Re: [patch 2/2] mm: memcontrol: fix missed end-writeback page accounting

Michal Hocko
2014-10-23T13:03:31Z

On Wed 22-10-14 14:29:28, Johannes Weiner wrote:
> 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API") changed page
> migration to uncharge the old page right away. The page is locked,
> unmapped, truncated, and off the LRU, but it could race with writeback
> ending, which then doesn't unaccount the page properly:
>
> test_clear_page_writeback() migration
> acquire pc->mem_cgroup->move_lock

I do not think that mentioning move_lock is important/helpful here
because the hot path which is taken all the time (except when there is a
task move in progress) doesn't take it.
Besides that it is not even relevant for the race.

> wait_on_page_writeback()
> TestClearPageWriteback()
> mem_cgroup_migrate()
> clear PCG_USED
> if (PageCgroupUsed(pc))
> decrease memcg pages under writeback
> release pc->mem_cgroup->move_lock
>
> The per-page statistics interface is heavily optimized to avoid a
> function call and a lookup_page_cgroup() in the file unmap fast path,
> which means it doesn't verify whether a page is still charged before
> clearing PageWriteback() and it has to do it in the stat update later.
>
> Rework it so that it looks up the page's memcg once at the beginning
> of the transaction and then uses it throughout. The charge will be
> verified before clearing PageWriteback() and migration can't uncharge
> the page as long as that is still set. The RCU lock will protect the
> memcg past uncharge.
>
> As far as losing the optimization goes, the following test results are
> from a microbenchmark that maps, faults, and unmaps a 4GB sparse file
> three times in a nested fashion, so that there are two negative passes
> that don't account but still go through the new transaction overhead.
> There is no actual difference:
>
> old: 33.195102545 seconds time elapsed ( +- 0.01% )
> new: 33.199231369 seconds time elapsed ( +- 0.03% )
>
> The time spent in page_remove_rmap()'s callees still adds up to the
> same, but the time spent in the function itself seems reduced:
>
> # Children Self Command Shared Object Symbol
> old: 0.12% 0.11% filemapstress [kernel.kallsyms] [k] page_remove_rmap
> new: 0.12% 0.08% filemapstress [kernel.kallsyms] [k] page_remove_rmap
>
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: "3.17" <stable@kernel.org>

Acked-by: Michal Hocko <mhocko@suse.cz>

Thanks!

