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: <CAO9qdTGp3KxpGjwfsKj3rADXsZLPbBY_XYyHJTy6NVgBF9U+0w@mail.gmail.com>
Date: Wed, 7 Jan 2026 00:23:28 +0900
From: Jeongjun Park <aha310510@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, 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>, 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

Lorenzo Stoakes <lorenzo.stoakes@...cle.com> 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
> ---

Reviewed-by: Jeongjun Park <aha310510@...il.com>

>  mm/vma.c | 27 +++++++++++++++------------
>  1 file changed, 15 insertions(+), 12 deletions(-)
>
> diff --git a/mm/vma.c b/mm/vma.c
> index 660f4732f8a5..fb45a6be7417 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -67,18 +67,13 @@ struct mmap_state {
>                 .state = VMA_MERGE_START,                               \
>         }
>
> -/*
> - * If, at any point, the VMA had unCoW'd mappings from parents, it will maintain
> - * more than one anon_vma_chain connecting it to more than one anon_vma. A merge
> - * would mean a wider range of folios sharing the root anon_vma lock, and thus
> - * potential lock contention, we do not wish to encourage merging such that this
> - * scales to a problem.
> - */
> -static bool vma_had_uncowed_parents(struct vm_area_struct *vma)
> +/* Was this VMA ever forked from a parent, i.e. maybe contains CoW mappings? */
> +static bool vma_is_fork_child(struct vm_area_struct *vma)
>  {
>         /*
>          * The list_is_singular() test is to avoid merging VMA cloned from
> -        * parents. This can improve scalability caused by anon_vma lock.
> +        * parents. This can improve scalability caused by the anon_vma root
> +        * lock.
>          */
>         return vma && vma->anon_vma && !list_is_singular(&vma->anon_vma_chain);
>  }
> @@ -115,11 +110,19 @@ static bool is_mergeable_anon_vma(struct vma_merge_struct *vmg, bool merge_next)
>         VM_WARN_ON(src && src_anon != src->anon_vma);
>
>         /* Case 1 - we will dup_anon_vma() from src into tgt. */
> -       if (!tgt_anon && src_anon)
> -               return !vma_had_uncowed_parents(src);
> +       if (!tgt_anon && src_anon) {
> +               struct vm_area_struct *copied_from = vmg->copied_from;
> +
> +               if (vma_is_fork_child(src))
> +                       return false;
> +               if (vma_is_fork_child(copied_from))
> +                       return false;
> +
> +               return true;
> +       }
>         /* Case 2 - we will simply use tgt's anon_vma. */
>         if (tgt_anon && !src_anon)
> -               return !vma_had_uncowed_parents(tgt);
> +               return !vma_is_fork_child(tgt);
>         /* Case 3 - the anon_vma's are already shared. */
>         return src_anon == tgt_anon;
>  }
> --
> 2.52.0
>