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Date:   Fri, 29 Sep 2023 12:29:02 +0200
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Subject: Re: [PATCH v2 3/3] mmap: Add clarifying comment to vma_merge() code
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To:     "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     maple-tree@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Jann Horn <jannh@google.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Matthew Wilcox <willy@infradead.org>, stable@vger.kernel.org
References: <20230928171634.2245042-1-Liam.Howlett@oracle.com>
 <20230928171634.2245042-4-Liam.Howlett@oracle.com>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <20230928171634.2245042-4-Liam.Howlett@oracle.com>
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On 9/28/23 19:16, 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@google.com>
> Link: https://lore.kernel.org/linux-mm/CAG48ez3iDwFPR=Ed1BfrNuyUJPMK_=StjxhUsCkL6po1s7bONg@mail.gmail.com/
> Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>

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

> ---
>  mm/mmap.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2f0ee489db8a..3c78afb707cf 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -937,6 +937,11 @@ 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);
>  		}