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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 29 Apr 2024 08:32:39 -0700
Message-ID: <CAHk-=whEMvpPLrzsi6BoH=o+-ScRKuuqxrdWSnrTtGEi=JvcNA@mail.gmail.com>
Subject: Re: [PATCH] bounds: Use the right number of bits for power-of-two CONFIG_NR_CPUS
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: linux-kernel@vger.kernel.org, 
	=?UTF-8?B?0JzQuNGF0LDQuNC7INCd0L7QstC+0YHQtdC70L7Qsg==?= <m.novosyolov@rosalinux.ru>, 
	=?UTF-8?B?0JjQu9GM0YTQsNGCINCT0LDQv9GC0YDQsNGF0LzQsNC90L7Qsg==?= <i.gaptrakhmanov@rosalinux.ru>, 
	stable@vger.kernel.org, Rik van Riel <riel@surriel.com>, 
	Mel Gorman <mgorman@techsingularity.net>, Peter Zijlstra <peterz@infradead.org>, 
	Ingo Molnar <mingo@kernel.org>, Andrew Morton <akpm@linux-foundation.org>
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On Mon, 29 Apr 2024 at 07:48, Matthew Wilcox (Oracle)
<willy@infradead.org> wrote:
>
> bits_per() rounds up to the next power of two when passed a power of
> two.  This causes crashes on some machines and configurations.

Bah. Your patch is *still* wrong, because bits_per() thinks you need
one bit for a zero value, so when you do

        bits_per(CONFIG_NR_CPUS - 1)

and some insane person has enabled SMP and managed to set
CONFIG_NR_CPUS to 1, the math is *still* broken.

The right thing to do is

        order_base_2(CONFIG_NR_CPUS)

and 'bits_per()' should be avoided, having completely crazy semantics
(you can tell how almost all users actually do "x-1" as the argument).

We should probably get rid of that horrid bits_per(() entirely.

I applied your patch with that fixed (which admittedly make it all
*my* patch, but applying it as yours just to get the changelog).

               Linus