mm, mempolicy: clean up __GFP_THISNODE confusion in policy_zonelist

__GFP_THISNODE is documented to enforce the allocation to be satisified
from the requested node with no fallbacks or placement policy
enforcements.  policy_zonelist seemingly breaks this semantic if the
current policy is MPOL_MBIND and instead of taking the node it will
fallback to the first node in the mask if the requested one is not in
the mask.  This is confusing to say the least because it fact we
shouldn't ever go that path.  First tasks shouldn't be scheduled on CPUs
with nodes outside of their mempolicy binding.  And secondly
policy_zonelist is called only from 3 places:

 - huge_zonelist - never should do __GFP_THISNODE when going this path

 - alloc_pages_vma - which shouldn't depend on __GFP_THISNODE either

 - alloc_pages_current - which uses default_policy id __GFP_THISNODE is
   used

So we shouldn't even need to care about this possibility and can drop
the confusing code.  Let's keep a WARN_ON_ONCE in place to catch
potential users and fix them up properly (aka use a different allocation
function which ignores mempolicy).

[akpm@linux-foundation.org: coding-style fixes]
Link: http://lkml.kernel.org/r/20161013125958.32155-1-mhocko@kernel.orgSigned-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@suse.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -1679,25 +1679,17 @@ static nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
 static struct zonelist *policy_zonelist(gfp_t gfp, struct mempolicy *policy,
 	int nd)
 {
-	switch (policy->mode) {
-	case MPOL_PREFERRED:
-		if (!(policy->flags & MPOL_F_LOCAL))
+	if (policy->mode == MPOL_PREFERRED && !(policy->flags & MPOL_F_LOCAL))
 		nd = policy->v.preferred_node;
-		break;
-	case MPOL_BIND:
+	else {
 		/*
-		 * Normally, MPOL_BIND allocations are node-local within the
-		 * allowed nodemask.  However, if __GFP_THISNODE is set and the
-		 * current node isn't part of the mask, we use the zonelist for
-		 * the first node in the mask instead.
-		 */
-		if (unlikely(gfp & __GFP_THISNODE) &&
-				unlikely(!node_isset(nd, policy->v.nodes)))
-			nd = first_node(policy->v.nodes);
-		break;
-	default:
-		BUG();
+		 * __GFP_THISNODE shouldn't even be used with the bind policy
+		 * because we might easily break the expectation to stay on the
+		 * requested node and not break the policy.
+		 */
+		WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE));
 	}
+
 	return node_zonelist(nd, gfp);
 }