From: Shakeel Butt <shakeelb@google.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <js1304@gmail.com>,
Alex Shi <alex.shi@linux.alibaba.com>,
Hugh Dickins <hughd@google.com>, Michal Hocko <mhocko@suse.com>,
"Kirill A. Shutemov" <kirill@shutemov.name>,
Roman Gushchin <guro@fb.com>, Linux MM <linux-mm@kvack.org>,
Cgroups <cgroups@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH 02/18] mm: memcontrol: fix theoretical race in charge moving
Date: Wed, 22 Apr 2020 11:01:42 -0700 [thread overview]
Message-ID: <CALvZod63SORMunUa5nqNqLoKuz--tvcURs-sqHD0ds0uaM7cDg@mail.gmail.com> (raw)
In-Reply-To: <20200422174229.GD362484@cmpxchg.org>
On Wed, Apr 22, 2020 at 10:42 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Wed, Apr 22, 2020 at 09:51:20AM -0700, Shakeel Butt wrote:
> > On Mon, Apr 20, 2020 at 3:11 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > > @@ -5426,15 +5420,23 @@ static int mem_cgroup_move_account(struct page *page,
> > > }
> > >
> > > /*
> > > + * All state has been migrated, let's switch to the new memcg.
> > > + *
> > > * It is safe to change page->mem_cgroup here because the page
> > > - * is referenced, charged, and isolated - we can't race with
> > > - * uncharging, charging, migration, or LRU putback.
> > > + * is referenced, charged, isolated, and locked: we can't race
> > > + * with (un)charging, migration, LRU putback, or anything else
> > > + * that would rely on a stable page->mem_cgroup.
> > > + *
> > > + * Note that lock_page_memcg is a memcg lock, not a page lock,
> > > + * to save space. As soon as we switch page->mem_cgroup to a
> > > + * new memcg that isn't locked, the above state can change
> > > + * concurrently again. Make sure we're truly done with it.
> > > */
> > > + smp_mb();
> >
> > You said theoretical race in the subject but the above comment
> > convinced me that smp_mb() is required. So, why is the race still
> > theoretical?
>
> Sorry about the confusion.
>
> I said theoretical because I spotted it while thinking about the
> code. I'm not aware of any real users that suffered the consequences
> of this race condition. But they could exist in theory :-)
>
> I think it's a real bug that needs fixing.
Thanks for the clarification. I would suggest removing "theoretical"
from the subject as it undermines that a real bug is fixed by the
patch.
next prev parent reply other threads:[~2020-04-22 18:01 UTC|newest]
Thread overview: 76+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-20 22:11 [PATCH 00/18] mm: memcontrol: charge swapin pages on instantiation Johannes Weiner
2020-04-20 22:11 ` [PATCH 01/18] mm: fix NUMA node file count error in replace_page_cache() Johannes Weiner
2020-04-21 8:28 ` Alex Shi
2020-04-21 19:13 ` Shakeel Butt
2020-04-22 6:34 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 02/18] mm: memcontrol: fix theoretical race in charge moving Johannes Weiner
2020-04-22 6:36 ` Joonsoo Kim
2020-04-22 16:51 ` Shakeel Butt
2020-04-22 17:42 ` Johannes Weiner
2020-04-22 18:01 ` Shakeel Butt [this message]
2020-04-22 18:02 ` Shakeel Butt
2020-04-20 22:11 ` [PATCH 03/18] mm: memcontrol: drop @compound parameter from memcg charging API Johannes Weiner
2020-04-21 9:11 ` Alex Shi
2020-04-22 6:37 ` Joonsoo Kim
2020-04-22 17:30 ` Shakeel Butt
2020-04-20 22:11 ` [PATCH 04/18] mm: memcontrol: move out cgroup swaprate throttling Johannes Weiner
2020-04-21 9:11 ` Alex Shi
2020-04-22 6:37 ` Joonsoo Kim
2020-04-22 22:20 ` Shakeel Butt
2020-04-20 22:11 ` [PATCH 05/18] mm: memcontrol: convert page cache to a new mem_cgroup_charge() API Johannes Weiner
2020-04-21 9:12 ` Alex Shi
2020-04-22 6:40 ` Joonsoo Kim
2020-04-22 12:09 ` Johannes Weiner
2020-04-23 5:25 ` Joonsoo Kim
2020-05-08 16:01 ` Johannes Weiner
2020-05-11 1:57 ` Joonsoo Kim
2020-05-11 7:38 ` Hugh Dickins
2020-05-11 15:06 ` Johannes Weiner
2020-05-11 16:32 ` Hugh Dickins
2020-05-11 18:10 ` Johannes Weiner
2020-05-11 18:12 ` Johannes Weiner
2020-05-11 18:44 ` Hugh Dickins
2020-04-20 22:11 ` [PATCH 06/18] mm: memcontrol: prepare uncharging for removal of private page type counters Johannes Weiner
2020-04-21 9:12 ` Alex Shi
2020-04-22 6:41 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 07/18] mm: memcontrol: prepare move_account " Johannes Weiner
2020-04-21 9:13 ` Alex Shi
2020-04-22 6:41 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 08/18] mm: memcontrol: prepare cgroup vmstat infrastructure for native anon counters Johannes Weiner
2020-04-22 6:42 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 09/18] mm: memcontrol: switch to native NR_FILE_PAGES and NR_SHMEM counters Johannes Weiner
2020-04-22 6:42 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 10/18] mm: memcontrol: switch to native NR_ANON_MAPPED counter Johannes Weiner
2020-04-22 6:51 ` Joonsoo Kim
2020-04-22 12:28 ` Johannes Weiner
2020-04-23 5:27 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 11/18] mm: memcontrol: switch to native NR_ANON_THPS counter Johannes Weiner
2020-04-24 0:29 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 12/18] mm: memcontrol: convert anon and file-thp to new mem_cgroup_charge() API Johannes Weiner
2020-04-24 0:29 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 13/18] mm: memcontrol: drop unused try/commit/cancel charge API Johannes Weiner
2020-04-24 0:30 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 14/18] mm: memcontrol: prepare swap controller setup for integration Johannes Weiner
2020-04-24 0:30 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 15/18] mm: memcontrol: make swap tracking an integral part of memory control Johannes Weiner
2020-04-21 9:27 ` Alex Shi
2020-04-21 14:39 ` Johannes Weiner
2020-04-22 3:14 ` Alex Shi
2020-04-22 13:30 ` Johannes Weiner
2020-04-22 13:40 ` Alex Shi
2020-04-22 13:43 ` Alex Shi
2020-04-24 0:30 ` Joonsoo Kim
2020-04-24 3:01 ` Johannes Weiner
2020-04-20 22:11 ` [PATCH 16/18] mm: memcontrol: charge swapin pages on instantiation Johannes Weiner
2020-04-21 9:21 ` Alex Shi
2020-04-24 0:44 ` Joonsoo Kim
2020-04-24 2:51 ` Johannes Weiner
2020-04-28 6:49 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 17/18] mm: memcontrol: delete unused lrucare handling Johannes Weiner
2020-04-24 0:46 ` Joonsoo Kim
2020-04-20 22:11 ` [PATCH 18/18] mm: memcontrol: update page->mem_cgroup stability rules Johannes Weiner
2020-04-21 9:20 ` Alex Shi
2020-04-24 0:48 ` Joonsoo Kim
2020-04-21 9:10 ` Hillf Danton
2020-04-21 14:34 ` Johannes Weiner
2020-04-21 9:32 ` [PATCH 00/18] mm: memcontrol: charge swapin pages on instantiation Alex Shi
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