Re: [PATCH] mm: proc: smaps_rollup: Fix pss_locked calculation

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 20 Jan 2019 17:10:49 -0800 Sandeep Patil <sspatil@android.com> wrote:

> The 'pss_locked' field of smaps_rollup was being calculated incorrectly
> as it accumulated the current pss everytime a locked VMA was found.
> 
> Fix that by making sure we record the current pss value before each VMA
> is walked. So, we can only add the delta if the VMA was found to be
> VM_LOCKED.
> 
> ...
>
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -709,6 +709,7 @@ static void smap_gather_stats(struct vm_area_struct *vma,
>  #endif
>  		.mm = vma->vm_mm,
>  	};
> +	unsigned long pss;
>  
>  	smaps_walk.private = mss;
>  
> @@ -737,11 +738,12 @@ static void smap_gather_stats(struct vm_area_struct *vma,
>  		}
>  	}
>  #endif
> -
> +	/* record current pss so we can calculate the delta after page walk */
> +	pss = mss->pss;
>  	/* mmap_sem is held in m_start */
>  	walk_page_vma(vma, &smaps_walk);
>  	if (vma->vm_flags & VM_LOCKED)
> -		mss->pss_locked += mss->pss;
> +		mss->pss_locked += mss->pss - pss;
>  }

This seems to be a rather obscure way of accumulating
mem_size_stats.pss_locked.  Wouldn't it make more sense to do this in
smaps_account(), wherever we increment mem_size_stats.pss?

It would be a tiny bit less efficient but I think that the code cleanup
justifies such a cost?