Commit 0610c25d authored by Greg Thelen's avatar Greg Thelen Committed by Linus Torvalds

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.

 - 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

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.

    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
    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 %
    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
    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: default avatarGreg Thelen <>
Reported-by: default avatarDave Hansen <>
Acked-by: default avatarMichal Hocko <>
Acked-by: default avatarJohannes Weiner <>
Cc: <>	[4.2+]
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 8346c416
......@@ -905,6 +905,27 @@ static inline void set_page_links(struct page *page, enum zone_type zone,
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;
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)
* Some inline functions in vmstat.h depend on page_zone()
......@@ -740,6 +740,15 @@ static int move_to_new_page(struct page *newpage, struct page *page,
if (PageSwapBacked(page))
* 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);
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;
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