Re: [PATCH] mm: memcontrol: provide shmem statistics

From: Michal Hocko
Date: Wed Feb 22 2017 - 03:12:51 EST

On Tue 21-02-17 11:43:43, Johannes Weiner wrote:
> Cgroups currently don't report how much shmem they use, which can be
> useful data to have, in particular since shmem is included in the
> cache/file item while being reclaimed like anonymous memory.
>
> Add a counter to track shmem pages during charging and uncharging.

Yes this is indeed useful. Accounting shmem to the page cache was a
mistake because this is more than confusing. Sad we cannot fix that.
I would have just one concern with this patch. You are relying on
PageSwapBacked check but it looks like we are going to implement
MADV_FREE by dropping this flag. I know we do not support MADV_FREE
on shared mappings but if we ever do then the accounting will become
subtly broken. Can/Should we rely on shmem_mapping() check instead?

Other than that the patch looks good to me.

> Reported-by: Chris Down <cdown@xxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
> Documentation/cgroup-v2.txt | 5 +++++
> include/linux/memcontrol.h | 1 +
> mm/memcontrol.c | 28 ++++++++++++++++++++--------
> 3 files changed, 26 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
> index 4cc07ce3b8dd..d99389ce7b01 100644
> --- a/Documentation/cgroup-v2.txt
> +++ b/Documentation/cgroup-v2.txt
> @@ -867,6 +867,11 @@ PAGE_SIZE multiple when read back.
>
> Amount of memory used in network transmission buffers
>
> + shmem
> +
> + Amount of cached filesystem data that is swap-backed,
> + such as tmpfs, shm segments, shared anonymous mmap()s
> +
> file_mapped
>
> Amount of cached filesystem data mapped with mmap()
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 61d20c17f3b7..47bdf727d1ad 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -46,6 +46,7 @@ enum mem_cgroup_stat_index {
> MEM_CGROUP_STAT_CACHE, /* # of pages charged as cache */
> MEM_CGROUP_STAT_RSS, /* # of pages charged as anon rss */
> MEM_CGROUP_STAT_RSS_HUGE, /* # of pages charged as anon huge */
> + MEM_CGROUP_STAT_SHMEM, /* # of pages charged as shmem */
> MEM_CGROUP_STAT_FILE_MAPPED, /* # of pages charged as file rss */
> MEM_CGROUP_STAT_DIRTY, /* # of dirty pages in page cache */
> MEM_CGROUP_STAT_WRITEBACK, /* # of pages under writeback */
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9c9cde768d40..49409f5c0238 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -102,6 +102,7 @@ static const char * const mem_cgroup_stat_names[] = {
> "cache",
> "rss",
> "rss_huge",
> + "shmem",
> "mapped_file",
> "dirty",
> "writeback",
> @@ -601,9 +602,13 @@ static void mem_cgroup_charge_statistics(struct mem_cgroup *memcg,
> if (PageAnon(page))
> __this_cpu_add(memcg->stat->count[MEM_CGROUP_STAT_RSS],
> nr_pages);
> - else
> + else {
> __this_cpu_add(memcg->stat->count[MEM_CGROUP_STAT_CACHE],
> nr_pages);
> + if (PageSwapBacked(page))
> + __this_cpu_add(memcg->stat->count[MEM_CGROUP_STAT_SHMEM],
> + nr_pages);
> + }
>
> if (compound) {
> VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> @@ -5200,6 +5205,8 @@ static int memory_stat_show(struct seq_file *m, void *v)
> seq_printf(m, "sock %llu\n",
> (u64)stat[MEMCG_SOCK] * PAGE_SIZE);
>
> + seq_printf(m, "shmem %llu\n",
> + (u64)stat[MEM_CGROUP_STAT_SHMEM] * PAGE_SIZE);
> seq_printf(m, "file_mapped %llu\n",
> (u64)stat[MEM_CGROUP_STAT_FILE_MAPPED] * PAGE_SIZE);
> seq_printf(m, "file_dirty %llu\n",
> @@ -5468,8 +5475,8 @@ void mem_cgroup_cancel_charge(struct page *page, struct mem_cgroup *memcg,
>
> static void uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,
> unsigned long nr_anon, unsigned long nr_file,
> - unsigned long nr_huge, unsigned long nr_kmem,
> - struct page *dummy_page)
> + unsigned long nr_kmem, unsigned long nr_huge,
> + unsigned long nr_shmem, struct page *dummy_page)
> {
> unsigned long nr_pages = nr_anon + nr_file + nr_kmem;
> unsigned long flags;
> @@ -5487,6 +5494,7 @@ static void uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,
> __this_cpu_sub(memcg->stat->count[MEM_CGROUP_STAT_RSS], nr_anon);
> __this_cpu_sub(memcg->stat->count[MEM_CGROUP_STAT_CACHE], nr_file);
> __this_cpu_sub(memcg->stat->count[MEM_CGROUP_STAT_RSS_HUGE], nr_huge);
> + __this_cpu_sub(memcg->stat->count[MEM_CGROUP_STAT_SHMEM], nr_shmem);
> __this_cpu_add(memcg->stat->events[MEM_CGROUP_EVENTS_PGPGOUT], pgpgout);
> __this_cpu_add(memcg->stat->nr_page_events, nr_pages);
> memcg_check_events(memcg, dummy_page);
> @@ -5499,6 +5507,7 @@ static void uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,
> static void uncharge_list(struct list_head *page_list)
> {
> struct mem_cgroup *memcg = NULL;
> + unsigned long nr_shmem = 0;
> unsigned long nr_anon = 0;
> unsigned long nr_file = 0;
> unsigned long nr_huge = 0;
> @@ -5531,9 +5540,9 @@ static void uncharge_list(struct list_head *page_list)
> if (memcg != page->mem_cgroup) {
> if (memcg) {
> uncharge_batch(memcg, pgpgout, nr_anon, nr_file,
> - nr_huge, nr_kmem, page);
> - pgpgout = nr_anon = nr_file =
> - nr_huge = nr_kmem = 0;
> + nr_kmem, nr_huge, nr_shmem, page);
> + pgpgout = nr_anon = nr_file = nr_kmem = 0;
> + nr_huge = nr_shmem = 0;
> }
> memcg = page->mem_cgroup;
> }
> @@ -5547,8 +5556,11 @@ static void uncharge_list(struct list_head *page_list)
> }
> if (PageAnon(page))
> nr_anon += nr_pages;
> - else
> + else {
> nr_file += nr_pages;
> + if (PageSwapBacked(page))
> + nr_shmem += nr_pages;
> + }
> pgpgout++;
> } else {
> nr_kmem += 1 << compound_order(page);
> @@ -5560,7 +5572,7 @@ static void uncharge_list(struct list_head *page_list)
>
> if (memcg)
> uncharge_batch(memcg, pgpgout, nr_anon, nr_file,
> - nr_huge, nr_kmem, page);
> + nr_kmem, nr_huge, nr_shmem, page);
> }
>
> /**
> --
> 2.11.1

--
Michal Hocko
SUSE Labs