linux-kernel - Re: [PATCH v4 3/9] mm: update vma_modify_flags() to handle residual flags, document

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Message-ID: <vr45iyduvak3wpzmos5l4jfpzticerlnxuhnpvbzb7fpsvanqx@rsuogv5s6brh>
Date: Wed, 19 Nov 2025 16:36:10 +0000
From: Pedro Falcato <pfalcato@...e.de>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Jonathan Corbet <corbet@....net>, David Hildenbrand <david@...hat.com>, 
	"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>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Jann Horn <jannh@...gle.com>, Zi Yan <ziy@...dia.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, Nico Pache <npache@...hat.com>, 
	Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>, 
	Lance Yang <lance.yang@...ux.dev>, linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, Andrei Vagin <avagin@...il.com>
Subject: Re: [PATCH v4 3/9] mm: update vma_modify_flags() to handle residual
 flags, document

On Tue, Nov 18, 2025 at 10:17:45AM +0000, Lorenzo Stoakes wrote:
> The vma_modify_*() family of functions each either perform splits, a merge
> or no changes at all in preparation for the requested modification to
> occur.
> 
> When doing so for a VMA flags change, we currently don't account for any
> flags which may remain (for instance, VM_SOFTDIRTY) despite the requested
> change in the case that a merge succeeded.
> 
> This is made more important by subsequent patches which will introduce the
> concept of sticky VMA flags which rely on this behaviour.
> 
> This patch fixes this by passing the VMA flags parameter as a pointer and
> updating it accordingly on merge and updating callers to accommodate for
> this.
> 
> Additionally, while we are here, we add kdocs for each of the
> vma_modify_*() functions, as the fact that the requested modification is
> not performed is confusing so it is useful to make this abundantly
> clear.
> 
> We also update the VMA userland tests to account for this change.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

Reviewed-by: Pedro Falcato <pfalcato@...e.de>

Quite ugly change, but for the sake of brevity I think we can live it
Temporarily(tm).

-- 
Pedro