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Message-Id: <20100414144015.0a0d2bd2.kamezawa.hiroyu@jp.fujitsu.com>
Date: Wed, 14 Apr 2010 14:40:15 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc: LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
Minchan Kim <minchan.kim@...il.com>,
Balbir Singh <balbir@...ux.vnet.ibm.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Christoph Lameter <cl@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][BUGFIX][PATCH] memcg: fix underflow of mapped_file stat
On Wed, 14 Apr 2010 14:31:32 +0900
Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:
> > Reported-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > ---
> > include/linux/memcontrol.h | 6 +-
> > mm/memcontrol.c | 95 ++++++++++++++++++++++++---------------------
> > mm/migrate.c | 2
> > 3 files changed, 56 insertions(+), 47 deletions(-)
> >
> > Index: mmotm-temp/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-temp.orig/mm/memcontrol.c
> > +++ mmotm-temp/mm/memcontrol.c
> > @@ -2501,10 +2501,12 @@ static inline int mem_cgroup_move_swap_a
> > * Before starting migration, account PAGE_SIZE to mem_cgroup that the old
> > * page belongs to.
> > */
> > -int mem_cgroup_prepare_migration(struct page *page, struct mem_cgroup **ptr)
> > +int mem_cgroup_prepare_migration(struct page *page,
> > + struct page *newpage, struct mem_cgroup **ptr)
> > {
> > struct page_cgroup *pc;
> > struct mem_cgroup *mem = NULL;
> > + enum charge_type ctype;
> > int ret = 0;
> >
> > if (mem_cgroup_disabled())
> > @@ -2517,65 +2519,70 @@ int mem_cgroup_prepare_migration(struct
> > css_get(&mem->css);
> > }
> > unlock_page_cgroup(pc);
> > -
> > - if (mem) {
> > - ret = __mem_cgroup_try_charge(NULL, GFP_KERNEL, &mem, false);
> > - css_put(&mem->css);
> > - }
> > - *ptr = mem;
> > + /*
> > + * If the page is uncharged before migration (removed from radix-tree)
> > + * we return here.
> > + */
> > + if (!mem)
> > + return 0;
> > + ret = __mem_cgroup_try_charge(NULL, GFP_KERNEL, &mem, false);
> > + css_put(&mem->css); /* drop extra refcnt */
> it should be:
>
> *ptr = mem;
> ret = __mem_cgroup_try_charge(NULL, GFP_KERNEL, ptr, false);
> css_put(&mem->css);
>
> as Andrea has fixed already.
>
Ah, yes. I'll rebase this onto Andrea's fix.
> > + if (ret)
> > + return ret;
> > + /*
> > + * The old page is under lock_page().
> > + * If the old_page is uncharged and freed while migration, page migration
> > + * will fail and newpage will properly uncharged by end_migration.
> > + * And commit_charge against newpage never fails.
> > + */
> > + pc = lookup_page_cgroup(newpage);
> > + if (PageAnon(page))
> > + ctype = MEM_CGROUP_CHARGE_TYPE_MAPPED;
> > + else if (!PageSwapBacked(page))
> I think using page_is_file_cache() would be better.
>
Right.
> > + ctype = MEM_CGROUP_CHARGE_TYPE_CACHE;
> > + else
> > + ctype = MEM_CGROUP_CHARGE_TYPE_SHMEM;
> > + __mem_cgroup_commit_charge(mem, pc, ctype);
> > + /* FILE_MAPPED of this page will be updated at remap routine */
> > return ret;
> > }
> >
> > /* remove redundant charge if migration failed*/
> > void mem_cgroup_end_migration(struct mem_cgroup *mem,
> > - struct page *oldpage, struct page *newpage)
> > + struct page *oldpage, struct page *newpage)
> > {
> > - struct page *target, *unused;
> > - struct page_cgroup *pc;
> > - enum charge_type ctype;
> > + struct page *used, *unused;
> >
> > if (!mem)
> > return;
> > cgroup_exclude_rmdir(&mem->css);
> > +
> > +
> unnecessary extra line :)
>
will remove.
> > /* at migration success, oldpage->mapping is NULL. */
> > if (oldpage->mapping) {
> > - target = oldpage;
> > - unused = NULL;
> > + used = oldpage;
> > + unused = newpage;
> > } else {
> > - target = newpage;
> > + used = newpage;
> > unused = oldpage;
> > }
> > -
> > - if (PageAnon(target))
> > - ctype = MEM_CGROUP_CHARGE_TYPE_MAPPED;
> > - else if (page_is_file_cache(target))
> > - ctype = MEM_CGROUP_CHARGE_TYPE_CACHE;
> > - else
> > - ctype = MEM_CGROUP_CHARGE_TYPE_SHMEM;
> > -
> > - /* unused page is not on radix-tree now. */
> > - if (unused)
> > - __mem_cgroup_uncharge_common(unused, ctype);
> > -
> > - pc = lookup_page_cgroup(target);
> > - /*
> > - * __mem_cgroup_commit_charge() check PCG_USED bit of page_cgroup.
> > - * So, double-counting is effectively avoided.
> > - */
> > - __mem_cgroup_commit_charge(mem, pc, ctype);
> > -
> > + /* PageCgroupUsed() flag check will do all we want */
> > + mem_cgroup_uncharge_page(unused);
> hmm... using mem_cgroup_uncharge_page() would be enough, but I think it doesn't
> show what we want: we must uncharge "unused" by all means in PageCgroupUsed case,
> and I feel it strange a bit to uncharge "unused" by mem_cgroup_uncharge_page(),
> if it *was* a cache page.
> So I think __mem_cgroup_uncharge_common(unused, MEM_CGROUP_CHARGE_TYPE_FORCE)
> would be better, otherwise we need more comments to explain why
> mem_cgroup_uncharge_page() is enough.
>
Hmm. ok. consider this part again.
> > /*
> > - * Both of oldpage and newpage are still under lock_page().
> > - * Then, we don't have to care about race in radix-tree.
> > - * But we have to be careful that this page is unmapped or not.
> > - *
> > - * There is a case for !page_mapped(). At the start of
> > - * migration, oldpage was mapped. But now, it's zapped.
> > - * But we know *target* page is not freed/reused under us.
> > - * mem_cgroup_uncharge_page() does all necessary checks.
> > - */
> > - if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
> > - mem_cgroup_uncharge_page(target);
> > + * If old page was file cache, and removed from radix-tree
> > + * before lock_page(), perepare_migration doesn't charge and we never
> > + * reach here.
> > + *
> And if newpage was removed from radix-tree after unlock_page(),
> the context which removed it from radix-tree uncharges it properly, because
> it is charged at prepare_migration.
>
> right?
>
yes. I'll add more texts.
> > + * Considering ANON pages, we can't depend on lock_page.
> > + * If a page may be unmapped before it's remapped, new page's
> > + * mapcount will not increase. (case that mapcount 0->1 never occur.)
> > + * PageCgroupUsed() and SwapCache checks will be done.
> > + *
> > + * Once mapcount goes to 1, our hook to page_remove_rmap will do
> > + * enough jobs.
> > + */
> > + if (PageAnon(used) && !page_mapped(used))
> > + mem_cgroup_uncharge_page(used);
> mem_cgroup_uncharge_page() does the same check :)
>
Ok. I'll fix.
Thanks,
-Kame
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