> ---
> include/linux/memcontrol.h | 58 ++++++++++++++--------------------------------
> mm/memcontrol.c | 54 ++++++++++++++++++------------------------
> mm/page-writeback.c | 22 ++++++++++--------
> mm/rmap.c | 20 ++++++++--------
> 4 files changed, 61 insertions(+), 93 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 0daf383f8f1c..ea007615e8f9 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -139,48 +139,23 @@ static inline bool mem_cgroup_disabled(void)
> return false;
> }
>
> -void __mem_cgroup_begin_update_page_stat(struct page *page, bool *locked,
> - unsigned long *flags);
> -
> -extern atomic_t memcg_moving;
> -
> -static inline void mem_cgroup_begin_update_page_stat(struct page *page,
> - bool *locked, unsigned long *flags)
> -{
> - if (mem_cgroup_disabled())
> - return;
> - rcu_read_lock();
> - *locked = false;
> - if (atomic_read(&memcg_moving))
> - __mem_cgroup_begin_update_page_stat(page, locked, flags);
> -}
> -
> -void __mem_cgroup_end_update_page_stat(struct page *page,
> - unsigned long *flags);
> -static inline void mem_cgroup_end_update_page_stat(struct page *page,
> - bool *locked, unsigned long *flags)
> -{
> - if (mem_cgroup_disabled())
> - return;
> - if (*locked)
> - __mem_cgroup_end_update_page_stat(page, flags);
> - rcu_read_unlock();
> -}
> -
> -void mem_cgroup_update_page_stat(struct page *page,
> - enum mem_cgroup_stat_index idx,
> - int val);
> -
> -static inline void mem_cgroup_inc_page_stat(struct page *page,
> +struct mem_cgroup *mem_cgroup_begin_page_stat(struct page *page, bool *locked,
> + unsigned long *flags);
> +void mem_cgroup_end_page_stat(struct mem_cgroup *memcg, bool locked,
> + unsigned long flags);
> +void mem_cgroup_update_page_stat(struct mem_cgroup *memcg,
> + enum mem_cgroup_stat_index idx, int val);
> +
> +static inline void mem_cgroup_inc_page_stat(struct mem_cgroup *memcg,
> enum mem_cgroup_stat_index idx)
> {
> - mem_cgroup_update_page_stat(page, idx, 1);
> + mem_cgroup_update_page_stat(memcg, idx, 1);
> }
>
> -static inline void mem_cgroup_dec_page_stat(struct page *page,
> +static inline void mem_cgroup_dec_page_stat(struct mem_cgroup *memcg,
> enum mem_cgroup_stat_index idx)
> {
> - mem_cgroup_update_page_stat(page, idx, -1);
> + mem_cgroup_update_page_stat(memcg, idx, -1);
> }
>
> unsigned long mem_cgroup_soft_limit_reclaim(struct zone *zone, int order,
> @@ -315,13 +290,14 @@ mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
> {
> }
>
> -static inline void mem_cgroup_begin_update_page_stat(struct page *page,
> +static inline struct mem_cgroup *mem_cgroup_begin_page_stat(struct page *page,
> bool *locked, unsigned long *flags)
> {
> + return NULL;
> }
>
> -static inline void mem_cgroup_end_update_page_stat(struct page *page,
> - bool *locked, unsigned long *flags)
> +static inline void mem_cgroup_end_page_stat(struct mem_cgroup *memcg,
> + bool locked, unsigned long flags)
> {
> }
>
> @@ -343,12 +319,12 @@ static inline bool mem_cgroup_oom_synchronize(bool wait)
> return false;
> }
>
> -static inline void mem_cgroup_inc_page_stat(struct page *page,
> +static inline void mem_cgroup_inc_page_stat(struct mem_cgroup *memcg,
> enum mem_cgroup_stat_index idx)
> {
> }
>
> -static inline void mem_cgroup_dec_page_stat(struct page *page,
> +static inline void mem_cgroup_dec_page_stat(struct mem_cgroup *memcg,
> enum mem_cgroup_stat_index idx)
> {
> }
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 3a203c7ec6c7..d84f316ac901 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1463,12 +1463,8 @@ int mem_cgroup_swappiness(struct mem_cgroup *memcg)
> * start move here.
> */
>
