memcg: simplify LRU handling by new rule

Now, at LRU handling, memory cgroup needs to do complicated works to see
valid pc->mem_cgroup, which may be overwritten.

This patch is for relaxing the protocol. This patch guarantees
   - when pc->mem_cgroup is overwritten, page must not be on LRU.

By this, LRU routine can believe pc->mem_cgroup and don't need to check
bits on pc->flags.  This new rule may adds small overheads to swapin.  But
in most case, lru handling gets faster.

After this patch, PCG_ACCT_LRU bit is obsolete and removed.

[akpm@linux-foundation.org: remove unneeded VM_BUG_ON(), restore hannes's christmas tree]
[akpm@linux-foundation.org: clean up code comment]
[hughd@google.com: fix NULL mem_cgroup_try_charge]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 54 insertions, 77 deletions

diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
index 1153095ee457..a2d11771c84b 100644
--- a/include/linux/page_cgroup.h
+++ b/include/linux/page_cgroup.h
@@ -10,8 +10,6 @@ enum {
         /* flags for mem_cgroup and file and I/O status */
         PCG_MOVE_LOCK, /* For race between move_account v.s. following bits */
         PCG_FILE_MAPPED, /* page is accounted as "mapped" */
-        /* No lock in page_cgroup */
-        PCG_ACCT_LRU, /* page has been accounted for (under lru_lock) */
         __NR_PCG_FLAGS,
 };
 
@@ -75,12 +73,6 @@ TESTPCGFLAG(Used, USED)
 CLEARPCGFLAG(Used, USED)
 SETPCGFLAG(Used, USED)
 
-SETPCGFLAG(AcctLRU, ACCT_LRU)
-CLEARPCGFLAG(AcctLRU, ACCT_LRU)
-TESTPCGFLAG(AcctLRU, ACCT_LRU)
-TESTCLEARPCGFLAG(AcctLRU, ACCT_LRU)
-
-
 SETPCGFLAG(FileMapped, FILE_MAPPED)
 CLEARPCGFLAG(FileMapped, FILE_MAPPED)
 TESTPCGFLAG(FileMapped, FILE_MAPPED)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c74102d6eb5a..ff051ee8fb4b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1040,30 +1040,7 @@ struct lruvec *mem_cgroup_lru_add_list(struct zone *zone, struct page *page,
                 return &zone->lruvec;
 
         pc = lookup_page_cgroup(page);
-        VM_BUG_ON(PageCgroupAcctLRU(pc));
-        /*
-         * putback:                             charge:
-         * SetPageLRU                           SetPageCgroupUsed
-         * smp_mb                               smp_mb
-         * PageCgroupUsed && add to memcg LRU   PageLRU && add to memcg LRU
-         *
-         * Ensure that one of the two sides adds the page to the memcg
-         * LRU during a race.
-         */
-        smp_mb();
-        /*
-         * If the page is uncharged, it may be freed soon, but it
-         * could also be swap cache (readahead, swapoff) that needs to
-         * be reclaimable in the future.  root_mem_cgroup will babysit
-         * it for the time being.
-         */
-        if (PageCgroupUsed(pc)) {
-                /* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
-                smp_rmb();
-                memcg = pc->mem_cgroup;
-                SetPageCgroupAcctLRU(pc);
-        } else
-                memcg = root_mem_cgroup;
+        memcg = pc->mem_cgroup;
         mz = page_cgroup_zoneinfo(memcg, page);
         /* compound_order() is stabilized through lru_lock */
         MEM_CGROUP_ZSTAT(mz, lru) += 1 << compound_order(page);
@@ -1090,18 +1067,8 @@ void mem_cgroup_lru_del_list(struct page *page, enum lru_list lru)
                 return;
 
         pc = lookup_page_cgroup(page);
-        /*
-         * root_mem_cgroup babysits uncharged LRU pages, but
-         * PageCgroupUsed is cleared when the page is about to get
-         * freed.  PageCgroupAcctLRU remembers whether the
-         * LRU-accounting happened against pc->mem_cgroup or
-         * root_mem_cgroup.
-         */
-        if (TestClearPageCgroupAcctLRU(pc)) {
-                VM_BUG_ON(!pc->mem_cgroup);
-                memcg = pc->mem_cgroup;
-        } else
-                memcg = root_mem_cgroup;
+        memcg = pc->mem_cgroup;
+        VM_BUG_ON(!memcg);
         mz = page_cgroup_zoneinfo(memcg, page);
         /* huge page split is done under lru_lock. so, we have no races. */
         MEM_CGROUP_ZSTAT(mz, lru) -= 1 << compound_order(page);
@@ -2217,8 +2184,25 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 }
 
