memcg: simple migration handling

Now, management of "charge" under page migration is done under following
manner. (Assume migrate page contents from oldpage to newpage)

 before
  - "newpage" is charged before migration.
 at success.
  - "oldpage" is uncharged at somewhere(unmap, radix-tree-replace)
 at failure
  - "newpage" is uncharged.
  - "oldpage" is charged if necessary (*1)

But (*1) is not reliable....because of GFP_ATOMIC.

This patch tries to change behavior as following by charge/commit/cancel ops.

 before
  - charge PAGE_SIZE (no target page)
 success
  - commit charge against "newpage".
 failure
  - commit charge against "oldpage".
    (PCG_USED bit works effectively to avoid double-counting)
  - if "oldpage" is obsolete, cancel charge of PAGE_SIZE.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 28 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 246dcb973ae7..a30ea5fcf9f1 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -121,20 +121,6 @@ static void remove_migration_pte(struct vm_area_struct *vma,
         if (!is_migration_entry(entry) || migration_entry_to_page(entry) != old)
                 goto out;
 
-        /*
-         * Yes, ignore the return value from a GFP_ATOMIC mem_cgroup_charge.
-         * Failure is not an option here: we're now expected to remove every
-         * migration pte, and will cause crashes otherwise.  Normally this
-         * is not an issue: mem_cgroup_prepare_migration bumped up the old
-         * page_cgroup count for safety, that's now attached to the new page,
-         * so this charge should just be another incrementation of the count,
-         * to keep in balance with rmap.c's mem_cgroup_uncharging.  But if
-         * there's been a force_empty, those reference counts may no longer
-         * be reliable, and this charge can actually fail: oh well, we don't
-         * make the situation any worse by proceeding as if it had succeeded.
-         */
-        mem_cgroup_charge_migrate_fixup(new, mm, GFP_ATOMIC);
-
         get_page(new);
         pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
         if (is_write_migration_entry(entry))
@@ -378,9 +364,6 @@ static void migrate_page_copy(struct page *newpage, struct page *page)
         anon = PageAnon(page);
         page->mapping = NULL;
 
-        if (!anon) /* This page was removed from radix-tree. */
-                mem_cgroup_uncharge_cache_page(page);
-
         /*
          * If any waiters have accumulated on the new page then
          * wake them up.
@@ -614,6 +597,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
         struct page *newpage = get_new_page(page, private, &result);
         int rcu_locked = 0;
         int charge = 0;
+        struct mem_cgroup *mem;
 
         if (!newpage)
                 return -ENOMEM;
@@ -623,24 +607,26 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
                 goto move_newpage;
         }
 
-        charge = mem_cgroup_prepare_migration(page, newpage);
-        if (charge == -ENOMEM) {
-                rc = -ENOMEM;
-                goto move_newpage;
-        }
         /* prepare cgroup just returns 0 or -ENOMEM */
-        BUG_ON(charge);
-
         rc = -EAGAIN;
+
         if (!trylock_page(page)) {
                 if (!force)
                         goto move_newpage;
                 lock_page(page);
         }
 
+        /* charge against new page */
+        charge = mem_cgroup_prepare_migration(page, &mem);
+        if (charge == -ENOMEM) {
+                rc = -ENOMEM;
+                goto unlock;
+        }
+        BUG_ON(charge);
+
         if (PageWriteback(page)) {
                 if (!force)
-                        goto unlock;
+                        goto uncharge;
                 wait_on_page_writeback(page);
         }
         /*
@@ -693,7 +679,9 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 rcu_unlock:
         if (rcu_locked)
                 rcu_read_unlock();
-
+uncharge:
+        if (!charge)
+                mem_cgroup_end_migration(mem, page, newpage);
 unlock:
         unlock_page(page);
 
@@ -709,8 +697,6 @@ unlock:
         }
 
 move_newpage:
-        if (!charge)
-                mem_cgroup_end_migration(newpage);
 
         /*
          * Move the new page to the LRU. If migration was not successful