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

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Date:   Sat, 2 Feb 2019 22:21:14 -0800
From:   Sandeep Patil <sspatil@...roid.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, 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 Tue, Jan 29, 2019 at 04:52:21PM +0100, Vlastimil Babka wrote:
> On 1/29/19 1:15 AM, Andrew Morton wrote:
> > 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?
> 
> Yeah, Sandeep could you add 'bool locked' param to smaps_account() and check it
> there? We probably don't need the whole vma param yet.

Agree, I will send -v2 shortly.

- ssp