linux-kernel - Re: [PATCH v3 3/3] mmap: Add clarifying comment to vma

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Message-ID: <1c7c1869-ef9c-487f-9c8e-08a51d9f8efe@lucifer.local>
Date:   Fri, 29 Sep 2023 23:30:01 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 3/3] mmap: Add clarifying comment to vma_merge() code

On Fri, Sep 29, 2023 at 02:30:41PM -0400, Liam R. Howlett wrote:
> When tracing through the code in vma_merge(), it was not completely
> clear why the error return to a dup_anon_vma() call would not overwrite
> a previous attempt to the same function.  This commit adds a comment
> specifying why it is safe.
>
> Suggested-by: Jann Horn <jannh@...gle.com>
> Link: https://lore.kernel.org/linux-mm/CAG48ez3iDwFPR=Ed1BfrNuyUJPMK_=StjxhUsCkL6po1s7bONg@mail.gmail.com/
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
>  mm/mmap.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f9f0a5fe4db4..9967acbd070f 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -943,6 +943,11 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  			vma_start_write(curr);
>  			remove = curr;
>  			remove2 = next;
> +			/*
> +			 * Note that the dup_anon_vma below cannot overwrite err
> +			 * since the first caller would do nothing unless next
> +			 * has an anon_vma.
> +			 */
>  			if (!next->anon_vma)
>  				err = dup_anon_vma(prev, curr, &anon_dup);
>  		}
> --
> 2.40.1
>

Nice comment! It causes me to sick up a bit in my mouth that this is a thing,
but it's good to have it documented.

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