Re: [PATCH v2 3/3] mmap: Add clarifying comment to vma_merge() code

[Vlastimil Babka]

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@xxxxxxxxxx>
> Link: https://lore.kernel.org/linux-mm/CAG48ez3iDwFPR=Ed1BfrNuyUJPMK_=StjxhUsCkL6po1s7bONg@xxxxxxxxxxxxxx/
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  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);
>  		}