linux-kernel - Re: [PATCH v3] ksm: use range-walk function to jump over holes in scan_get_next_rmap

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Message-Id: <20251016140735.d7d4c282f0fbf22954a4b4ee@linux-foundation.org>
Date: Thu, 16 Oct 2025 14:07:35 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Pedro Demarchi Gomes <pedrodemargomes@...il.com>
Cc: David Hildenbrand <david@...hat.com>, Xu Xin <xu.xin16@....com.cn>,
 craftfever <craftfever@...mail.cc>, Chengming Zhou
 <chengming.zhou@...ux.dev>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] ksm: use range-walk function to jump over holes in
 scan_get_next_rmap_item

On Wed, 15 Oct 2025 22:22:36 -0300 Pedro Demarchi Gomes <pedrodemargomes@...il.com> wrote:

> Currently, scan_get_next_rmap_item() walks every page address in a VMA
> to locate mergeable pages. This becomes highly inefficient when scanning
> large virtual memory areas that contain mostly unmapped regions.
> 
> This patch replaces the per-address lookup with a range walk using
> walk_page_range(). The range walker allows KSM to skip over entire
> unmapped holes in a VMA, avoiding unnecessary lookups.
> This problem was previously discussed in [1].
> 
> ...
>
> Reported-by: craftfever <craftfever@...mail.cc>
> Closes: https://lkml.kernel.org/r/020cf8de6e773bb78ba7614ef250129f11a63781@murena.io
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@...il.com>

Is Fixes: b1d3e9bbccb4 ("mm/ksm: convert scan_get_next_rmap_item() from
follow_page() to folio_walk") appropriate?  

The problem which is being addressed seems pretty serious.  What do
people think about proposing a -stable backport of this fix?

It would be better if this changelog were to describe the user-visible
effects of the problem.  A copy-n-paste from
https://bugzilla.kernel.org/show_bug.cgi?id=220599 would suffice.