memcg: make mem_cgroup_split_huge_fixup() more efficient

In split_huge_page(), mem_cgroup_split_huge_fixup() is called to handle
page_cgroup modifcations.  It takes move_lock_page_cgroup() and modifies
page_cgroup and LRU accounting jobs and called HPAGE_PMD_SIZE - 1 times.

But thinking again,
  - compound_lock() is held at move_accout...then, it's not necessary
    to take move_lock_page_cgroup().
  - LRU is locked and all tail pages will go into the same LRU as
    head is now on.
  - page_cgroup is contiguous in huge page range.

This patch fixes mem_cgroup_split_huge_fixup() as to be called once per
hugepage and reduce costs for spliting.

[akpm@linux-foundation.org: fix typo, per Michal]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 17 insertions, 17 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 972878b648c2..42174612cc0b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2553,39 +2553,39 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
                         (1 << PCG_ACCT_LRU) | (1 << PCG_MIGRATION))
 /*
  * Because tail pages are not marked as "used", set it. We're under
- * zone->lru_lock, 'splitting on pmd' and compund_lock.
+ * zone->lru_lock, 'splitting on pmd' and compound_lock.
+ * charge/uncharge will be never happen and move_account() is done under
+ * compound_lock(), so we don't have to take care of races.
  */
-void mem_cgroup_split_huge_fixup(struct page *head, struct page *tail)
+void mem_cgroup_split_huge_fixup(struct page *head)
 {
         struct page_cgroup *head_pc = lookup_page_cgroup(head);
-        struct page_cgroup *tail_pc = lookup_page_cgroup(tail);
-        unsigned long flags;
+        struct page_cgroup *pc;
+        int i;
 
         if (mem_cgroup_disabled())
                 return;
-        /*
-         * We have no races with charge/uncharge but will have races with
-         * page state accounting.
-         */
-        move_lock_page_cgroup(head_pc, &flags);
+        for (i = 1; i < HPAGE_PMD_NR; i++) {
+                pc = head_pc + i;
+                pc->mem_cgroup = head_pc->mem_cgroup;
+                smp_wmb();/* see __commit_charge() */
+                /*
+                 * LRU flags cannot be copied because we need to add tail
+                 * page to LRU by generic call and our hooks will be called.
+                 */
+                pc->flags = head_pc->flags & ~PCGF_NOCOPY_AT_SPLIT;
+        }
 
-        tail_pc->mem_cgroup = head_pc->mem_cgroup;
-        smp_wmb(); /* see __commit_charge() */
         if (PageCgroupAcctLRU(head_pc)) {
                 enum lru_list lru;
                 struct mem_cgroup_per_zone *mz;
-
                 /*
-                 * LRU flags cannot be copied because we need to add tail
-                 *.page to LRU by generic call and our hook will be called.
                  * We hold lru_lock, then, reduce counter directly.
                  */
                 lru = page_lru(head);
                 mz = page_cgroup_zoneinfo(head_pc->mem_cgroup, head);
-                MEM_CGROUP_ZSTAT(mz, lru) -= 1;
+                MEM_CGROUP_ZSTAT(mz, lru) -= HPAGE_PMD_NR - 1;
         }
-        tail_pc->flags = head_pc->flags & ~PCGF_NOCOPY_AT_SPLIT;
-        move_unlock_page_cgroup(head_pc, &flags);
 }
 #endif