linux-kernel - Re: [RFC PATCH 1/5] mm/mmap: Correctly position vma_iterator in __split

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Message-ID: <119b01ac-a57a-43cf-90ca-093e850c4b7e@lucifer.local>
Date: Mon, 10 Jun 2024 13:09:01 +0100
From: Lorenzo Stoakes <lstoakes@...il.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Vlastimil Babka <vbabka@...e.cz>, sidhartha.kumar@...cle.com,
	Matthew Wilcox <willy@...radead.org>, linux-fsdevel@...r.kernel.org,
	bpf@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/5] mm/mmap: Correctly position vma_iterator in
 __split_vma()

On Fri, May 31, 2024 at 12:32:13PM -0400, Liam R. Howlett wrote:
> The vma iterator may be left pointing to the newly created vma.  This
> happens when inserting the new vma at the end of the old vma
> (!new_below).
>
> The incorrect position in the vma iterator is not exposed currently
> since the vma iterator is repositioned in the munmap path and is not
> reused in any of the other paths.
>
> This has limited impact in the current code, but is required for future
> changes.
>
> Fixes: b2b3b886738f ("mm: don't use __vma_adjust() in __split_vma()")
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
>  mm/mmap.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 83b4682ec85c..31d464e6a656 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2442,6 +2442,9 @@ static int __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	/* Success. */
>  	if (new_below)
>  		vma_next(vmi);
> +	else
> +		vma_prev(vmi);
> +
>  	return 0;
>
>  out_free_mpol:
> --
> 2.43.0
>

Looks good to me.

As Suren alludes to, I agree that it's important to comment to indicate
that you want to move the iterator to point to the VMA that's been
shrunk rather than the newly inserted VMA.

Reviewed-by: Lorenzo Stoakes <lstoakes@...il.com>