linux-kernel - Re: [PATCH 2/3] mm: implement sticky, copy on fork VMA flags

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Message-ID: <jmyrkglrrdxtdkcnovmkcxbk64zgfpp6r3e33bquixkvoxl45r@zcnwp3v2ucbp>
Date: Thu, 30 Oct 2025 16:25:54 +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>, 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 2/3] mm: implement sticky, copy on fork VMA flags

On Wed, Oct 29, 2025 at 04:50:32PM +0000, Lorenzo Stoakes wrote:
> It's useful to be able to force a VMA to be copied on fork outside of the
> parameters specified by vma_needs_copy(), which otherwise only copies page
> tables if:
> 
> * The destination VMA has VM_UFFD_WP set
> * The mapping is a PFN or mixed map
> * The mapping is anonymous and forked in (i.e. vma->anon_vma is non-NULL)
> 
> Setting this flag implies that the page tables mapping the VMA are such
> that simply re-faulting the VMA will not re-establish them in identical
> form.
> 
> We introduce VM_COPY_ON_FORK to clearly identify which flags require this
> behaviour, which currently is only VM_MAYBE_GUARD.

Do we want this to be sticky though? If you're looking for more granularity
with this flag, the best option might be to stop merges from happening there.
If not, I can imagine a VMA that merges with other VMAs far past the original
guarded range, and thus you get no granularity (possibly, not even useful).

If you're _not_ looking for granularity, then maybe using a per-mm flag for
guard ranges or some other solution would be superior?

The rest of the patch (superficially) looks good to me, though.

-- 
Pedro