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Date: Wed, 27 Mar 2024 14:10:10 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Sasha Levin <sashal@kernel.org>
Cc: 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>,
	linux-kernel@vger.kernel.org
Subject: Re: FAILED: Patch "bounds: support non-power-of-two CONFIG_NR_CPUS"
 failed to apply to 5.4-stable tree
Message-ID: <ZgQowqqGf-E7Cpcz@casper.infradead.org>
References: <20240327122125.2836828-1-sashal@kernel.org>
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In-Reply-To: <20240327122125.2836828-1-sashal@kernel.org>

On Wed, Mar 27, 2024 at 08:21:25AM -0400, Sasha Levin wrote:
> The patch below does not apply to the 5.4-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.

Looks like you just need a little more fuzz on the patch.

diff --git a/kernel/bounds.c b/kernel/bounds.c
index 9795d75b09b2..a94e3769347e 100644
--- a/kernel/bounds.c
+++ b/kernel/bounds.c
@@ -19,7 +19,7 @@ int main(void)
 	DEFINE(NR_PAGEFLAGS, __NR_PAGEFLAGS);
 	DEFINE(MAX_NR_ZONES, __MAX_NR_ZONES);
 #ifdef CONFIG_SMP
-	DEFINE(NR_CPUS_BITS, ilog2(CONFIG_NR_CPUS));
+	DEFINE(NR_CPUS_BITS, bits_per(CONFIG_NR_CPUS));
 #endif
 	DEFINE(SPINLOCK_SIZE, sizeof(spinlock_t));
 	/* End of constants */

> Thanks,
> Sasha
> 
> ------------------ original commit in Linus's tree ------------------
> 
> >From f2d5dcb48f7ba9e3ff249d58fc1fa963d374e66a Mon Sep 17 00:00:00 2001
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Date: Tue, 10 Oct 2023 15:55:49 +0100
> Subject: [PATCH] bounds: support non-power-of-two CONFIG_NR_CPUS
> 
> ilog2() rounds down, so for example when PowerPC 85xx sets CONFIG_NR_CPUS
> to 24, we will only allocate 4 bits to store the number of CPUs instead of
> 5.  Use bits_per() instead, which rounds up.  Found by code inspection.
> The effect of this would probably be a misaccounting when doing NUMA
> balancing, so to a user, it would only be a performance penalty.  The
> effects may be more wide-spread; it's hard to tell.
> 
> Link: https://lkml.kernel.org/r/20231010145549.1244748-1-willy@infradead.org
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Fixes: 90572890d202 ("mm: numa: Change page last {nid,pid} into {cpu,pid}")
> Reviewed-by: Rik van Riel <riel@surriel.com>
> Acked-by: Mel Gorman <mgorman@techsingularity.net>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>  kernel/bounds.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/bounds.c b/kernel/bounds.c
> index b529182e8b04f..c5a9fcd2d6228 100644
> --- a/kernel/bounds.c
> +++ b/kernel/bounds.c
> @@ -19,7 +19,7 @@ int main(void)
>  	DEFINE(NR_PAGEFLAGS, __NR_PAGEFLAGS);
>  	DEFINE(MAX_NR_ZONES, __MAX_NR_ZONES);
>  #ifdef CONFIG_SMP
> -	DEFINE(NR_CPUS_BITS, ilog2(CONFIG_NR_CPUS));
> +	DEFINE(NR_CPUS_BITS, bits_per(CONFIG_NR_CPUS));
>  #endif
>  	DEFINE(SPINLOCK_SIZE, sizeof(spinlock_t));
>  #ifdef CONFIG_LRU_GEN
> -- 
> 2.43.0
> 
> 
> 
>