[patch v4] mm, thp: only collapse hugepages to nodes with affinity for zone_reclaim_mode

[David Rientjes <rientjes@google.com>]

Commit 9f1b868a13ac ("mm: thp: khugepaged: add policy for finding target 
node") improved the previous khugepaged logic which allocated a 
transparent hugepages from the node of the first page being collapsed.

However, it is still possible to collapse pages to remote memory which may 
suffer from additional access latency.  With the current policy, it is 
possible that 255 pages (with PAGE_SHIFT == 12) will be collapsed remotely 
if the majority are allocated from that node.

When zone_reclaim_mode is enabled, it means the VM should make every attempt
to allocate locally to prevent NUMA performance degradation.  In this case,
we do not want to collapse hugepages to remote nodes that would suffer from
increased access latency.  Thus, when zone_reclaim_mode is enabled, only
allow collapsing to nodes with RECLAIM_DISTANCE or less.

There is no functional change for systems that disable zone_reclaim_mode.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 v2: only change behavior for zone_reclaim_mode per Dave Hansen
 v3: optimization based on previous node counts per Vlastimil Babka
 v4: unconditionally define khugepaged_scan_abort per Dave Hansen
     no increase in .text size for CONFIG_NUMA=n

 mm/huge_memory.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2233,6 +2233,30 @@ static void khugepaged_alloc_sleep(void)
 
 static int khugepaged_node_load[MAX_NUMNODES];
 
+static bool khugepaged_scan_abort(int nid)
+{
+	int i;
+
+	/*
+	 * If zone_reclaim_mode is disabled, then no extra effort is made to
+	 * allocate memory locally.
+	 */
+	if (!zone_reclaim_mode)
+		return false;
+
+	/* If there is a count for this node already, it must be acceptable */
+	if (khugepaged_node_load[nid])
+		return false;
+
+	for (i = 0; i < MAX_NUMNODES; i++) {
+		if (!khugepaged_node_load[i])
+			continue;
+		if (node_distance(nid, i) > RECLAIM_DISTANCE)
+			return true;
+	}
+	return false;
+}
+
 #ifdef CONFIG_NUMA
 static int khugepaged_find_target_node(void)
 {
@@ -2545,6 +2569,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 		 * hit record.
 		 */
 		node = page_to_nid(page);
+		if (khugepaged_scan_abort(node))
+			goto out_unmap;
 		khugepaged_node_load[node]++;
 		VM_BUG_ON_PAGE(PageCompound(page), page);
 		if (!PageLRU(page) || PageLocked(page) || !PageAnon(page))

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