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  - if "oldpage" is obsolete, cancel charge of PAGE_SIZE.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>

2 files changed, 66 insertions, 84 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c34eb52bdc3..b71195e8198 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -627,34 +627,6 @@ int mem_cgroup_newpage_charge(struct page *page,
                                 MEM_CGROUP_CHARGE_TYPE_MAPPED, NULL);
 }
 
-/*
- * same as mem_cgroup_newpage_charge(), now.
- * But what we assume is different from newpage, and this is special case.
- * treat this in special function. easy for maintenance.
- */
-
-int mem_cgroup_charge_migrate_fixup(struct page *page,
-                                struct mm_struct *mm, gfp_t gfp_mask)
-{
-        if (mem_cgroup_subsys.disabled)
-                return 0;
-
-        if (PageCompound(page))
-                return 0;
-
-        if (page_mapped(page) || (page->mapping && !PageAnon(page)))
-                return 0;
-
-        if (unlikely(!mm))
-                mm = &init_mm;
-
-        return mem_cgroup_charge_common(page, mm, gfp_mask,
-                                MEM_CGROUP_CHARGE_TYPE_MAPPED, NULL);
-}
-
-
-
-
 int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
                                 gfp_t gfp_mask)
 {
@@ -697,7 +669,6 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
                                 MEM_CGROUP_CHARGE_TYPE_SHMEM, NULL);
 }
 
-
 void mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr)
 {
         struct page_cgroup *pc;
@@ -782,13 +753,13 @@ void mem_cgroup_uncharge_cache_page(struct page *page)
 }
 
 /*
- * Before starting migration, account against new page.
+ * Before starting migration, account PAGE_SIZE to mem_cgroup that the old
+ * page belongs to.
  */
-int mem_cgroup_prepare_migration(struct page *page, struct page *newpage)
+int mem_cgroup_prepare_migration(struct page *page, struct mem_cgroup **ptr)
 {
         struct page_cgroup *pc;
         struct mem_cgroup *mem = NULL;
-        enum charge_type ctype = MEM_CGROUP_CHARGE_TYPE_MAPPED;
         int ret = 0;
 
         if (mem_cgroup_subsys.disabled)
@@ -799,42 +770,67 @@ int mem_cgroup_prepare_migration(struct page *page, struct page *newpage)
         if (PageCgroupUsed(pc)) {
                 mem = pc->mem_cgroup;
                 css_get(&mem->css);
-                if (PageCgroupCache(pc)) {
-                        if (page_is_file_cache(page))
-                                ctype = MEM_CGROUP_CHARGE_TYPE_CACHE;
-                        else
-                                ctype = MEM_CGROUP_CHARGE_TYPE_SHMEM;
-                }
         }
         unlock_page_cgroup(pc);
+
         if (mem) {
-                ret = mem_cgroup_charge_common(newpage, NULL,
-                                        GFP_HIGHUSER_MOVABLE,
-                                        ctype, mem);
+                ret = mem_cgroup_try_charge(NULL, GFP_HIGHUSER_MOVABLE, &mem);
                 css_put(&mem->css);
         }
+        *ptr = mem;
         return ret;
 }
 
 /* remove redundant charge if migration failed*/
-void mem_cgroup_end_migration(struct page *newpage)
+void mem_cgroup_end_migration(struct mem_cgroup *mem,
+                struct page *oldpage, struct page *newpage)
 {
+        struct page *target, *unused;
+        struct page_cgroup *pc;
+        enum charge_type ctype;
+
+        if (!mem)
+                return;
+
+        /* at migration success, oldpage->mapping is NULL. */
+        if (oldpage->mapping) {
+                target = oldpage;
+                unused = NULL;
+        } else {
+                target = newpage;
+                unused = oldpage;
+        }
+
+        if (PageAnon(target))
+                ctype = MEM_CGROUP_CHARGE_TYPE_MAPPED;
+        else if (page_is_file_cache(target))
+                ctype = MEM_CGROUP_CHARGE_TYPE_CACHE;
+        else
+                ctype = MEM_CGROUP_CHARGE_TYPE_SHMEM;
+
+        /* unused page is not on radix-tree now. */
+        if (unused && ctype != MEM_CGROUP_CHARGE_TYPE_MAPPED)
+                __mem_cgroup_uncharge_common(unused, ctype);
+
+        pc = lookup_page_cgroup(target);
         /*
-         * At success, page->mapping is not NULL.
-         * special rollback care is necessary when
-         * 1. at migration failure. (newpage->mapping is cleared in this case)
-         * 2. the newpage was moved but not remapped again because the task
-         *    exits and the newpage is obsolete. In this case, the new page
-         *    may be a swapcache. So, we just call mem_cgroup_uncharge_page()
-         *    always for avoiding mess. The  page_cgroup will be removed if
-         *    unnecessary. File cache pages is still on radix-tree. Don't
-         *    care it.
+         * __mem_cgroup_commit_charge() check PCG_USED bit of page_cgroup.
+         * So, double-counting is effectively avoided.
+         */
+        __mem_cgroup_commit_charge(mem, pc, ctype);
+
+        /*
+         * Both of oldpage and newpage are still under lock_page().
+         * Then, we don't have to care about race in radix-tree.
+         * But we have to be careful that this page is unmapped or not.
+         *
+         * There is a case for !page_mapped(). At the start of
+         * migration, oldpage was mapped. But now, it's zapped.
+         * But we know *target* page is not freed/reused under us.
+         * mem_cgroup_uncharge_page() does all necessary checks.
          */
-        if (!newpage->mapping)
-                __mem_cgroup_uncharge_common(newpage,
-                                MEM_CGROUP_CHARGE_TYPE_FORCE);
-        else if (PageAnon(newpage))
-                mem_cgroup_uncharge_page(newpage);
+        if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
+                mem_cgroup_uncharge_page(target);
 }
 
 /*
diff --git a/mm/migrate.c b/mm/migrate.c
index 246dcb973ae..a30ea5fcf9f 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