Re: [PATCH v2 2/8] mm/rmap: skip unfaulted VMAs on anon_vma clone, unlink

[Liam R. Howlett]

* Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [260106 10:04]:
> 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@xxxxxxxxxx>

Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

> ---
>  mm/rmap.c | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 54ccf884d90a..de2cbe860566 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -314,6 +314,9 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>  
>  	check_anon_vma_clone(dst, src);
>  
> +	if (!src->anon_vma)
> +		return 0;
> +
>  	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
>  		struct anon_vma *anon_vma;
>  
> @@ -474,7 +477,10 @@ 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) {
> +		VM_WARN_ON_ONCE(!list_empty(&vma->anon_vma_chain));
> +		return;
> +	}
>  
>  	/*
>  	 * Unlink each anon_vma chained to the VMA.  This list is ordered
> @@ -498,15 +504,13 @@ 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
>