[PATCH 2/2] memcg: reduce usage at change limit

[KAMEZAWA Hiroyuki]

Reduce the usage of res_counter at the change of limit.

Changelog v4 -> v5.
 - moved "feedback" alogrithm from res_counter to memcg.

Background:
 - Now, mem->usage is not reduced at limit change. So, the users will see
   usage > limit case in memcg every time. This patch fixes it.

 Before:
 - no usage change at setting limit.
 - setting limit always returns 0 even if usage can never be less than zero.
   (This can happen when memory is locked or there is no swap.)
 - This is BUG, I think.
 After:
 - usage will be less than new limit at limit change.
 - set limit returns -EBUSY when the usage cannot be reduced.


Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

---
 Documentation/controllers/memory.txt |    3 -
 mm/memcontrol.c                      |   68 ++++++++++++++++++++++++++++-------
 2 files changed, 56 insertions(+), 15 deletions(-)

Index: mm-2.6.26-rc5-mm3/mm/memcontrol.c
===================================================================
--- mm-2.6.26-rc5-mm3.orig/mm/memcontrol.c
+++ mm-2.6.26-rc5-mm3/mm/memcontrol.c
@@ -852,18 +852,30 @@ out:
 	css_put(&mem->css);
 	return ret;
 }
+/*
+ * try to set limit and reduce usage if necessary.
+ * returns 0 at success.
+ * returns -EBUSY if memory cannot be dropped.
+ */
 
-static int mem_cgroup_write_strategy(char *buf, unsigned long long *tmp)
+static inline int mem_cgroup_resize_limit(struct cgroup *cont,
+					unsigned long long val)
 {
-	*tmp = memparse(buf, &buf);
-	if (*buf != '\0')
-		return -EINVAL;
+	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
+	int retry_count = 0;
+	int progress;
 
-	/*
-	 * Round up the value to the closest page size
-	 */
-	*tmp = ((*tmp + PAGE_SIZE - 1) >> PAGE_SHIFT) << PAGE_SHIFT;
-	return 0;
+retry:
+	if (!res_counter_set_limit(&memcg->res, val))
+		return 0;
+	if (retry_count == MEM_CGROUP_RECLAIM_RETRIES)
+		return -EBUSY;
+
+	cond_resched();
+	progress = try_to_free_mem_cgroup_pages(memcg, GFP_KERNEL);
+	if (!progress)
+		retry_count++;
+	goto retry;
 }
 
 static u64 mem_cgroup_read(struct cgroup *cont, struct cftype *cft)
@@ -874,11 +886,41 @@ static u64 mem_cgroup_read(struct cgroup
 
 static ssize_t mem_cgroup_write(struct cgroup *cont, struct cftype *cft,
 				struct file *file, const char __user *userbuf,
-				size_t nbytes, loff_t *ppos)
+				size_t bbytes, loff_t *ppos)
 {
-	return res_counter_write(&mem_cgroup_from_cont(cont)->res,
-				cft->private, userbuf, nbytes, ppos,
-				mem_cgroup_write_strategy);
+	char *buf, *end;
+	unsigned long long val;
+	int ret;
+
+	buf = kmalloc(bbytes + 1, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+	buf[bbytes] = '\0';
+	ret = -EFAULT;
+	if (copy_from_user(buf, userbuf, bbytes))
+		goto out;
+
+	ret = -EINVAL;
+	strstrip(buf);
+	val = memparse(buf, &end);
+	if (*end != '\0')
+		goto out;
+	/* Round to page size */
+	val = ((val + PAGE_SIZE - 1) >> PAGE_SHIFT) << PAGE_SHIFT;
+
+	switch(cft->private) {
+	case RES_LIMIT:
+		ret = mem_cgroup_resize_limit(cont, val);
+		break;
+	default:
+		ret = -EINVAL;
+		goto out;
+	}
+	if (!ret)
+		ret = bbytes;
+out:
+	kfree(buf);
+	return ret;
 }
 
 static int mem_cgroup_reset(struct cgroup *cont, unsigned int event)
Index: mm-2.6.26-rc5-mm3/Documentation/controllers/memory.txt
===================================================================
--- mm-2.6.26-rc5-mm3.orig/Documentation/controllers/memory.txt
+++ mm-2.6.26-rc5-mm3/Documentation/controllers/memory.txt
@@ -242,8 +242,7 @@ rmdir() if there are no tasks.
 1. Add support for accounting huge pages (as a separate controller)
 2. Make per-cgroup scanner reclaim not-shared pages first
 3. Teach controller to account for shared-pages
-4. Start reclamation when the limit is lowered
-5. Start reclamation in the background when the limit is
+4. Start reclamation in the background when the limit is
    not yet hit but the usage is getting closer
 
 Summary

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