linux-kernel - Re: [PATCH] mm: proc: smaps_rollup: Fix pss

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Date:   Mon, 28 Jan 2019 16:15:09 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Sandeep Patil <sspatil@...roid.com>
Cc:     vbabka@...e.cz, adobriyan@...il.com, avagin@...nvz.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        kernel-team@...roid.com, dancol@...gle.com
Subject: Re: [PATCH] mm: proc: smaps_rollup: Fix pss_locked calculation

On Sun, 20 Jan 2019 17:10:49 -0800 Sandeep Patil <sspatil@...roid.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?