linux-kernel - Re: [PATCH 1/2] mm: propagate VM

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Message-ID: <069adbf6-181f-420f-b2b1-0ca22894d904@kernel.org>
Date: Mon, 17 Nov 2025 15:25:05 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: propagate VM_SOFTDIRTY on merge

On 14.11.25 18:53, Lorenzo Stoakes wrote:
> Currently we set VM_SOFTDIRTY when a new mapping is set up (whether by
> establishing a new VMA, or via merge) as implemented in __mmap_complete()
> and do_brk_flags().
> 
> However, when performing a merge of existing mappings such as when
> performing mprotect(), we may lose the VM_SOFTDIRTY flag.
> 
> This is because currently we simply ignore VM_SOFTDIRTY for the purposes of
> merge, so one VMA may possess the flag and another not, and whichever
> happens to be the target VMA will be the one upon which the merge is
> performed which may or may not have VM_SOFTDIRTY set.
> 
> Now we have the concept of 'sticky' VMA flags, let's make VM_SOFTDIRTY one
> which solves this issue.
> 
> Additionally update VMA userland tests to propagate changes.
> 
> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---

Looks reasonable to me. I thought that we had that behavior in the past 
... but I also remember scenarios where we would have imprecise 
soft-dirty handling. So I assume this was semi-broken for a while 
(soft-broken :) )

Acked-by: David Hildenbrand (Red Hat) <david@...nel.org>

-- 
Cheers

David