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

Andrew Morton
2014-10-22T20:39:36Z

On Wed, 22 Oct 2014 14:29:28 -0400 Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org> 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
> 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
>
> ...
>
> @@ -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)

It would be useful to document the args here (especially `locked').
Also the new rcu_read_locking protocol is worth a mention: that it
exists, what it does, why it persists as long as it does.

> {
> 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;
> }
>
>
> ...
>
> @@ -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);
> }

The anon and file paths have as much unique code as they do common
code. I wonder if page_remove_rmap() would come out better if split
into two functions? I gave that a quick try and it came out OK-looking.