Re: [PATCH 2/8] mm/rmap: skip unfaulted VMAs on anon_vma clone, unlink

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Lorenzo Stoakes <lorenzo.stoakes@oracle.com> [251217 07:27]:
> For both anon_vma_clone() and unlink_anon_vmas(), if the source VMA or the
> VMA to be linked are unfaulted (e.g. !vma->anon_vma), then the functions do
> nothing. Simply exit early in these cases.
> 
> In the unlink_anon_vmas() case we can also remove a conditional that checks
> whether vma->anon_vma is set.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>  mm/rmap.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 0e34c0a69fbc..9332d1cbc643 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -309,6 +309,9 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>  	struct anon_vma_chain *avc, *pavc;
>  	struct anon_vma *root = NULL;
>  
> +	if (!src->anon_vma)
> +		return 0;
> +
>  	check_anon_vma_clone(dst, src);
>  
>  	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
> @@ -441,7 +444,8 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
>  	mmap_assert_locked(vma->vm_mm);
>  
>  	/* Unfaulted is a no-op. */
> -	VM_WARN_ON_ONCE(!vma->anon_vma && !list_empty(&vma->anon_vma_chain));
> +	if (!vma->anon_vma)
> +		return;

I guess it doesn't matter because you just added the !list_empty()
check, but did you mean to drop that part?

>  
>  	/*
>  	 * Unlink each anon_vma chained to the VMA.  This list is ordered
> @@ -465,15 +469,13 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
>  		list_del(&avc->same_vma);
>  		anon_vma_chain_free(avc);
>  	}
> -	if (vma->anon_vma) {
> -		vma->anon_vma->num_active_vmas--;
>  
> -		/*
> -		 * vma would still be needed after unlink, and anon_vma will be prepared
> -		 * when handle fault.
> -		 */
> -		vma->anon_vma = NULL;
> -	}
> +	vma->anon_vma->num_active_vmas--;
> +	/*
> +	 * vma would still be needed after unlink, and anon_vma will be prepared
> +	 * when handle fault.
> +	 */
> +	vma->anon_vma = NULL;
>  	unlock_anon_vma_root(root);
>  
>  	/*
> -- 
> 2.52.0
>