netdev - Re: [PATCH 04/31] mm: tag reseve pages

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Date:	Fri, 2 Oct 2009 14:43:34 +1000
From:	Neil Brown <neilb@...e.de>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Suresh Jayaraman <sjayaraman@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	netdev@...r.kernel.org, Miklos Szeredi <mszeredi@...e.cz>,
	Wouter Verhelst <w@...r.be>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	trond.myklebust@....uio.no
Subject: Re: [PATCH 04/31] mm: tag reseve pages

On Thursday October 1, rientjes@...gle.com wrote:
> On Thu, 1 Oct 2009, Suresh Jayaraman wrote:
> 
> > Index: mmotm/mm/page_alloc.c
> > ===================================================================
> > --- mmotm.orig/mm/page_alloc.c
> > +++ mmotm/mm/page_alloc.c
> > @@ -1501,8 +1501,10 @@ zonelist_scan:
> >  try_this_zone:
> >  		page = buffered_rmqueue(preferred_zone, zone, order,
> >  						gfp_mask, migratetype);
> > -		if (page)
> > +		if (page) {
> > +			page->reserve = !!(alloc_flags & ALLOC_NO_WATERMARKS);
> >  			break;
> > +		}
> >  this_zone_full:
> >  		if (NUMA_BUILD)
> >  			zlc_mark_zone_full(zonelist, z);
> 
> page->reserve won't necessary indicate that access to reserves was 
> _necessary_ for the allocation to succeed, though.  This will mark any 
> page being allocated under PF_MEMALLOC as reserve when all zones may be 
> well above their min watermarks.

Normally if zones are above their watermarks, page->reserve will not
be set.
This is because __alloc_page_nodemask (which seems to be the main
non-inline entrypoint) first calls get_page_from_freelist with
alloc_flags set to ALLOC_WMARK_LOW|ALLOC_CPUSET.
Only if this fails does __alloc_page_nodemask call
__alloc_pages_slowpath which potentially sets ALLOC_NO_WATERMARKS in
alloc_flags.

So page->reserved being set actually tells us:
  PF_MEMALLOC or GFP_MEMALLOC were used, and
  a WMARK_LOW allocation attempt failed very recently

which is close enough to "the emergency reserves were used" I think.

Thanks,
NeilBrown
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