memory cgroup enhancements: force_empty interface for dropping all account in empty cgroup

This patch adds an interface "memory.force_empty".  Any write to this file
will drop all charges in this cgroup if there is no task under.

%echo 1 > /....../memory.force_empty

will drop all charges of memory cgroup if cgroup's tasks is empty.

This is useful to invoke rmdir() against memory cgroup successfully.

Tested and worked well on x86_64/fake-NUMA system.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Pavel Emelianov <xemul@openvz.org>
Cc: Paul Menage <menage@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Kirill Korotaev <dev@sw.ru>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Cc: David Rientjes <rientjes@google.com>
Cc: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 103 insertions, 7 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9793873d5a90..c867612d9c04 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -471,6 +471,7 @@ void mem_cgroup_uncharge(struct page_cgroup *pc)
                 page = pc->page;
                 /*
                  * get page->cgroup and clear it under lock.
+                 * force_empty can drop page->cgroup without checking refcnt.
                  */
                 if (clear_page_cgroup(page, pc) == pc) {
                         mem = pc->mem_cgroup;
@@ -480,13 +481,6 @@ void mem_cgroup_uncharge(struct page_cgroup *pc)
                         list_del_init(&pc->lru);
                         spin_unlock_irqrestore(&mem->lru_lock, flags);
                         kfree(pc);
-                } else {
-                        /*
-                         * Note:This will be removed when force-empty patch is
-                         * applied. just show warning here.
-                         */
-                        printk(KERN_ERR "Race in mem_cgroup_uncharge() ?");
-                        dump_stack();
                 }
         }
 }
@@ -534,6 +528,76 @@ retry:
         return;
 }
 
+/*
+ * This routine traverse page_cgroup in given list and drop them all.
+ * This routine ignores page_cgroup->ref_cnt.
+ * *And* this routine doesn't reclaim page itself, just removes page_cgroup.
+ */
+#define FORCE_UNCHARGE_BATCH    (128)
+static void
+mem_cgroup_force_empty_list(struct mem_cgroup *mem, struct list_head *list)
+{
+        struct page_cgroup *pc;
+        struct page *page;
+        int count;
+        unsigned long flags;
+
+retry:
+        count = FORCE_UNCHARGE_BATCH;
+        spin_lock_irqsave(&mem->lru_lock, flags);
+
+        while (--count && !list_empty(list)) {
+                pc = list_entry(list->prev, struct page_cgroup, lru);
+                page = pc->page;
+                /* Avoid race with charge */
+                atomic_set(&pc->ref_cnt, 0);
+                if (clear_page_cgroup(page, pc) == pc) {
+                        css_put(&mem->css);
+                        res_counter_uncharge(&mem->res, PAGE_SIZE);
+                        list_del_init(&pc->lru);
+                        kfree(pc);
+                } else  /* being uncharged ? ...do relax */
+                        break;
+        }
+        spin_unlock_irqrestore(&mem->lru_lock, flags);
+        if (!list_empty(list)) {
+                cond_resched();
+                goto retry;
+        }
+        return;
+}
+
+/*
+ * make mem_cgroup's charge to be 0 if there is no task.
+ * This enables deleting this mem_cgroup.
+ */
+
+int mem_cgroup_force_empty(struct mem_cgroup *mem)
+{
+        int ret = -EBUSY;
+        css_get(&mem->css);
+        /*
+         * page reclaim code (kswapd etc..) will move pages between
+`        * active_list <-> inactive_list while we don't take a lock.
+         * So, we have to do loop here until all lists are empty.
+         */
+        while (!(list_empty(&mem->active_list) &&
+                 list_empty(&mem->inactive_list))) {
+                if (atomic_read(&mem->css.cgroup->count) > 0)
+                        goto out;
+                /* drop all page_cgroup in active_list */
+                mem_cgroup_force_empty_list(mem, &mem->active_list);
+                /* drop all page_cgroup in inactive_list */
+                mem_cgroup_force_empty_list(mem, &mem->inactive_list);
+        }
+        ret = 0;
+out:
+        css_put(&mem->css);
+        return ret;
+}
+
+
+
 int mem_cgroup_write_strategy(char *buf, unsigned long long *tmp)
 {
         *tmp = memparse(buf, &buf);
@@ -619,6 +683,33 @@ static ssize_t mem_control_type_read(struct cgroup *cont,
                         ppos, buf, s - buf);
 }
 
+
+static ssize_t mem_force_empty_write(struct cgroup *cont,
+                                struct cftype *cft, struct file *file,
+                                const char __user *userbuf,
+                                size_t nbytes, loff_t *ppos)
+{
+        struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
+        int ret;
+        ret = mem_cgroup_force_empty(mem);
+        if (!ret)
+                ret = nbytes;
+        return ret;
+}
+
+/*
+ * Note: This should be removed if cgroup supports write-only file.
+ */
+
+static ssize_t mem_force_empty_read(struct cgroup *cont,
+                                struct cftype *cft,
+                                struct file *file, char __user *userbuf,
+                                size_t nbytes, loff_t *ppos)
+{
+        return -EINVAL;
+}
+
+
 static struct cftype mem_cgroup_files[] = {
         {
                 .name = "usage_in_bytes",
@@ -641,6 +732,11 @@ static struct cftype mem_cgroup_files[] = {
                 .write = mem_control_type_write,
                 .read = mem_control_type_read,
         },
+        {
+                .name = "force_empty",
+                .write = mem_force_empty_write,
+                .read = mem_force_empty_read,
+        },
 };
 
 static struct mem_cgroup init_mem_cgroup;