> -/* for quick checking without looking up memcg */
> -atomic_t memcg_moving __read_mostly;
> -
> static void mem_cgroup_start_move(struct mem_cgroup *memcg)
> {
> - atomic_inc(&memcg_moving);
> atomic_inc(&memcg->moving_account);
> synchronize_rcu();
> }
> @@ -1479,10 +1475,8 @@ static void mem_cgroup_end_move(struct mem_cgroup *memcg)
> * Now, mem_cgroup_clear_mc() may call this function with NULL.
> * We check NULL in callee rather than caller.
> */
> - if (memcg) {
> - atomic_dec(&memcg_moving);
> + if (memcg)
> atomic_dec(&memcg->moving_account);
> - }
> }
>
> /*
> @@ -2132,26 +2126,32 @@ cleanup:
> * account and taking the move_lock in the slowpath.
> */
>
> -void __mem_cgroup_begin_update_page_stat(struct page *page,
> - bool *locked, unsigned long *flags)
> +struct mem_cgroup *mem_cgroup_begin_page_stat(struct page *page,
> + bool *locked,
> + unsigned long *flags)
> {
> struct mem_cgroup *memcg;
> struct page_cgroup *pc;
>
> + rcu_read_lock();
> +
> + if (mem_cgroup_disabled())
> + return NULL;
> +
> pc = lookup_page_cgroup(page);
> again:
> memcg = pc->mem_cgroup;
> if (unlikely(!memcg || !PageCgroupUsed(pc)))
> - return;
> + return NULL;
> /*
> * If this memory cgroup is not under account moving, we don't
> * need to take move_lock_mem_cgroup(). Because we already hold
> * rcu_read_lock(), any calls to move_account will be delayed until
> * rcu_read_unlock().
> */
> - VM_BUG_ON(!rcu_read_lock_held());
> + *locked = false;
> if (atomic_read(&memcg->moving_account) <= 0)
> - return;
> + return memcg;
>
> move_lock_mem_cgroup(memcg, flags);
> if (memcg != pc->mem_cgroup || !PageCgroupUsed(pc)) {
> @@ -2159,36 +2159,26 @@ again:
> goto again;
> }
> *locked = true;
> +
> + return memcg;
> }
>
> -void __mem_cgroup_end_update_page_stat(struct page *page, unsigned long *flags)
> +void mem_cgroup_end_page_stat(struct mem_cgroup *memcg, bool locked,
> + unsigned long flags)
> {
> - struct page_cgroup *pc = lookup_page_cgroup(page);
> + if (memcg && locked)
> + move_unlock_mem_cgroup(memcg, &flags);
>
> - /*
> - * It's guaranteed that pc->mem_cgroup never changes while
> - * lock is held because a routine modifies pc->mem_cgroup
> - * should take move_lock_mem_cgroup().
> - */
> - move_unlock_mem_cgroup(pc->mem_cgroup, flags);
> + rcu_read_unlock();
> }
>
> -void mem_cgroup_update_page_stat(struct page *page,
> +void mem_cgroup_update_page_stat(struct mem_cgroup *memcg,
> enum mem_cgroup_stat_index idx, int val)
> {
> - struct mem_cgroup *memcg;
> - struct page_cgroup *pc = lookup_page_cgroup(page);
> - unsigned long uninitialized_var(flags);
> -
> - if (mem_cgroup_disabled())
> - return;
> -
> VM_BUG_ON(!rcu_read_lock_held());
> - memcg = pc->mem_cgroup;
> - if (unlikely(!memcg || !PageCgroupUsed(pc)))
> - return;
>
> - this_cpu_add(memcg->stat->count[idx], val);
> + if (memcg)
> + this_cpu_add(memcg->stat->count[idx], val);
> }
>
> /*
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index ff6a5b07211e..19ceae87522d 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2327,11 +2327,12 @@ EXPORT_SYMBOL(clear_page_dirty_for_io);
> int test_clear_page_writeback(struct page *page)
> {
> struct address_space *mapping = page_mapping(page);
> - int ret;
> - bool locked;
> unsigned long memcg_flags;
> + struct mem_cgroup *memcg;
> + bool locked;
> + int ret;
>
> - mem_cgroup_begin_update_page_stat(page, &locked, &memcg_flags);
> + memcg = mem_cgroup_begin_page_stat(page, &locked, &memcg_flags);
> if (mapping) {
> struct backing_dev_info *bdi = mapping->backing_dev_info;
> unsigned long flags;
> @@ -2352,22 +2353,23 @@ int test_clear_page_writeback(struct page *page)
> ret = TestClearPageWriteback(page);
