linux-kernel - Re: [PATCH 1/1] mm/ksm: fix spurious soft-dirty bit on zero-filled page merging

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Message-ID: <e74ae9da-9dfb-4673-ae9f-c0b27b796cf0@redhat.com>
Date: Mon, 29 Sep 2025 10:25:04 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org
Cc: xu.xin16@....com.cn, chengming.zhou@...ux.dev, ran.xiaokai@....com.cn,
 yang.yang29@....com.cn, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 ioworker0@...il.com
Subject: Re: [PATCH 1/1] mm/ksm: fix spurious soft-dirty bit on zero-filled
 page merging

On 28.09.25 06:52, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
> 
> When KSM merges a zero-filled page with the shared zeropage, it uses
> pte_mkdirty() to mark the new PTE for internal accounting. However,
> pte_mkdirty() unconditionally sets both the hardware dirty bit and the
> soft-dirty bit.
> 

Right, that's one think we should clean up at one point.

> This behavior causes false positives in userspace tools like CRIU that
> rely on the soft-dirty mechanism for tracking memory changes.

IIRC, false positives are not a problem. We get them all of the time 
when merging VMAs etc.

So I am not sure if this here is really worth fixing. Soft-dirty is not, 
and never will be false-positive free.

-- 
Cheers

David / dhildenb