linux-kernel - Re: [PATCH v2 3/4] mm/vma: enforce VMA fork limit on unfaulted,faulted mremap merge too

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Message-ID: <0cca73ec-00bf-4ff3-8b6a-48c723d01c35@suse.cz>
Date: Fri, 9 Jan 2026 19:10:41 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: "Liam R . Howlett" <Liam.Howlett@...cle.com>, Jann Horn
 <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
 Yeoreum Yun <yeoreum.yun@....com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, David Hildenbrand <david@...nel.org>,
 Jeongjun Park <aha310510@...il.com>, Rik van Riel <riel@...riel.com>,
 Harry Yoo <harry.yoo@...cle.com>
Subject: Re: [PATCH v2 3/4] mm/vma: enforce VMA fork limit on
 unfaulted,faulted mremap merge too

On 1/5/26 21:11, Lorenzo Stoakes wrote:
> The is_mergeable_anon_vma() function uses vmg->middle as the source
> VMA. However when merging a new VMA, this field is NULL.
> 
> In all cases except mremap(), the new VMA will either be newly established
> and thus lack an anon_vma, or will be an expansion of an existing VMA thus
> we do not care about whether VMA is CoW'd or not.
> 
> In the case of an mremap(), we can end up in a situation where we can
> accidentally allow an unfaulted/faulted merge with a VMA that has been
> forked, violating the general rule that we do not permit this for reasons
> of anon_vma lock scalability.
> 
> Now we have the ability to be aware of the fact we are copying a VMA and
> also know which VMA that is, we can explicitly check for this, so do so.
> 
> This is pertinent since commit 879bca0a2c4f ("mm/vma: fix incorrectly
> disallowed anonymous VMA merges"), as this patch permits unfaulted/faulted
> merges that were previously disallowed running afoul of this issue.
> 
> While we are here, vma_had_uncowed_parents() is a confusing name, so make
> it simple and rename it to vma_is_fork_child().
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Fixes: 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA merges")
> Cc: stable@...nel.org

Acked-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!