Re: [patch 3/5] mm: try to distribute dirty pages fairly across zones

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Nov 24, 2011 at 10:07:55AM +0900, KAMEZAWA Hiroyuki wrote:
> 
> 
> Can I make a question ?
> 
> On Wed, 23 Nov 2011 14:34:16 +0100
> Johannes Weiner <hannes@cmpxchg.org> wrote:
> 
> 
> > +		/*
> > +		 * When allocating a page cache page for writing, we
> > +		 * want to get it from a zone that is within its dirty
> > +		 * limit, such that no single zone holds more than its
> > +		 * proportional share of globally allowed dirty pages.
> > +		 * The dirty limits take into account the zone's
> > +		 * lowmem reserves and high watermark so that kswapd
> > +		 * should be able to balance it without having to
> > +		 * write pages from its LRU list.
> > +		 *
> > +		 * This may look like it could increase pressure on
> > +		 * lower zones by failing allocations in higher zones
> > +		 * before they are full.  But the pages that do spill
> > +		 * over are limited as the lower zones are protected
> > +		 * by this very same mechanism.  It should not become
> > +		 * a practical burden to them.
> > +		 *
> > +		 * XXX: For now, allow allocations to potentially
> > +		 * exceed the per-zone dirty limit in the slowpath
> > +		 * (ALLOC_WMARK_LOW unset) before going into reclaim,
> > +		 * which is important when on a NUMA setup the allowed
> > +		 * zones are together not big enough to reach the
> > +		 * global limit.  The proper fix for these situations
> > +		 * will require awareness of zones in the
> > +		 * dirty-throttling and the flusher threads.
> > +		 */
> > +		if ((alloc_flags & ALLOC_WMARK_LOW) &&
> > +		    (gfp_mask & __GFP_WRITE) && !zone_dirty_ok(zone))
> > +			goto this_zone_full;
> >  
> >  		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
> >  		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
> 
> This wil call 
> 
>                 if (NUMA_BUILD)
>                         zlc_mark_zone_full(zonelist, z);
> 
> And this zone will be marked as full. 
> 
> IIUC, zlc_clear_zones_full() is called only when direct reclaim ends.
> So, if no one calls direct-reclaim, 'full' mark may never be cleared
> even when number of dirty pages goes down to safe level ?
> I'm sorry if this is alread discussed.

It does not remember which zones are marked full for longer than a
second - see zlc_setup() - and also ignores this information when an
iteration over the zonelist with the cache enabled came up
empty-handed.

I thought it would make sense to take advantage of the cache and save
the zone_dirty_ok() checks against ineligible zones too on subsequent
iterations.

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