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

[Neil Brown <neilb@suse.de>]

On Thursday October 1, rientjes@google.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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