 /*
- * Unlike exported interface, "oom" parameter is added. if oom==true,
- * oom-killer can be invoked.
+ * __mem_cgroup_try_charge() does
+ * 1. detect memcg to be charged against from passed *mm and *ptr,
+ * 2. update res_counter
+ * 3. call memory reclaim if necessary.
+ *
+ * In some special case, if the task is fatal, fatal_signal_pending() or
+ * has TIF_MEMDIE, this function returns -EINTR while writing root_mem_cgroup
+ * to *ptr. There are two reasons for this. 1: fatal threads should quit as soon
+ * as possible without any hazards. 2: all pages should have a valid
+ * pc->mem_cgroup. If mm is NULL and the caller doesn't pass a valid memcg
+ * pointer, that is treated as a charge to root_mem_cgroup.
+ *
+ * So __mem_cgroup_try_charge() will return
+ *  0       ...  on success, filling *ptr with a valid memcg pointer.
+ *  -ENOMEM ...  charge failure because of resource limits.
+ *  -EINTR  ...  if thread is fatal. *ptr is filled with root_mem_cgroup.
+ *
+ * Unlike the exported interface, an "oom" parameter is added. if oom==true,
+ * the oom-killer can be invoked.
  */
 static int __mem_cgroup_try_charge(struct mm_struct *mm,
                                    gfp_t gfp_mask,
@@ -2247,7 +2231,7 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
          * set, if so charge the init_mm (happens for pagecache usage).
          */
         if (!*ptr && !mm)
-                goto bypass;
+                *ptr = root_mem_cgroup;
 again:
         if (*ptr) { /* css should be a valid one */
                 memcg = *ptr;
@@ -2273,7 +2257,9 @@ again:
                  * task-struct. So, mm->owner can be NULL.
                  */
                 memcg = mem_cgroup_from_task(p);
-                if (!memcg || mem_cgroup_is_root(memcg)) {
+                if (!memcg)
+                        memcg = root_mem_cgroup;
+                if (mem_cgroup_is_root(memcg)) {
                         rcu_read_unlock();
                         goto done;
                 }
@@ -2348,8 +2334,8 @@ nomem:
         *ptr = NULL;
         return -ENOMEM;
 bypass:
-        *ptr = NULL;
-        return 0;
+        *ptr = root_mem_cgroup;
+        return -EINTR;
 }
 
 /*
@@ -2457,6 +2443,7 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
 
         mem_cgroup_charge_statistics(memcg, PageCgroupCache(pc), nr_pages);
         unlock_page_cgroup(pc);
+        WARN_ON_ONCE(PageLRU(page));
         /*
          * "charge_statistics" updated event counter. Then, check it.
          * Insert ancestor (and ancestor's ancestors), to softlimit RB-tree.
@@ -2468,7 +2455,7 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 
 #define PCGF_NOCOPY_AT_SPLIT ((1 << PCG_LOCK) | (1 << PCG_MOVE_LOCK) |\
-                        (1 << PCG_ACCT_LRU) | (1 << PCG_MIGRATION))
+                        (1 << PCG_MIGRATION))
 /*
  * Because tail pages are not marked as "used", set it. We're under
  * zone->lru_lock, 'splitting on pmd' and compound_lock.
@@ -2478,7 +2465,9 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
 void mem_cgroup_split_huge_fixup(struct page *head)
 {
         struct page_cgroup *head_pc = lookup_page_cgroup(head);
+        struct mem_cgroup_per_zone *mz;
         struct page_cgroup *pc;
+        enum lru_list lru;
         int i;
 
         if (mem_cgroup_disabled())
@@ -2487,23 +2476,15 @@ void mem_cgroup_split_huge_fixup(struct page *head)
                 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;
         }
-
-        if (PageCgroupAcctLRU(head_pc)) {
-                enum lru_list lru;
-                struct mem_cgroup_per_zone *mz;
-                /*
-                 * 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) -= HPAGE_PMD_NR - 1;
-        }
+        /*
+         * Tail pages will be added to LRU.
+         * 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) -= HPAGE_PMD_NR - 1;
 }
 #endif
 
