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

[Harry Yoo]

On Mon, Jan 05, 2026 at 08:11:49PM +0000, 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@xxxxxxxxxx>
> Fixes: 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA merges")
> Cc: stable@xxxxxxxxxx
> ---

LGTM, so:
Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>

-- 
Cheers,
Harry / Hyeonggon