linux-kernel - Re: [PATCH -mmotm-2009-12-10-17-19] Fix wrong rss count of smaps

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Date:	Tue, 29 Dec 2009 14:08:59 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>
Subject: Re: [PATCH -mmotm-2009-12-10-17-19] Fix wrong rss count of smaps

On Mon, 2009-12-28 at 13:46 +0900, Minchan Kim wrote:
> I am not sure we have to account zero page with file_rss. 
> Hugh and Kame's new zero page doesn't do it. 
> As side effect of this, we can prevent innocent process which have a lot
> of zero page when OOM happens. 
> (But I am not sure there is a process like this :)
> So I think not file_rss counting is not bad. 
> 
> RSS counting zero page with file_rss helps any program using smaps?
> If we have to keep the old behavior, I have to remake this patch. 
> 
> == CUT_HERE ==
> 
> Long time ago, We regards zero page as file_rss and
> vm_normal_page doesn't return NULL.
> 
> But now, we reinstated ZERO_PAGE and vm_normal_page's implementation
> can return NULL in case of zero page. Also we don't count it with
> file_rss any more.
> 
> Then, RSS and PSS can't be matched.
> For consistency, Let's ignore zero page in smaps_pte_range.
> 

Not counting the zero page in RSS is fine with me. But will this patch
make the total from smaps agree with get_mm_rss()?

Regarding OOM handling: arguably RSS should play no role in OOM as it's
practically meaningless in a shared memory system. If we were instead
used per-process unshared pages as the metric (aka USS), we'd have a
much better notion of how much memory an OOM kill would recover.
Unfortunately, that's not trivial to track as the accounting on COW
operations is not lightweight.

> CC: Matt Mackall <mpm@...enic.com>
> Signed-off-by: Minchan Kim <minchan.kim@...il.com>
> ---
>  fs/proc/task_mmu.c |    3 +--
>  1 files changed, 1 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 47c03f4..f277c4a 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -361,12 +361,11 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>  		if (!pte_present(ptent))
>  			continue;
>  
> -		mss->resident += PAGE_SIZE;
> -
>  		page = vm_normal_page(vma, addr, ptent);
>  		if (!page)
>  			continue;
>  
> +		mss->resident += PAGE_SIZE;
>  		/* Accumulate the size in pages that have been accessed. */
>  		if (pte_young(ptent) || PageReferenced(page))
>  			mss->referenced += PAGE_SIZE;
> -- 
> 1.5.6.3
> 
> 
> 



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