On Thursday October 1, rientjes@google.com wrote:

On Thu, 1 Oct 2009, Suresh Jayaraman wrote:

quoted

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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