mem-hotplug: use GFP_HIGHUSER_MOVABLE and alloc from next node in alloc_migrate_target()

alloc_migrate_target() is called from migrate_pages(), and the page is always from user space, so we can add __GFP_HIGHMEM directly.

Second, when we offline a node, the new page should alloced from other nodes instead of the current node, because re-migrate is a waste of time.

Signed-off-by Xishi Qiu <[email protected]>

---
 mm/page_isolation.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

mm/page_isolation.c +3 -3

index 612122b..83848dc 100644
@@ -282,20 +282,16 @@ int test_pages_isolated(unsigned long start_pfn, unsigned long end_pfn,
 struct page *alloc_migrate_target(struct page *page, unsigned long private,
 				  int **resultp)
 {
-	gfp_t gfp_mask = GFP_USER | __GFP_MOVABLE;
-
 	/*
-	 * TODO: allocate a destination hugepage from a nearest neighbor node,
+	 * TODO: allocate a destination page from a nearest neighbor node,
 	 * accordance with memory policy of the user process if possible. For
 	 * now as a simple work-around, we use the next node for destination.
 	 */
+	int nid = next_node_in(page_to_nid(page), node_online_map);
+
 	if (PageHuge(page))
 		return alloc_huge_page_node(page_hstate(compound_head(page)),

Quoted 7 lines · Xishi Qiu

alloc_migrate_target() is called from migrate_pages(), and the page is always from user space, so we can add __GFP_HIGHMEM directly.

Second, when we offline a node, the new page should alloced from other nodes instead of the current node, because re-migrate is a waste of time.

alloc_migrate_target() is not only used from memory hotplug, it is also used for CMA: we won't be isolating PageHuge() pages in isolate_migratepages_range(), so this would cause a regression where we'd be migrating memory to a remote NUMA node rather than preferring to allocate locally.

You may find it useful to use the 'private' field of the migrate_pages() callback to specify the node the page should preferably be migrated to.

Quoted 25 lines

Signed-off-by Xishi Qiu <[email protected]>

---
 mm/page_isolation.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

mm/page_isolation.c +3 -3

index 612122b..83848dc 100644
@@ -282,20 +282,16 @@ int test_pages_isolated(unsigned long start_pfn, unsigned long end_pfn,
 struct page *alloc_migrate_target(struct page *page, unsigned long private,
 				  int **resultp)
 {
-	gfp_t gfp_mask = GFP_USER | __GFP_MOVABLE;
-
 	/*
-	 * TODO: allocate a destination hugepage from a nearest neighbor node,
+	 * TODO: allocate a destination page from a nearest neighbor node,
 	 * accordance with memory policy of the user process if possible. For
 	 * now as a simple work-around, we use the next node for destination.
 	 */
+	int nid = next_node_in(page_to_nid(page), node_online_map);
+
 	if (PageHuge(page))
 		return alloc_huge_page_node(page_hstate(compound_head(page)),

mm/page_isolation.c int test_pages_isolated(unsigned long start_pfn, unsigned long end_pfn, · line 296

Quoted 10 lines

mm/page_isolation.c +3 -7

-					    next_node_in(page_to_nid(page),
-							 node_online_map));
-
-	if (PageHighMem(page))
-		gfp_mask |= __GFP_HIGHMEM;
-
-	return alloc_page(gfp_mask);
+						 nid);
+	else
+		return __alloc_pages_node(nid, GFP_HIGHUSER_MOVABLE, 0);

I don't think this __alloc_pages_node() does what you think it does, it only prefers nid here and will readily fallback to other nodes if necessary. That is different than alloc_huge_page_node() which does no fallback. So there's two issues with this change: (1) inconsistency between PageHuge() and !PageHuge() behavior, and (2) the use of __alloc_pages_node() does not match the commit description which states "re-migrate is a waste of time."

.