> }
> if (ret) {
> - mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_WRITEBACK);
> + mem_cgroup_dec_page_stat(memcg, MEM_CGROUP_STAT_WRITEBACK);
> dec_zone_page_state(page, NR_WRITEBACK);
> inc_zone_page_state(page, NR_WRITTEN);
> }
> - mem_cgroup_end_update_page_stat(page, &locked, &memcg_flags);
> + mem_cgroup_end_page_stat(memcg, locked, memcg_flags);
> return ret;
> }
>
> int __test_set_page_writeback(struct page *page, bool keep_write)
> {
> struct address_space *mapping = page_mapping(page);
> - int ret;
> - bool locked;
> unsigned long memcg_flags;
> + struct mem_cgroup *memcg;
> + bool locked;
> + int ret;
>
> - mem_cgroup_begin_update_page_stat(page, &locked, &memcg_flags);
> + memcg = mem_cgroup_begin_page_stat(page, &locked, &memcg_flags);
> if (mapping) {
> struct backing_dev_info *bdi = mapping->backing_dev_info;
> unsigned long flags;
> @@ -2394,10 +2396,10 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
> ret = TestSetPageWriteback(page);
> }
> if (!ret) {
> - mem_cgroup_inc_page_stat(page, MEM_CGROUP_STAT_WRITEBACK);
> + mem_cgroup_inc_page_stat(memcg, MEM_CGROUP_STAT_WRITEBACK);
> inc_zone_page_state(page, NR_WRITEBACK);
> }
> - mem_cgroup_end_update_page_stat(page, &locked, &memcg_flags);
> + mem_cgroup_end_page_stat(memcg, locked, memcg_flags);
> return ret;
>
> }
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 116a5053415b..f574046f77d4 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1042,15 +1042,16 @@ void page_add_new_anon_rmap(struct page *page,
> */
> void page_add_file_rmap(struct page *page)
> {
> - bool locked;
> + struct mem_cgroup *memcg;
> unsigned long flags;
> + bool locked;
>
> - mem_cgroup_begin_update_page_stat(page, &locked, &flags);
> + memcg = mem_cgroup_begin_page_stat(page, &locked, &flags);
> if (atomic_inc_and_test(&page->_mapcount)) {
> __inc_zone_page_state(page, NR_FILE_MAPPED);
> - mem_cgroup_inc_page_stat(page, MEM_CGROUP_STAT_FILE_MAPPED);
> + mem_cgroup_inc_page_stat(memcg, MEM_CGROUP_STAT_FILE_MAPPED);
> }
> - mem_cgroup_end_update_page_stat(page, &locked, &flags);
> + mem_cgroup_end_page_stat(memcg, locked, flags);
> }
>
> /**
> @@ -1061,9 +1062,10 @@ void page_add_file_rmap(struct page *page)
> */
> void page_remove_rmap(struct page *page)
> {
> + struct mem_cgroup *uninitialized_var(memcg);
> bool anon = PageAnon(page);
> - bool locked;
> unsigned long flags;
> + bool locked;
>
> /*
> * The anon case has no mem_cgroup page_stat to update; but may
> @@ -1071,7 +1073,7 @@ void page_remove_rmap(struct page *page)
> * we hold the lock against page_stat move: so avoid it on anon.
> */
> if (!anon)
> - mem_cgroup_begin_update_page_stat(page, &locked, &flags);
> + memcg = mem_cgroup_begin_page_stat(page, &locked, &flags);
>
> /* page still mapped by someone else? */
> if (!atomic_add_negative(-1, &page->_mapcount))
> @@ -1096,8 +1098,7 @@ void page_remove_rmap(struct page *page)
> -hpage_nr_pages(page));
> } else {
> __dec_zone_page_state(page, NR_FILE_MAPPED);
> - mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_FILE_MAPPED);
> - mem_cgroup_end_update_page_stat(page, &locked, &flags);
> + mem_cgroup_dec_page_stat(memcg, MEM_CGROUP_STAT_FILE_MAPPED);
> }
> if (unlikely(PageMlocked(page)))
> clear_page_mlock(page);
> @@ -1110,10 +1111,9 @@ void page_remove_rmap(struct page *page)
> * Leaving it set also helps swapoff to reinstate ptes
> * faster for those pages still in swapcache.
> */
> - return;
> out:
> if (!anon)
> - mem_cgroup_end_update_page_stat(page, &locked, &flags);
> + mem_cgroup_end_page_stat(memcg, locked, flags);
> }
>
> /*
> --
> 2.1.2
>

--
Michal Hocko
SUSE Labs