Commit 0610c25d authored Oct 01, 2015 by Greg Thelen Committed by Linus Torvalds Oct 01, 2015

memcg: fix dirty page migration



The problem starts with a file backed dirty page which is charged to a
memcg.  Then page migration is used to move oldpage to newpage.

Migration:
 - copies the oldpage's data to newpage
 - clears oldpage.PG_dirty
 - sets newpage.PG_dirty
 - uncharges oldpage from memcg
 - charges newpage to memcg

Clearing oldpage.PG_dirty decrements the charged memcg's dirty page
count.

However, because newpage is not yet charged, setting newpage.PG_dirty
does not increment the memcg's dirty page count.  After migration
completes newpage.PG_dirty is eventually cleared, often in
account_page_cleaned().  At this time newpage is charged to a memcg so
the memcg's dirty page count is decremented which causes underflow
because the count was not previously incremented by migration.  This
underflow causes balance_dirty_pages() to see a very large unsigned
number of dirty memcg pages which leads to aggressive throttling of
buffered writes by processes in non root memcg.

This issue:
 - can harm performance of non root memcg buffered writes.
 - can report too small (even negative) values in
   memory.stat[(total_)dirty] counters of all memcg, including the root.

To avoid polluting migrate.c with #ifdef CONFIG_MEMCG checks, introduce
page_memcg() and set_page_memcg() helpers.

Test:
    0) setup and enter limited memcg
    mkdir /sys/fs/cgroup/test
    echo 1G > /sys/fs/cgroup/test/memory.limit_in_bytes
    echo $$ > /sys/fs/cgroup/test/cgroup.procs

    1) buffered writes baseline
    dd if=/dev/zero of=/data/tmp/foo bs=1M count=1k
    sync
    grep ^dirty /sys/fs/cgroup/test/memory.stat

    2) buffered writes with compaction antagonist to induce migration
    yes 1 > /proc/sys/vm/compact_memory &
    rm -rf /data/tmp/foo
    dd if=/dev/zero of=/data/tmp/foo bs=1M count=1k
    kill %
    sync
    grep ^dirty /sys/fs/cgroup/test/memory.stat

    3) buffered writes without antagonist, should match baseline
    rm -rf /data/tmp/foo
    dd if=/dev/zero of=/data/tmp/foo bs=1M count=1k
    sync
    grep ^dirty /sys/fs/cgroup/test/memory.stat

                       (speed, dirty residue)
             unpatched                       patched
    1) 841 MB/s 0 dirty pages          886 MB/s 0 dirty pages
    2) 611 MB/s -33427456 dirty pages  793 MB/s 0 dirty pages
    3) 114 MB/s -33427456 dirty pages  891 MB/s 0 dirty pages

    Notice that unpatched baseline performance (1) fell after
    migration (3): 841 -> 114 MB/s.  In the patched kernel, post
    migration performance matches baseline.

Fixes: c4843a75 ("memcg: add per cgroup dirty page accounting")
Signed-off-by: Greg Thelen <gthelen@google.com>
Reported-by: Dave Hansen <dave.hansen@intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>	[4.2+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 8346c416

include/linux/mm.h

+21 −0

Original line number	Diff line number	Diff line
		@@ -905,6 +905,27 @@ static inline void set_page_links(struct page *page, enum zone_type zone,
		#endif
		}

		#ifdef CONFIG_MEMCG
		static inline struct mem_cgroup page_memcg(struct page page)
		{
		return page->mem_cgroup;
		}

		static inline void set_page_memcg(struct page page, struct mem_cgroup memcg)
		{
		page->mem_cgroup = memcg;
		}
		#else
		static inline struct mem_cgroup page_memcg(struct page page)
		{
		return NULL;
		}

		static inline void set_page_memcg(struct page page, struct mem_cgroup memcg)
		{
		}
		#endif

		/*
		* Some inline functions in vmstat.h depend on page_zone()
		*/

mm/migrate.c

+11 −1

Original line number	Diff line number	Diff line
		@@ -740,6 +740,15 @@ static int move_to_new_page(struct page newpage, struct page page,
		if (PageSwapBacked(page))
		SetPageSwapBacked(newpage);

		/*
		* Indirectly called below, migrate_page_copy() copies PG_dirty and thus
		* needs newpage's memcg set to transfer memcg dirty page accounting.
		* So perform memcg migration in two steps:
		* 1. set newpage->mem_cgroup (here)
		* 2. clear page->mem_cgroup (below)
		*/
		set_page_memcg(newpage, page_memcg(page));

		mapping = page_mapping(page);
		if (!mapping)
		rc = migrate_page(mapping, newpage, page, mode);
		@@ -756,9 +765,10 @@ static int move_to_new_page(struct page newpage, struct page page,
		rc = fallback_migrate_page(mapping, newpage, page, mode);

		if (rc != MIGRATEPAGE_SUCCESS) {
		set_page_memcg(newpage, NULL);
		newpage->mapping = NULL;
		} else {
		mem_cgroup_migrate(page, newpage, false);
		set_page_memcg(page, NULL);
		if (page_was_mapped)
		remove_migration_ptes(page, newpage);
		page->mapping = NULL;