On Wed 20-12-17 16:21:13, Andrey Ryabinin wrote:
> mem_cgroup_resize_[memsw]_limit() tries to free only 32 (SWAP_CLUSTER_MAX)
> pages on each iteration. This makes practically impossible to decrease
> limit of memory cgroup. Tasks could easily allocate back 32 pages,
> so we can't reduce memory usage, and once retry_count reaches zero we return
> -EBUSY.
>
> Easy to reproduce the problem by running the following commands:
>
> mkdir /sys/fs/cgroup/memory/test
> echo $$ >> /sys/fs/cgroup/memory/test/tasks
> cat big_file > /dev/null &
> sleep 1 && echo $((100*1024*1024)) > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
> -bash: echo: write error: Device or resource busy
>
> Instead of relying on retry_count, keep trying to free required amount of pages
> until reclaimer makes any progress.
The wording of the changelog has some room for improvements. The last
sentence should read something like "Instead of relying on retry_count,
keep retrying the reclaim until the desired limit is reached or fail
if the reclaim doesn't make any progress or a signal is pending."
I am bussy as hell today so I will look closer tomorrow or on Friday.
But from a very quick glance the patch seems reasonable.
> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> ---
> mm/memcontrol.c | 70 +++++++++++++--------------------------------------------
> 1 file changed, 16 insertions(+), 54 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f40b5ad3f959..0d26db9a665d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1176,20 +1176,6 @@ void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
> }
>
> /*
> - * This function returns the number of memcg under hierarchy tree. Returns
> - * 1(self count) if no children.
> - */
> -static int mem_cgroup_count_children(struct mem_cgroup *memcg)
> -{
> - int num = 0;
> - struct mem_cgroup *iter;
> -
> - for_each_mem_cgroup_tree(iter, memcg)
> - num++;
> - return num;
> -}
> -
> -/*
> * Return the memory (and swap, if configured) limit for a memcg.
> */
> unsigned long mem_cgroup_get_limit(struct mem_cgroup *memcg)
> @@ -2462,22 +2448,10 @@ static DEFINE_MUTEX(memcg_limit_mutex);
> static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
> unsigned long limit)
> {
> - unsigned long curusage;
> - unsigned long oldusage;
> + unsigned long usage;
> bool enlarge = false;
> - int retry_count;
> int ret;
>
> - /*
> - * For keeping hierarchical_reclaim simple, how long we should retry
> - * is depends on callers. We set our retry-count to be function
> - * of # of children which we should visit in this loop.
> - */
> - retry_count = MEM_CGROUP_RECLAIM_RETRIES *
> - mem_cgroup_count_children(memcg);
> -
> - oldusage = page_counter_read(&memcg->memory);
> -
> do {
> if (signal_pending(current)) {
> ret = -EINTR;
> @@ -2498,15 +2472,13 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
> if (!ret)
> break;
>
> - try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, true);
> -
> - curusage = page_counter_read(&memcg->memory);
> - /* Usage is reduced ? */
> - if (curusage >= oldusage)
> - retry_count--;
> - else
> - oldusage = curusage;
> - } while (retry_count);
> + usage = page_counter_read(&memcg->memory);
> + if (!try_to_free_mem_cgroup_pages(memcg, usage - limit,
> + GFP_KERNEL, true)) {
> + ret = -EBUSY;
> + break;
> + }
> + } while (true);
>
> if (!ret && enlarge)
> memcg_oom_recover(memcg);
> @@ -2517,18 +2489,10 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
> static int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
> unsigned long limit)
> {
> - unsigned long curusage;
> - unsigned long oldusage;
> + unsigned long usage;
> bool enlarge = false;
> - int retry_count;
> int ret;
>
> - /* see mem_cgroup_resize_res_limit */
> - retry_count = MEM_CGROUP_RECLAIM_RETRIES *
> - mem_cgroup_count_children(memcg);
> -
> - oldusage = page_counter_read(&memcg->memsw);
> -
> do {
> if (signal_pending(current)) {
> ret = -EINTR;
> @@ -2549,15 +2513,13 @@ static int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
> if (!ret)
> break;
>
> - try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, false);
> -
> - curusage = page_counter_read(&memcg->memsw);
> - /* Usage is reduced ? */
> - if (curusage >= oldusage)
> - retry_count--;
> - else
> - oldusage = curusage;
> - } while (retry_count);
> + usage = page_counter_read(&memcg->memsw);
> + if (!try_to_free_mem_cgroup_pages(memcg, usage - limit,
> + GFP_KERNEL, false)) {
> + ret = -EBUSY;
> + break;
> + }
> + } while (true);
>
> if (!ret && enlarge)
> memcg_oom_recover(memcg);
> --
> 2.13.6
>
> --
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--
Michal Hocko
SUSE Labs
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