Re: [PATCH v2 3/5] proc/task_mmu: remove special-casing of smap_gather_stats() start parameter

[David Hildenbrand (Arm)]

On 9/7/26 08:39, Suren Baghdasaryan wrote:
> smap_gather_stats() interprets its start parameter to mean vma->vm_start
> when it's set to 0. Eliminate this special interpretation and pass
> vma->vm_start explicitly when needed.
> 
> Since smap_gather_stats() operates within a single VMA, we can replace
> walk_page_vma()/walk_page_range() calls with walk_page_range_vma()
> which is simpler and also can be called while holding per-VMA lock.
> 
> No functional change intended.
> 
> Suggested by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
>  fs/proc/task_mmu.c | 40 ++++++++++++++++++++++------------------
>  1 file changed, 22 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 9908ba32f180..3351decd1172 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1246,20 +1246,27 @@ get_smaps_shmem_walk_ops(struct proc_maps_private *priv)
>  	return &smaps_shmem_walk_vma_lock_ops;
>  }
>  
> -/*
> - * Gather mem stats from @vma with the indicated beginning
> - * address @start, and keep them in @mss.
> +/**
> + * smap_gather_stats() - Gather mem stats from @vma.
> + * @priv: proc maps private state.
> + * @vma: The VMA to gather stats for.
> + * @mss: The accumulated stats.
> + * @start: The address from which to start.
>   *
> - * Use vm_start of @vma as the beginning address if @start is 0.
> + * This gathers stats for the whole of the VMA unless the lock was dropped
> + * and VMA grew or got merged and we found it again, in which case we only
> + * gather stats for the remainder of the VMA range.
>   */
>  static void smap_gather_stats(struct proc_maps_private *priv,
>  			      struct vm_area_struct *vma,
> -			      struct mem_size_stats *mss, unsigned long start)
> +			      struct mem_size_stats *mss,
> +			      unsigned long start)
>  {
>  	const struct mm_walk_ops *ops = get_smaps_walk_ops(priv);
> +	const bool is_partial = start > vma->vm_start;
>  
>  	/* Invalid start */
> -	if (start >= vma->vm_end)
> +	if (start < vma->vm_start || start >= vma->vm_end)
>  		return;
>  
>  	if (vma == get_gate_vma(priv->lock_ctx.mm))
> @@ -1279,20 +1286,17 @@ static void smap_gather_stats(struct proc_maps_private *priv,
>  		 * 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 shared_or_ro = vma_test(vma, VMA_SHARED_BIT) ||
> +					  !vma_test(vma, VMA_WRITE_BIT);
>  
> -		if (!start && (!shmem_swapped || (vma->vm_flags & VM_SHARED) ||
> -					!(vma->vm_flags & VM_WRITE))) {
> +		if (!is_partial && (!shmem_swapped || shared_or_ro))
>  			mss->swap += shmem_swapped;
> -		} else {
> +		else
>  			ops = get_smaps_shmem_walk_ops(priv);
> -		}

Horrible, horrible code, really. But not your fault :)

I think we can just make the shared_or_ro less odd by just checking for cow
mappings (as described in the comment).

	const bool is_cow = vma_is_cow_mapping(vma);

...

	if (is_partial || (shmem_swapped && is_cow))
		ops = get_smaps_shmem_walk_ops(priv);
	else
		mss->swap += shmem_swapped;

That's almost in a form that I could understand what's happening.

-- 
Cheers,

David