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Commit 484ebb3b authored by Greg Thelen's avatar Greg Thelen Committed by Linus Torvalds
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memcg: make mem_cgroup_read_stat() unsigned 



mem_cgroup_read_stat() returns a page count by summing per cpu page
counters.  The summing is racy wrt.  updates, so a transient negative
sum is possible.  Callers don't want negative values:

 - mem_cgroup_wb_stats() doesn't want negative nr_dirty or nr_writeback.
   This could confuse dirty throttling.

 - oom reports and memory.stat shouldn't show confusing negative usage.

 - tree_usage() already avoids negatives.

Avoid returning negative page counts from mem_cgroup_read_stat() and
convert it to unsigned.

[akpm@linux-foundation.org: fix old typo while we're in there]
Signed-off-by: default avatarGreg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: default avatarMichal Hocko <mhocko@suse.com>
Cc: <stable@vger.kernel.org>	[4.2+]
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 0610c25d

mm/memcontrol.c

+18 −12
Original line number Diff line number Diff line
@@ -644,12 +644,14 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_zone *mctz) 
}



/*

 * Return page count for single (non recursive) @memcg.

 *

 * Implementation Note: reading percpu statistics for memcg.

 *

 * Both of vmstat[] and percpu_counter has threshold and do periodic

 * synchronization to implement "quick" read. There are trade-off between

 * reading cost and precision of value. Then, we may have a chance to implement

 * a periodic synchronizion of counter in memcg's counter.

 * a periodic synchronization of counter in memcg's counter.

 *

 * But this _read() function is used for user interface now. The user accounts

 * memory usage by memory cgroup and he _always_ requires exact value because
@@ -659,17 +661,24 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_zone *mctz) 
 *

 * If there are kernel internal actions which can make use of some not-exact

 * value, and reading all cpu value can be performance bottleneck in some

 * common workload, threashold and synchonization as vmstat[] should be

 * common workload, threshold and synchronization as vmstat[] should be

 * implemented.

 */

static long mem_cgroup_read_stat(struct mem_cgroup *memcg,

				 enum mem_cgroup_stat_index idx)

static unsigned long

mem_cgroup_read_stat(struct mem_cgroup *memcg, enum mem_cgroup_stat_index idx)

{

	long val = 0;

	int cpu;



	/* Per-cpu values can be negative, use a signed accumulator */

	for_each_possible_cpu(cpu)

		val += per_cpu(memcg->stat->count[idx], cpu);

	/*

	 * Summing races with updates, so val may be negative.  Avoid exposing

	 * transient negative values.

	 */

	if (val < 0)

		val = 0;

	return val;

}
@@ -1254,7 +1263,7 @@ void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p) 
		for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++) {

			if (i == MEM_CGROUP_STAT_SWAP && !do_swap_account)

				continue;

			pr_cont(" %s:%ldKB", mem_cgroup_stat_names[i],

			pr_cont(" %s:%luKB", mem_cgroup_stat_names[i],

				K(mem_cgroup_read_stat(iter, i)));

		}
@@ -2819,14 +2828,11 @@ static unsigned long tree_stat(struct mem_cgroup *memcg, 
			       enum mem_cgroup_stat_index idx)

{

	struct mem_cgroup *iter;

	long val = 0;

	unsigned long val = 0;



	/* Per-cpu values can be negative, use a signed accumulator */

	for_each_mem_cgroup_tree(iter, memcg)

		val += mem_cgroup_read_stat(iter, idx);



	if (val < 0) /* race ? */

		val = 0;

	return val;

}
@@ -3169,7 +3175,7 @@ static int memcg_stat_show(struct seq_file *m, void *v) 
	for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++) {

		if (i == MEM_CGROUP_STAT_SWAP && !do_swap_account)

			continue;

		seq_printf(m, "%s %ld\n", mem_cgroup_stat_names[i],

		seq_printf(m, "%s %lu\n", mem_cgroup_stat_names[i],

			   mem_cgroup_read_stat(memcg, i) * PAGE_SIZE);

	}
@@ -3194,13 +3200,13 @@ static int memcg_stat_show(struct seq_file *m, void *v) 
			   (u64)memsw * PAGE_SIZE);



	for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++) {

		long long val = 0;

		unsigned long long val = 0;



		if (i == MEM_CGROUP_STAT_SWAP && !do_swap_account)

			continue;

		for_each_mem_cgroup_tree(mi, memcg)

			val += mem_cgroup_read_stat(mi, i) * PAGE_SIZE;

		seq_printf(m, "total_%s %lld\n", mem_cgroup_stat_names[i], val);

		seq_printf(m, "total_%s %llu\n", mem_cgroup_stat_names[i], val);

	}



	for (i = 0; i < MEM_CGROUP_EVENTS_NSTATS; i++) {