From: Johannes Weiner <hannes@cmpxchg.org>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
Michal Hocko <mhocko@suse.cz>,
Balbir Singh <bsingharora@gmail.com>,
cgroups@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [patch 7/8] mm: memcg: modify PageCgroupAcctLRU non-atomically
Date: Thu, 24 Nov 2011 09:53:57 +0100 [thread overview]
Message-ID: <20111124085357.GA6843@cmpxchg.org> (raw)
In-Reply-To: <alpine.LSU.2.00.1111231039390.2175@sister.anvils>
On Wed, Nov 23, 2011 at 10:52:39AM -0800, Hugh Dickins wrote:
> On Wed, 23 Nov 2011, Johannes Weiner wrote:
>
> > From: Johannes Weiner <jweiner@redhat.com>
> >
> > This bit is protected by zone->lru_lock, there is no need for locked
> > operations when setting and clearing it.
> >
> > Signed-off-by: Johannes Weiner <jweiner@redhat.com>
>
> Unless there are special considerations which you have not mentioned at
> all in the description above, this 7/8 and the similar 8/8 are mistaken.
>
> The atomic operation is not for guaranteeing the setting and clearing
> of the bit in question: it's for guaranteeing that you don't accidentally
> set or clear any of the other bits in the same word when you're doing so,
> if another task is updating them at the same time as you're doing this.
>
> There are circumstances when non-atomic shortcuts can be taken, when
> you're sure the field cannot yet be visible to other tasks (we do that
> when setting PageLocked on a freshly allocated page, for example - but
> even then have to rely on others using get_page_unless_zero properly).
> But I don't think that's the case here.
I have no idea how I could oversee this. You are, of course, right.
That said, I *think* that it is safe for PageCgroupCache because
nobody else should be modifying any pc->flags concurrently:
PCG_LOCK: by definition exclusive and held during setting and clearing
PCG_CACHE
PCG_CACHE: serialized by PCG_LOCK
PCG_USED: serialized by PCG_LOCK
PCG_MIGRATION: serialized by PCG_LOCK
PCG_MOVE_LOCK: 1. update_page_stat() is only called against file
pages, and the page lock serializes charging against mapping. the
page is also charged before establishing a pte mapping, so an unmap
can not race with a charge. 2. split_huge_fixup runs against already
charged pages. 3. move_account is serialized both by LRU-isolation
during charging and PCG_LOCK
PCG_FILE_MAPPED: same as PCG_MOVE_LOCK's 1.
PCG_ACCT_LRU: pages are isolated from the LRU during charging
But all this is obviously anything but robust, and so I retract the
broken 7/8 and the might-actually-work 8/8.
next prev parent reply other threads:[~2011-11-24 8:54 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-11-23 15:42 [patch 0/8] mm: memcg fixlets for 3.3 Johannes Weiner
2011-11-23 15:42 ` [patch 1/8] mm: oom_kill: remove memcg argument from oom_kill_task() Johannes Weiner
2011-11-23 22:35 ` David Rientjes
2011-11-23 23:57 ` KAMEZAWA Hiroyuki
2011-11-24 9:07 ` Michal Hocko
2011-11-28 0:37 ` Balbir Singh
2011-11-23 15:42 ` [patch 2/8] mm: unify remaining mem_cont, mem, etc. variable names to memcg Johannes Weiner
2011-11-23 22:40 ` David Rientjes
2011-11-23 23:58 ` KAMEZAWA Hiroyuki
2011-11-24 9:17 ` Michal Hocko
2011-11-28 0:42 ` Balbir Singh
2011-11-23 15:42 ` [patch 3/8] mm: memcg: clean up fault accounting Johannes Weiner
2011-11-24 0:00 ` KAMEZAWA Hiroyuki
2011-11-24 9:33 ` Michal Hocko
2011-11-24 9:51 ` Johannes Weiner
2011-11-28 0:45 ` Balbir Singh
2011-11-23 15:42 ` [patch 4/8] mm: memcg: lookup_page_cgroup (almost) never returns NULL Johannes Weiner
2011-11-24 0:01 ` KAMEZAWA Hiroyuki
2011-11-24 9:52 ` Michal Hocko
2011-11-24 10:05 ` Johannes Weiner
2011-11-24 10:26 ` Michal Hocko
2011-11-28 9:15 ` Johannes Weiner
2011-11-28 9:34 ` Johannes Weiner
2011-11-28 10:12 ` Michal Hocko
2011-11-28 7:03 ` Balbir Singh
2011-11-28 9:17 ` Johannes Weiner
2011-11-23 15:42 ` [patch 5/8] mm: memcg: remove unneeded checks from newpage_charge() Johannes Weiner
2011-11-24 0:04 ` KAMEZAWA Hiroyuki
2011-11-24 9:04 ` Johannes Weiner
2011-11-24 10:30 ` Michal Hocko
2011-11-24 11:58 ` Johannes Weiner
2011-11-23 15:42 ` [patch 6/8] mm: memcg: remove unneeded checks from uncharge_page() Johannes Weiner
2011-11-24 0:06 ` KAMEZAWA Hiroyuki
2011-11-24 9:06 ` Johannes Weiner
2011-11-24 10:34 ` Michal Hocko
2011-11-23 15:42 ` [patch 7/8] mm: memcg: modify PageCgroupAcctLRU non-atomically Johannes Weiner
2011-11-23 18:52 ` Hugh Dickins
2011-11-24 8:53 ` Johannes Weiner [this message]
2011-11-24 0:09 ` KAMEZAWA Hiroyuki
2011-11-24 8:55 ` Johannes Weiner
2011-11-23 15:42 ` [patch 8/8] mm: memcg: modify PageCgroupCache non-atomically Johannes Weiner
2011-11-24 0:13 ` KAMEZAWA Hiroyuki
2011-11-24 9:13 ` Johannes Weiner
2011-11-24 6:09 ` [patch 0/8] mm: memcg fixlets for 3.3 Balbir Singh
2011-11-24 9:45 ` Johannes Weiner
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