memcg: check under limit at shrink_usage

Current memory cgroup(both in mainline and -mm) doesn't account swap
caches as memory(swap cache support is dropped temporarily now).

So try_to_free_mem_cgroup_pages doesn't reflect the count of pages that
have been moved to swap cache.

But this makes mem_cgroup_shrink_usage fail easily if most of the pages
are anon/shmem, and then shmem_getpage returns -ENOMEM and the process
will be killed.

This patch adds res_counter_check_under_limit to avoid these cases.

BTW, even if swap cache support is enabled again, if a process is moved to
another cgroup, which has been just made, between precharge and
shrink_usage in shmem_getpage, shrink_usage may fail just because there is
no pages to reclaim.

So this change would make sense anyway.

Signed-off-by: Daisuke Nishimura <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/memcontrol.c

@@ -806,6 +806,7 @@ int mem_cgroup_shrink_usage(struct mm_struct *mm, gfp_t gfp_mask)
 
 	do {
 		progress = try_to_free_mem_cgroup_pages(mem, gfp_mask);
+		progress += res_counter_check_under_limit(&mem->res);
 	} while (!progress && --retry);
 
 	css_put(&mem->css);