 1 so the first record does not have a 0 handle. */
> -#define DEPOT_MAX_POOLS \
> -       (((1LL << (DEPOT_POOL_INDEX_BITS)) - 1 < DEPOT_POOLS_CAP) ? \
> -        (1LL << (DEPOT_POOL_INDEX_BITS)) - 1 : DEPOT_POOLS_CAP)
> -
> -static unsigned int stack_max_pools = DEPOT_MAX_POOLS;
> +/*
> + * The pool_index is offset by 1 so the first record does not have a 0 handle.
> + */
> +static unsigned int stack_max_pools __read_mostly =
> +       MIN((1LL << DEPOT_POOL_INDEX_BITS) - 1, 8192);
>
>  static bool stack_depot_disabled;
>  static bool __stack_depot_early_init_requested __initdata = IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT);
> _
>
> (please add this to the next version)

Nice. Will do.

> (but why do we do this min() at all?  Why not simply use (1<<DEPOT_POOL_INDEX_BITS)?)
>
> (shouldn't that 8192 be 8191?  Seems oddly inconsistent)

Yeah, I don't have a good answer here. I was hoping Marco or Alexander
would chime in.

> If user hits this they're going to tear hair figuring out the actual
> limit.  So how about "stack_depot_max_pools exceeds %d, using default
> of %d".

Good point. Will fix.