Re: MPOL_BIND on memory only nodes

[Michal Hocko]

On Wed 12-10-16 11:43:37, Michal Hocko wrote:
> On Wed 12-10-16 14:55:24, Anshuman Khandual wrote:
[...]
> > Why we insist on __GFP_THISNODE ?
> 
> AFAIU __GFP_THISNODE just overrides the given node to the policy
> nodemask in case the current node is not part of that node mask. In
> other words we are ignoring the given node and use what the policy says. 
> I can see how this can be confusing especially when confronting the
> documentation:
> 
>  * __GFP_THISNODE forces the allocation to be satisified from the requested
>  *   node with no fallbacks or placement policy enforcements.

You made me think and look into this deeper. I came to the conclusion
that this is actually a relict from the past. 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 AFAICS this is essentially a dead code or I am missing something. Mel
do you remember why we needed it in the past? I would be really tempted
to just get rid of this confusing code and this instead:
---
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index ad1c96ac313c..98beec47bba9 100644
--- a/mm/mempolicy.c
+++ b/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))
-			nd = policy->v.preferred_node;
-		break;
-	case MPOL_BIND:
+	if (policy->mode == MPOL_PREFERRED && !(policy->flags & MPOL_F_LOCAL))
+		nd = policy->v.preferred_node;
+	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.
+		 * __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.
 		 */
-		if (unlikely(gfp & __GFP_THISNODE) &&
-				unlikely(!node_isset(nd, policy->v.nodes)))
-			nd = first_node(policy->v.nodes);
-		break;
-	default:
-		BUG();
+		WARN_ON_ONCE(polic->mode == MPOL_BIND && (gfp && __GFP_THISNODE));
 	}
+
 	return node_zonelist(nd, gfp);
 }
 
-- 
Michal Hocko
SUSE Labs