From: Hugh Dickins <hughd@google.com>
To: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Petr Holasek <pholasek@redhat.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Izik Eidus <izik.eidus@ravellosystems.com>,
KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 11/11] ksm: stop hotremove lockdep warning
Date: Mon, 11 Feb 2013 14:13:48 -0800 (PST) [thread overview]
Message-ID: <alpine.LNX.2.00.1302111410290.1174@eggly.anvils> (raw)
In-Reply-To: <20130208194510.65fadd37@thinkpad.boeblingen.de.com>
On Fri, 8 Feb 2013, Gerald Schaefer wrote:
> On Fri, 25 Jan 2013 18:10:18 -0800 (PST)
> Hugh Dickins <hughd@google.com> wrote:
>
> > Complaints are rare, but lockdep still does not understand the way
> > ksm_memory_callback(MEM_GOING_OFFLINE) takes ksm_thread_mutex, and
> > holds it until the ksm_memory_callback(MEM_OFFLINE): that appears
> > to be a problem because notifier callbacks are made under down_read
> > of blocking_notifier_head->rwsem (so first the mutex is taken while
> > holding the rwsem, then later the rwsem is taken while still holding
> > the mutex); but is not in fact a problem because mem_hotplug_mutex
> > is held throughout the dance.
> >
> > There was an attempt to fix this with mutex_lock_nested(); but if that
> > happened to fool lockdep two years ago, apparently it does so no
> > longer.
> >
> > I had hoped to eradicate this issue in extending KSM page migration
> > not to need the ksm_thread_mutex. But then realized that although
> > the page migration itself is safe, we do still need to lock out ksmd
> > and other users of get_ksm_page() while offlining memory - at some
> > point between MEM_GOING_OFFLINE and MEM_OFFLINE, the struct pages
> > themselves may vanish, and get_ksm_page()'s accesses to them become a
> > violation.
> >
> > So, give up on holding ksm_thread_mutex itself from MEM_GOING_OFFLINE
> > to MEM_OFFLINE, and add a KSM_RUN_OFFLINE flag, and
> > wait_while_offlining() checks, to achieve the same lockout without
> > being caught by lockdep. This is less elegant for KSM, but it's more
> > important to keep lockdep useful to other users - and I apologize for
> > how long it took to fix.
>
> Thanks a lot for the patch! I verified that it fixes the lockdep warning
> that we got on memory hotremove.
>
> >
> > Reported-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
> > Signed-off-by: Hugh Dickins <hughd@google.com>
Thank you for reporting and testing and reporting back:
sorry again for taking so long to fix it.
Hugh
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next prev parent reply other threads:[~2013-02-11 22:13 UTC|newest]
Thread overview: 69+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-01-26 1:53 [PATCH 0/11] ksm: NUMA trees and page migration Hugh Dickins
2013-01-26 1:54 ` [PATCH 1/11] ksm: allow trees per NUMA node Hugh Dickins
2013-01-27 1:14 ` Simon Jeons
2013-01-27 2:54 ` Hugh Dickins
2013-01-27 3:16 ` Simon Jeons
2013-01-27 21:55 ` Hugh Dickins
2013-01-28 23:03 ` Andrew Morton
2013-01-29 1:17 ` Hugh Dickins
2013-01-28 23:08 ` Andrew Morton
2013-01-29 1:38 ` Hugh Dickins
2013-02-05 16:41 ` Mel Gorman
2013-02-07 23:57 ` Hugh Dickins
2013-01-26 1:56 ` [PATCH 2/11] ksm: add sysfs ABI Documentation Hugh Dickins
2013-01-26 1:58 ` [PATCH 3/11] ksm: trivial tidyups Hugh Dickins
2013-01-28 23:11 ` Andrew Morton
2013-01-29 1:44 ` Hugh Dickins
2013-01-26 1:59 ` [PATCH 4/11] ksm: reorganize ksm_check_stable_tree Hugh Dickins
2013-02-05 16:48 ` Mel Gorman
2013-02-08 0:07 ` Hugh Dickins
2013-02-14 11:30 ` Mel Gorman
2013-01-26 2:00 ` [PATCH 5/11] ksm: get_ksm_page locked Hugh Dickins
2013-01-27 2:36 ` Simon Jeons
2013-01-27 22:08 ` Hugh Dickins
2013-01-28 0:36 ` Simon Jeons
2013-01-28 3:35 ` Hugh Dickins
2013-01-27 2:48 ` Simon Jeons
2013-01-27 22:10 ` Hugh Dickins
2013-02-05 17:18 ` Mel Gorman
2013-02-08 0:33 ` Hugh Dickins
2013-02-14 11:34 ` Mel Gorman
2013-01-26 2:01 ` [PATCH 6/11] ksm: remove old stable nodes more thoroughly Hugh Dickins
2013-01-27 4:55 ` Simon Jeons
2013-01-27 23:05 ` Hugh Dickins
2013-01-28 1:42 ` Simon Jeons
2013-01-28 4:14 ` Hugh Dickins
2013-01-28 2:12 ` Simon Jeons
2013-01-28 4:19 ` Hugh Dickins
2013-01-28 6:36 ` Simon Jeons
2013-01-28 23:44 ` Andrew Morton
2013-01-29 2:03 ` Hugh Dickins
2013-02-05 17:55 ` Mel Gorman
2013-02-08 19:33 ` Hugh Dickins
2013-02-14 11:58 ` Mel Gorman
2013-02-14 22:19 ` Hugh Dickins
2013-01-26 2:03 ` [PATCH 7/11] ksm: make KSM page migration possible Hugh Dickins
2013-01-27 5:47 ` Simon Jeons
2013-01-27 23:12 ` Hugh Dickins
2013-01-28 0:41 ` Simon Jeons
2013-01-28 3:44 ` Hugh Dickins
2013-02-05 19:11 ` Mel Gorman
2013-02-08 20:52 ` Hugh Dickins
2013-01-26 2:05 ` [PATCH 8/11] ksm: make !merge_across_nodes migration safe Hugh Dickins
2013-01-27 8:49 ` Simon Jeons
2013-01-27 23:25 ` Hugh Dickins
2013-01-28 3:44 ` Simon Jeons
2013-01-26 2:06 ` [PATCH 9/11] ksm: enable KSM page migration Hugh Dickins
2013-01-26 2:07 ` [PATCH 10/11] mm: remove offlining arg to migrate_pages Hugh Dickins
2013-01-26 2:10 ` [PATCH 11/11] ksm: stop hotremove lockdep warning Hugh Dickins
2013-01-27 6:23 ` Simon Jeons
2013-01-27 23:35 ` Hugh Dickins
2013-02-08 18:45 ` Gerald Schaefer
2013-02-11 22:13 ` Hugh Dickins [this message]
2013-01-28 23:54 ` [PATCH 0/11] ksm: NUMA trees and page migration Andrew Morton
2013-01-29 0:49 ` Izik Eidus
2013-01-29 2:26 ` Izik Eidus
2013-01-29 16:51 ` Andrea Arcangeli
2013-01-31 0:05 ` Ric Mason
2013-01-29 1:07 ` Hugh Dickins
2013-01-29 10:45 ` Gleb Natapov
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