linux-kernel - Re: [PATCH for mmotm 2/5]

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Date:	Thu, 11 Jun 2009 12:13:42 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH for mmotm 2/5]

On Thu, Jun 11, 2009 at 07:26:48PM +0900, KOSAKI Motohiro wrote:
> Changes since Wu's original patch
>   - adding vmstat
>   - rename NR_TMPFS_MAPPED to NR_SWAP_BACKED_FILE_MAPPED
> 
> 
> ----------------------
> Subject: [PATCH] introduce NR_SWAP_BACKED_FILE_MAPPED zone stat

This got lost in the actual subject line.

> Desirable zone reclaim implementaion want to know the number of
> file-backed and unmapped pages.
> 

There needs to be more justification for this. We need an example
failure case that this addresses. For example, Patch 1 of my series was
to address the following problem included with the patchset leader

"The reported problem was that malloc() stalled for a long time (minutes
in some cases) if a large tmpfs mount was occupying a large percentage of
memory overall. The pages did not get cleaned or reclaimed by zone_reclaim()
because the zone_reclaim_mode was unsuitable, but the lists are uselessly
scanned frequencly making the CPU spin at near 100%."

We should have a similar case.

What "desirable" zone_reclaim() should be spelled out as well. Minimally
something like

"For zone_reclaim() to be efficient, it must be able to detect in advance
if the LRU scan will reclaim the necessary pages with the limitations of
the current zone_reclaim_mode. Otherwise, the CPU usage is increases as
zone_reclaim() uselessly scans the LRU list.

The problem with the heuristic is ....

This patch fixes the heuristic by ...."

etc?

I'm not trying to be awkward. I believe I provided similar reasoning
with my own patchset.

> Thus, we need to know number of swap-backed mapped pages for
> calculate above number.
> 
> 
> Cc: Mel Gorman <mel@....ul.ie>
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> ---
>  include/linux/mmzone.h |    2 ++
>  mm/rmap.c              |    7 +++++++
>  mm/vmstat.c            |    1 +
>  3 files changed, 10 insertions(+)
> 
> Index: b/include/linux/mmzone.h
> ===================================================================
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -88,6 +88,8 @@ enum zone_stat_item {
>  	NR_ANON_PAGES,	/* Mapped anonymous pages */
>  	NR_FILE_MAPPED,	/* pagecache pages mapped into pagetables.
>  			   only modified from process context */
> +	NR_SWAP_BACKED_FILE_MAPPED, /* Similar to NR_FILE_MAPPED. but
> +				       only account swap-backed pages */
>  	NR_FILE_PAGES,
>  	NR_FILE_DIRTY,
>  	NR_WRITEBACK,
> Index: b/mm/rmap.c
> ===================================================================
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -829,6 +829,10 @@ void page_add_file_rmap(struct page *pag
>  {
>  	if (atomic_inc_and_test(&page->_mapcount)) {
>  		__inc_zone_page_state(page, NR_FILE_MAPPED);
> +		if (PageSwapBacked(page))
> +			__inc_zone_page_state(page,
> +					      NR_SWAP_BACKED_FILE_MAPPED);
> +
>  		mem_cgroup_update_mapped_file_stat(page, 1);
>  	}
>  }
> @@ -884,6 +888,9 @@ void page_remove_rmap(struct page *page)
>  		__dec_zone_page_state(page, NR_ANON_PAGES);
>  	} else {
>  		__dec_zone_page_state(page, NR_FILE_MAPPED);
> +		if (PageSwapBacked(page))
> +			__dec_zone_page_state(page,
> +					NR_SWAP_BACKED_FILE_MAPPED);
>  	}
>  	mem_cgroup_update_mapped_file_stat(page, -1);
>  	/*
> Index: b/mm/vmstat.c
> ===================================================================
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -633,6 +633,7 @@ static const char * const vmstat_text[] 
>  	"nr_mlock",
>  	"nr_anon_pages",
>  	"nr_mapped",
> +	"nr_swap_backed_file_mapped",
>  	"nr_file_pages",
>  	"nr_dirty",
>  	"nr_writeback",
> 

Otherwise the patch seems reasonable.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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