Re: [PATCH for mmotm 2/5]

[KOSAKI Motohiro]

> 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.

You are right. my intention is not actual issue, it only fix
documentation lie.

Documentation/sysctl/vm.txt says
=============================================================

min_unmapped_ratio:

This is available only on NUMA kernels.

A percentage of the total pages in each zone.  Zone reclaim will only
occur if more than this percentage of pages are file backed and unmapped.
This is to insure that a minimal amount of local pages is still available for
file I/O even if the node is overallocated.

The default is 1 percent.
==============================================================

but actual code don't account "percentage of file backed and unmapped".
Administrator can't imazine current implementation form this documentation.

Plus, I don't think this patch is too messy. thus I did decide to make
this fix.

if anyone provide good documentation fix, my worry will vanish.



> > Thus, we need to know number of swap-backed mapped pages for
> > calculate above number.
> > 
> > 
> > Cc: Mel Gorman <mel@xxxxxxxxx>
> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> > ---
> >  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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