On Tue 22-09-15 15:16:32, Greg Thelen wrote:
> 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.
OK, this can confuse dirty throttling AFAIU
> - oom reports and memory.stat shouldn't show confusing negative usage.
I guess this is not earth shattering.
> - tree_usage() already avoids negatives.
>
> Avoid returning negative page counts from mem_cgroup_read_stat() and
> convert it to unsigned.
>
> Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
I guess we want that for stable 4.2 because of the dirty throttling
part. Longterm we should use generic per-cpu counter.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!
> ---
> mm/memcontrol.c | 30 ++++++++++++++++++------------
> 1 file changed, 18 insertions(+), 12 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6ddaeba34e09..2633e9be4a99 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -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, threashold 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++) {
> --
> 2.6.0.rc0.131.gf624c3d
--
Michal Hocko
SUSE Labs
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