cpusets: update mems allowed in page allocator

If all allowable memory is unreclaimable, it is possible to loop forever
in the page allocator for ~__GFP_NORETRY allocations.

During this time, it is also possible for a task's cpuset to expand its
set of allowable nodes so that it now includes free memory. The cached
copy of this set, current->mems_allowed, is stale, however, since there
has not been a subsequent call to cpuset_update_task_memory_state().

The cached copy of the set of allowable nodes is now updated in the page
allocator's slow path so the additional memory is available to
get_page_from_freelist().

[[e-mail]: add comment]
Signed-off-by: David Rientjes <[e-mail]>
Cc: Paul Menage <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/page_alloc.c

···
 
 	/* We now go into synchronous reclaim */
 	cpuset_memory_pressure_bump();
+	/*
+	 * The task's cpuset might have expanded its set of allowable nodes
+	 */
+	cpuset_update_task_memory_state();
 	p->flags |= PF_MEMALLOC;
 	reclaim_state.reclaimed_slab = 0;
 	p->reclaim_state = &reclaim_state;