linux-kernel - [PATCH v2 3/5] mm/page_alloc: move pages to tail in move_to_free

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Message-Id: <20201005121534.15649-4-david@redhat.com>
Date:   Mon,  5 Oct 2020 14:15:32 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, linux-hyperv@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-acpi@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mike Rapoport <rppt@...nel.org>,
        Scott Cheloha <cheloha@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: [PATCH v2 3/5] mm/page_alloc: move pages to tail in move_to_free_list()

Whenever we move pages between freelists via move_to_free_list()/
move_freepages_block(), we don't actually touch the pages:
1. Page isolation doesn't actually touch the pages, it simply isolates
   pageblocks and moves all free pages to the MIGRATE_ISOLATE freelist.
   When undoing isolation, we move the pages back to the target list.
2. Page stealing (steal_suitable_fallback()) moves free pages directly
   between lists without touching them.
3. reserve_highatomic_pageblock()/unreserve_highatomic_pageblock() moves
   free pages directly between freelists without touching them.

We already place pages to the tail of the freelists when undoing isolation
via __putback_isolated_page(), let's do it in any case (e.g., if order <=
pageblock_order) and document the behavior. To simplify, let's move the
pages to the tail for all move_to_free_list()/move_freepages_block() users.

In 2., the target list is empty, so there should be no change. In 3.,
we might observe a change, however, highatomic is more concerned about
allocations succeeding than cache hotness - if we ever realize this
change degrades a workload, we can special-case this instance and add a
proper comment.

This change results in all pages getting onlined via online_pages() to
be placed to the tail of the freelist.

Reviewed-by: Oscar Salvador <osalvador@...e.de>
Acked-by: Pankaj Gupta <pankaj.gupta.linux@...il.com>
Reviewed-by: Wei Yang <richard.weiyang@...ux.alibaba.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc: Mel Gorman <mgorman@...hsingularity.net>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: Wei Yang <richard.weiyang@...ux.alibaba.com>
Cc: Oscar Salvador <osalvador@...e.de>
Cc: Mike Rapoport <rppt@...nel.org>
Cc: Scott Cheloha <cheloha@...ux.ibm.com>
Cc: Michael Ellerman <mpe@...erman.id.au>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/page_alloc.c     | 10 +++++++---
 mm/page_isolation.c |  5 +++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index df5ff0cd6df1..b187e46cf640 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -901,13 +901,17 @@ static inline void add_to_free_list_tail(struct page *page, struct zone *zone,
 	area->nr_free++;
 }
 
-/* Used for pages which are on another list */
+/*
+ * Used for pages which are on another list. Move the pages to the tail
+ * of the list - so the moved pages won't immediately be considered for
+ * allocation again (e.g., optimization for memory onlining).
+ */
 static inline void move_to_free_list(struct page *page, struct zone *zone,
 				     unsigned int order, int migratetype)
 {
 	struct free_area *area = &zone->free_area[order];
 
-	list_move(&page->lru, &area->free_list[migratetype]);
+	list_move_tail(&page->lru, &area->free_list[migratetype]);
 }
 
 static inline void del_page_from_free_list(struct page *page, struct zone *zone,
@@ -2340,7 +2344,7 @@ static inline struct page *__rmqueue_cma_fallback(struct zone *zone,
 #endif
 
 /*
- * Move the free pages in a range to the free lists of the requested type.
+ * Move the free pages in a range to the freelist tail of the requested type.
  * Note that start_page and end_pages are not aligned on a pageblock
  * boundary. If alignment is required, use move_freepages_block()
  */
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index abfe26ad59fd..83692b937784 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -106,6 +106,11 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
 	 * If we isolate freepage with more than pageblock_order, there
 	 * should be no freepage in the range, so we could avoid costly
 	 * pageblock scanning for freepage moving.
+	 *
+	 * We didn't actually touch any of the isolated pages, so place them
+	 * to the tail of the freelist. This is an optimization for memory
+	 * onlining - just onlined memory won't immediately be considered for
+	 * allocation.
 	 */
 	if (!isolated_page) {
 		nr_pages = move_freepages_block(zone, page, migratetype, NULL);
-- 
2.26.2