[PATCH v2 4/5] mm/page_alloc: place pages to tail in __free_pages_core()

[David Hildenbrand <david@xxxxxxxxxx>]

__free_pages_core() is used when exposing fresh memory to the buddy
during system boot and when onlining memory in generic_online_page().

generic_online_page() is used in two cases:

1. Direct memory onlining in online_pages().
2. Deferred memory onlining in memory-ballooning-like mechanisms (HyperV
   balloon and virtio-mem), when parts of a section are kept
   fake-offline to be fake-onlined later on.

In 1, we already place pages to the tail of the freelist. Pages will be
freed to MIGRATE_ISOLATE lists first and moved to the tail of the freelists
via undo_isolate_page_range().

In 2, we currently don't implement a proper rule. In case of virtio-mem,
where we currently always online MAX_ORDER - 1 pages, the pages will be
placed to the HEAD of the freelist - undesireable. While the hyper-v
balloon calls generic_online_page() with single pages, usually it will
call it on successive single pages in a larger block.

The pages are fresh, so place them to the tail of the freelist and avoid
the PCP. In __free_pages_core(), remove the now superflouos call to
set_page_refcounted() and add a comment regarding page initialization and
the refcount.

Note: In 2. we currently don't shuffle. If ever relevant (page shuffling
is usually of limited use in virtualized environments), we might want to
shuffle after a sequence of generic_online_page() calls in the
relevant callers.

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
Acked-by: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>
Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
Cc: Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>
Cc: Wei Liu <wei.liu@xxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 mm/page_alloc.c | 33 +++++++++++++++++++++++----------
 1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b187e46cf640..3dadcc6d4009 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -275,7 +275,8 @@ bool pm_suspended_storage(void)
 unsigned int pageblock_order __read_mostly;
 #endif
 
-static void __free_pages_ok(struct page *page, unsigned int order);
+static void __free_pages_ok(struct page *page, unsigned int order,
+                           fpi_t fpi_flags);
 
 /*
  * results with 256, 32 in the lowmem_reserve sysctl:
@@ -687,7 +688,7 @@ static void bad_page(struct page *page, const char *reason)
 void free_compound_page(struct page *page)
 {
        mem_cgroup_uncharge(page);
-       __free_pages_ok(page, compound_order(page));
+       __free_pages_ok(page, compound_order(page), FPI_NONE);
 }
 
 void prep_compound_page(struct page *page, unsigned int order)
@@ -1423,14 +1424,14 @@ static void free_pcppages_bulk(struct zone *zone, int 
count,
 static void free_one_page(struct zone *zone,
                                struct page *page, unsigned long pfn,
                                unsigned int order,
-                               int migratetype)
+                               int migratetype, fpi_t fpi_flags)
 {
        spin_lock(&zone->lock);
        if (unlikely(has_isolate_pageblock(zone) ||
                is_migrate_isolate(migratetype))) {
                migratetype = get_pfnblock_migratetype(page, pfn);
        }
-       __free_one_page(page, pfn, zone, order, migratetype, FPI_NONE);
+       __free_one_page(page, pfn, zone, order, migratetype, fpi_flags);
        spin_unlock(&zone->lock);
 }
 
@@ -1508,7 +1509,8 @@ void __meminit reserve_bootmem_region(phys_addr_t start, 
phys_addr_t end)
        }
 }
 
-static void __free_pages_ok(struct page *page, unsigned int order)
+static void __free_pages_ok(struct page *page, unsigned int order,
+                           fpi_t fpi_flags)
 {
        unsigned long flags;
        int migratetype;
@@ -1520,7 +1522,8 @@ static void __free_pages_ok(struct page *page, unsigned 
int order)
        migratetype = get_pfnblock_migratetype(page, pfn);
        local_irq_save(flags);
        __count_vm_events(PGFREE, 1 << order);
-       free_one_page(page_zone(page), page, pfn, order, migratetype);
+       free_one_page(page_zone(page), page, pfn, order, migratetype,
+                     fpi_flags);
        local_irq_restore(flags);
 }
 
@@ -1530,6 +1533,11 @@ void __free_pages_core(struct page *page, unsigned int 
order)
        struct page *p = page;
        unsigned int loop;
 
+       /*
+        * When initializing the memmap, __init_single_page() sets the refcount
+        * of all pages to 1 ("allocated"/"not free"). We have to set the
+        * refcount of all involved pages to 0.
+        */
        prefetchw(p);
        for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
                prefetchw(p + 1);
@@ -1540,8 +1548,12 @@ void __free_pages_core(struct page *page, unsigned int 
order)
        set_page_count(p, 0);
 
        atomic_long_add(nr_pages, &page_zone(page)->managed_pages);
-       set_page_refcounted(page);
-       __free_pages(page, order);
+
+       /*
+        * Bypass PCP and place fresh pages right to the tail, primarily
+        * relevant for memory onlining.
+        */
+       __free_pages_ok(page, order, FPI_TO_TAIL);
 }
 
 #ifdef CONFIG_NEED_MULTIPLE_NODES
@@ -3168,7 +3180,8 @@ static void free_unref_page_commit(struct page *page, 
unsigned long pfn)
         */
        if (migratetype >= MIGRATE_PCPTYPES) {
                if (unlikely(is_migrate_isolate(migratetype))) {
-                       free_one_page(zone, page, pfn, 0, migratetype);
+                       free_one_page(zone, page, pfn, 0, migratetype,
+                                     FPI_NONE);
                        return;
                }
                migratetype = MIGRATE_MOVABLE;
@@ -4991,7 +5004,7 @@ static inline void free_the_page(struct page *page, 
unsigned int order)
        if (order == 0)         /* Via pcp? */
                free_unref_page(page);
        else
-               __free_pages_ok(page, order);
+               __free_pages_ok(page, order, FPI_NONE);
 }
 
 void __free_pages(struct page *page, unsigned int order)
-- 
2.26.2