Re: [PATCH v4 3/7] proc/task_mmu: clarify shmem mapping walk conditions in smap_gather_stats()

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Fri, Sep 11, 2026 at 12:41:41PM -0700, Suren Baghdasaryan wrote:
> smap_gather_stats() optimizes stats gathering by skipping the walk for
> shmem mappings in certain conditions. Update the comment to clarify
> these conditions and use vma_is_cow_mapping() for CoW identification
> instead of open-coding it.
>
> No functional change intended.
>
> Suggested by: David Hildenbrand (Arm) <david@kernel.org>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  fs/proc/task_mmu.c | 23 +++++++++--------------
>  1 file changed, 9 insertions(+), 14 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index cfc7af1b551d..0e53eb065a3e 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1270,23 +1270,18 @@ static void smap_gather_stats(struct proc_maps_private *priv,
>
>  	if (vma->vm_file && shmem_mapping(vma->vm_file->f_mapping)) {
>  		/*
> -		 * For shared or readonly shmem mappings we know that all
> -		 * swapped out pages belong to the shmem object, and we can
> -		 * obtain the swap value much more efficiently. For private
> -		 * writable mappings, we might have COW pages that are
> -		 * not affected by the parent swapped out pages of the shmem
> -		 * object, so we have to distinguish them during the page walk.
> -		 * Unless we know that the shmem object (or the part mapped by
> -		 * our VMA) has no swapped out pages at all.
> +		 * CoW mappings might map anon folios that do not belong to
> +		 * shmem. Perform a less efficient page table walk in this
> +		 * situation, unless we know that the shmem object (or the
> +		 * part mapped by our VMA) has no swapped out pages at all.
>  		 */
> -		unsigned long shmem_swapped = shmem_swap_usage(vma);
> +		const unsigned long shmem_swapped = shmem_swap_usage(vma);
> +		const bool is_cow = vma_is_cow_mapping(vma);
>
> -		if (!start && (!shmem_swapped || (vma->vm_flags & VM_SHARED) ||
> -					!(vma->vm_flags & VM_WRITE))) {
> -			mss->swap += shmem_swapped;
> -		} else {
> +		if (start || (shmem_swapped && is_cow))
>  			ops = get_smaps_shmem_walk_ops(priv);
> -		}
> +		else
> +			mss->swap += shmem_swapped;
>  	}
>
>  	if (!start)
> --
> 2.55.0.1007.g17ff1f9808-goog
>

--
Cheers, Lorenzo