linux-mm.kvack.org archive mirror
 help / color / mirror / Atom feed
From: Johannes Weiner <hannes@cmpxchg.org>
To: Shakeel Butt <shakeelb@google.com>
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 13:42:29 -0400	[thread overview]
Message-ID: <20200422174229.GD362484@cmpxchg.org> (raw)
In-Reply-To: <CALvZod4gFC1TDo8dtdaeQKj_ZEoOnQvRnw_dZANH7qQYCmnnGA@mail.gmail.com>

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.


  reply	other threads:[~2020-04-22 17:42 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 [this message]
2020-04-22 18:01       ` Shakeel Butt
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200422174229.GD362484@cmpxchg.org \
    --to=hannes@cmpxchg.org \
    --cc=alex.shi@linux.alibaba.com \
    --cc=cgroups@vger.kernel.org \
    --cc=guro@fb.com \
    --cc=hughd@google.com \
    --cc=js1304@gmail.com \
    --cc=kernel-team@fb.com \
    --cc=kirill@shutemov.name \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mhocko@suse.com \
    --cc=shakeelb@google.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).