@@ -2620,7 +2601,7 @@ static int mem_cgroup_move_parent(struct page *page,
 
         parent = mem_cgroup_from_cont(pcg);
         ret = __mem_cgroup_try_charge(NULL, gfp_mask, nr_pages, &parent, false);
-        if (ret || !parent)
+        if (ret)
                 goto put_back;
 
         if (nr_pages > 1)
@@ -2667,9 +2648,8 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
 
         pc = lookup_page_cgroup(page);
         ret = __mem_cgroup_try_charge(mm, gfp_mask, nr_pages, &memcg, oom);
-        if (ret || !memcg)
+        if (ret == -ENOMEM)
                 return ret;
-
         __mem_cgroup_commit_charge(memcg, page, nr_pages, pc, ctype);
         return 0;
 }
@@ -2736,10 +2716,9 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
         if (!page_is_file_cache(page))
                 type = MEM_CGROUP_CHARGE_TYPE_SHMEM;
 
-        if (!PageSwapCache(page)) {
+        if (!PageSwapCache(page))
                 ret = mem_cgroup_charge_common(page, mm, gfp_mask, type);
-                WARN_ON_ONCE(PageLRU(page));
-        } else { /* page is swapcache/shmem */
+        else { /* page is swapcache/shmem */
                 ret = mem_cgroup_try_charge_swapin(mm, page, gfp_mask, &memcg);
                 if (!ret)
                         __mem_cgroup_commit_charge_swapin(page, memcg, type);
@@ -2781,11 +2760,16 @@ int mem_cgroup_try_charge_swapin(struct mm_struct *mm,
         *memcgp = memcg;
         ret = __mem_cgroup_try_charge(NULL, mask, 1, memcgp, true);
         css_put(&memcg->css);
+        if (ret == -EINTR)
+                ret = 0;
         return ret;
 charge_cur_mm:
         if (unlikely(!mm))
                 mm = &init_mm;
-        return __mem_cgroup_try_charge(mm, mask, 1, memcgp, true);
+        ret = __mem_cgroup_try_charge(mm, mask, 1, memcgp, true);
+        if (ret == -EINTR)
+                ret = 0;
+        return ret;
 }
 
 static void
@@ -3245,7 +3229,7 @@ int mem_cgroup_prepare_migration(struct page *page,
         *memcgp = memcg;
         ret = __mem_cgroup_try_charge(NULL, gfp_mask, 1, memcgp, false);
         css_put(&memcg->css);/* drop extra refcnt */
-        if (ret || *memcgp == NULL) {
+        if (ret) {
                 if (PageAnon(page)) {
                         lock_page_cgroup(pc);
                         ClearPageCgroupMigration(pc);
@@ -3255,6 +3239,7 @@ int mem_cgroup_prepare_migration(struct page *page,
                          */
                         mem_cgroup_uncharge_page(page);
                 }
+                /* we'll need to revisit this error code (we have -EINTR) */
                 return -ENOMEM;
         }
         /*
@@ -3674,7 +3659,7 @@ static int mem_cgroup_force_empty_list(struct mem_cgroup *memcg,
                 pc = lookup_page_cgroup(page);
 
                 ret = mem_cgroup_move_parent(page, pc, memcg, GFP_KERNEL);
-                if (ret == -ENOMEM)
+                if (ret == -ENOMEM || ret == -EINTR)
                         break;
 
                 if (ret == -EBUSY || ret == -EINVAL) {
@@ -5065,9 +5050,9 @@ one_by_one:
                 }
                 ret = __mem_cgroup_try_charge(NULL,
                                         GFP_KERNEL, 1, &memcg, false);
-                if (ret || !memcg)
+                if (ret)
                         /* mem_cgroup_clear_mc() will do uncharge later */
-                        return -ENOMEM;
+                        return ret;
                 mc.precharge++;
         }
         return ret;