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centosplus patch bug#15115
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commit 6d8409580bee356ce418dcb94260b24dda639934
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Author: Michal Hocko <mhocko@suse.com>
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Date:   Mon Dec 12 16:42:23 2016 -0800
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    mm, mempolicy: clean up __GFP_THISNODE confusion in policy_zonelist
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    __GFP_THISNODE is documented to enforce the allocation to be satisified
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    from the requested node with no fallbacks or placement policy
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    enforcements.  policy_zonelist seemingly breaks this semantic if the
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    current policy is MPOL_MBIND and instead of taking the node it will
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    fallback to the first node in the mask if the requested one is not in
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    the mask.  This is confusing to say the least because it fact we
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    shouldn't ever go that path.  First tasks shouldn't be scheduled on CPUs
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    with nodes outside of their mempolicy binding.  And secondly
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    policy_zonelist is called only from 3 places:
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     - huge_zonelist - never should do __GFP_THISNODE when going this path
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     - alloc_pages_vma - which shouldn't depend on __GFP_THISNODE either
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     - alloc_pages_current - which uses default_policy id __GFP_THISNODE is
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       used
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    So we shouldn't even need to care about this possibility and can drop
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    the confusing code.  Let's keep a WARN_ON_ONCE in place to catch
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    potential users and fix them up properly (aka use a different allocation
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    function which ignores mempolicy).
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    [akpm@linux-foundation.org: coding-style fixes]
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    Link: http://lkml.kernel.org/r/20161013125958.32155-1-mhocko@kernel.org
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    Signed-off-by: Michal Hocko <mhocko@suse.com>
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    Acked-by: Vlastimil Babka <vbabka@suse.cz>
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    Cc: Mel Gorman <mgorman@suse.de>
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    Cc: David Rientjes <rientjes@google.com>
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    Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
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    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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    Applied-by: Akemi Yagi <toracat@centos.org>
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diff --git a/mm/mempolicy.c b/mm/mempolicy.c
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index a6a27e5..4d58021 100644
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--- a/mm/mempolicy.c
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+++ b/mm/mempolicy.c
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@@ -1679,25 +1679,17 @@ static nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
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 static struct zonelist *policy_zonelist(gfp_t gfp, struct mempolicy *policy,
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 	int nd)
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 {
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-	switch (policy->mode) {
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-	case MPOL_PREFERRED:
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-		if (!(policy->flags & MPOL_F_LOCAL))
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-			nd = policy->v.preferred_node;
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-		break;
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-	case MPOL_BIND:
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+	if (policy->mode == MPOL_PREFERRED && !(policy->flags & MPOL_F_LOCAL))
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+		nd = policy->v.preferred_node;
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+	else {
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 		/*
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-		 * Normally, MPOL_BIND allocations are node-local within the
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-		 * allowed nodemask.  However, if __GFP_THISNODE is set and the
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-		 * current node isn't part of the mask, we use the zonelist for
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-		 * the first node in the mask instead.
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+		 * __GFP_THISNODE shouldn't even be used with the bind policy
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+		 * because we might easily break the expectation to stay on the
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+		 * requested node and not break the policy.
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 		 */
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-		if (unlikely(gfp & __GFP_THISNODE) &&
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-				unlikely(!node_isset(nd, policy->v.nodes)))
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-			nd = first_node(policy->v.nodes);
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-		break;
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-	default:
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-		BUG();
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+		WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE));
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 	}
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+
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 	return node_zonelist(nd, gfp);
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 }
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