mm: memcontrol: try harder to set a new memory.high

Setting a memory.high limit below the usage makes almost no effort to
shrink the cgroup to the new target size.

While memory.high is a "soft" limit that isn't supposed to cause OOM
situations, we should still try harder to meet a user request through
persistent reclaim.

For example, after setting a 10M memory.high on an 800M cgroup full of
file cache, the usage shrinks to about 350M:

  + cat /cgroup/workingset/memory.current
  841568256
  + echo 10M
  + cat /cgroup/workingset/memory.current
  355729408

This isn't exactly what the user would expect to happen. Setting the
value a few more times eventually whittles the usage down to what we
are asking for:

  + echo 10M
  + cat /cgroup/workingset/memory.current
  104181760
  + echo 10M
  + cat /cgroup/workingset/memory.current
  31801344
  + echo 10M
  + cat /cgroup/workingset/memory.current
  10440704

To improve this, add reclaim retry loops to the memory.high write()
callback, similar to what we do for memory.max, to make a reasonable
effort that the usage meets the requested size after the call returns.

Afterwards, a single write() to memory.high is enough in all but extreme
cases:

  + cat /cgroup/workingset/memory.current
  841609216
  + echo 10M
  + cat /cgroup/workingset/memory.current
  10182656

790M is not a reasonable reclaim target to ask of a single reclaim
invocation.  And it wouldn't be reasonable to optimize the reclaim code
for it.  So asking for the full size but retrying is not a bad choice
here: we express our intent, and benefit if reclaim becomes better at
handling larger requests, but we also acknowledge that some of the
deltas we can encounter in memory_high_write() are just too ridiculously
big for a single reclaim invocation to manage.

Link: http://lkml.kernel.org/r/20191022201518.341216-2-hannes@cmpxchg.orgSigned-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -6091,7 +6091,8 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
 				 char *buf, size_t nbytes, loff_t off)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
-	unsigned long nr_pages;
+	unsigned int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
+	bool drained = false;
 	unsigned long high;
 	int err;
 
@@ -6102,12 +6103,29 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
 
 	memcg->high = high;
 
-	nr_pages = page_counter_read(&memcg->memory);
-	if (nr_pages > high)
-		try_to_free_mem_cgroup_pages(memcg, nr_pages - high,
+	for (;;) {
+		unsigned long nr_pages = page_counter_read(&memcg->memory);
+		unsigned long reclaimed;
+
+		if (nr_pages <= high)
+			break;
+
+		if (signal_pending(current))
+			break;
+
+		if (!drained) {
+			drain_all_stock(memcg);
+			drained = true;
+			continue;
+		}
+
+		reclaimed = try_to_free_mem_cgroup_pages(memcg, nr_pages - high,
 							 GFP_KERNEL, true);
 
-	memcg_wb_domain_size_changed(memcg);
+		if (!reclaimed && !nr_retries--)
+			break;
+	}
+
 	return nbytes;
 }