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Date: Fri, 20 Mar 2026 12:08:48 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Joonwon Kang <joonwonkang@google.com>
Cc: dennis@kernel.org, tj@kernel.org, cl@gentwo.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
Subject: Re: [PATCH] percpu: Fix hint invariant breakage
Message-Id: <20260320120848.ba7ddc21aaa4edab8c2b5b58@linux-foundation.org>
In-Reply-To: <20260320115214.143933-1-joonwonkang@google.com>
References: <20260320115214.143933-1-joonwonkang@google.com>
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On Fri, 20 Mar 2026 11:52:14 +0000 Joonwon Kang <joonwonkang@google.com> wrote:

> The invariant "scan_hint_start > contig_hint_start if and only if
> scan_hint == contig_hint" should be kept for hint management. However,
> it could be broken in some cases:
> 
>   - if (new contig == contig_hint == scan_hint) && (contig_hint_start <
>     scan_hint_start < new contig start) && the new contig is to become a
>     new contig_hint due to its better alignment, then scan_hint should
>     be invalidated instead of keeping it.
> 
>   - if (new contig == contig_hint > scan_hint) && (start <
>     contig_hint_start) && the new contig is not to become a new
>     contig_hint, then scan_hint should be invalidated instead of being
>     updated to the new contig.
> 
> This commit fixes this invariant breakage and also optimizes scan_hint
> by keeping it or updating it when acceptable:
> 
>   - if (new contig > contig_hint > scan_hint) && (scan_hint_start < new
>     contig start < contig_hint_start), then keep scan_hint instead of
>     invalidating it.
> 
>   - if (new contig > contig_hint == scan_hint) && (contig_hint_start <
>     new contig start < scan_hint_start), then update scan_hint to the
>     old contig_hint instead of invalidating it.
> 
>   - if (new contig == contig_hint > scan_hint) && (new contig start <
>     contig_hint_start) && the new contig is to become a new contig_hint
>     due to its better alignment, then update scan_hint to the old
>     contig_hint instead of invalidating or keeping it.
> 

Thanks.  Does the change have any observable runtime effects?

Was this patch inspired by code inspection?