From: Mel Gorman <mel@csn.ul.ie>
To: Rik van Riel <riel@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Minchan Kim <minchan.kim@gmail.com>,
Linux-MM <linux-mm@kvack.org>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
LKML <linux-kernel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 5/5] extend KSM refcounts to the anon_vma root
Date: Thu, 13 May 2010 12:26:04 +0100 [thread overview]
Message-ID: <20100513112603.GB27949@csn.ul.ie> (raw)
In-Reply-To: <4BEB18FE.1090808@redhat.com>
On Wed, May 12, 2010 at 05:09:18PM -0400, Rik van Riel wrote:
> On 05/12/2010 05:07 PM, Mel Gorman wrote:
>> On Wed, May 12, 2010 at 01:41:11PM -0400, Rik van Riel wrote:
>>> Subject: extend KSM refcounts to the anon_vma root
>>>
>>> KSM reference counts can cause an anon_vma to exist after the processe
>>> it belongs to have already exited. Because the anon_vma lock now lives
>>> in the root anon_vma, we need to ensure that the root anon_vma stays
>>> around until after all the "child" anon_vmas have been freed.
>>>
>>> The obvious way to do this is to have a "child" anon_vma take a
>>> reference to the root in anon_vma_fork. When the anon_vma is freed
>>> at munmap or process exit, we drop the refcount in anon_vma_unlink
>>> and possibly free the root anon_vma.
>>>
>>> The KSM anon_vma reference count function also needs to be modified
>>> to deal with the possibility of freeing 2 levels of anon_vma. The
>>> easiest way to do this is to break out the KSM magic and make it
>>> generic.
>>>
>>> When compiling without CONFIG_KSM, this code is compiled out.
>>>
>>> Signed-off-by: Rik van Riel<riel@redhat.com>
>>> ---
>>> include/linux/rmap.h | 12 ++++++++++++
>>> mm/ksm.c | 17 ++++++-----------
>>> mm/rmap.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
>>> 3 files changed, 62 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>>> index 33ffe14..387d40c 100644
>>> --- a/include/linux/rmap.h
>>> +++ b/include/linux/rmap.h
>>> @@ -126,6 +126,18 @@ int anon_vma_fork(struct vm_area_struct *, struct vm_area_struct *);
>>> void __anon_vma_link(struct vm_area_struct *);
>>> void anon_vma_free(struct anon_vma *);
>>>
>>> +#ifdef CONFIG_KSM
>>> +static inline void get_anon_vma(struct anon_vma *anon_vma)
>>> +{
>>> + atomic_inc(&anon_vma->ksm_refcount);
>>> +}
>>> +
>>> +void drop_anon_vma(struct anon_vma *);
>>> +#else
>>> +#define get_anon_vma(x) do {} while(0)
>>> +#define drop_anon_vma(x) do {} while(0)
>>> +#endif
>>> +
>>> static inline void anon_vma_merge(struct vm_area_struct *vma,
>>> struct vm_area_struct *next)
>>> {
>>> diff --git a/mm/ksm.c b/mm/ksm.c
>>> index 7ca0dd7..9f2acc9 100644
>>> --- a/mm/ksm.c
>>> +++ b/mm/ksm.c
>>> @@ -318,19 +318,14 @@ static void hold_anon_vma(struct rmap_item *rmap_item,
>>> struct anon_vma *anon_vma)
>>> {
>>> rmap_item->anon_vma = anon_vma;
>>> - atomic_inc(&anon_vma->ksm_refcount);
>>> + get_anon_vma(anon_vma);
>>> }
>>
>> I'm not quite getting this. Here, we get the local anon_vma so we
>> increment its reference count and later we drop it but without a
>> refcount taken on the root anon_vma, why is it guaranteed to stay
>> around?
>
> Because anon_vma_fork takes a reference count on the root anon_vma,
> the VMA we take a refcount on will either have a refcount on the
> root, or it is the root.
>
Sorry, I'm still not getting it. anon_vma_fork keeps the refcount around
during fork but what about during exit? Lets say anon_vma_unlink is called
on the following arrangement;
root_anon_vma->refcounted_anon_vma
We walk the list but the root_anon_vma doesn't have a refcount so it
gets freed. drop_anon_vma gets called on refcounted_anon_vma which does
if (atomic_dec_and_lock(&anon_vma->ksm_refcount, &anon_vma->root->lock))
but the root anon_vma is now gone. Are you depending on the lifecycle of
anon_vma's within KSM for this to work? If so, then the migration-related
fixes in mmotm that take a refcount on anon_vma during migration will also
need to take a refcount on the root.
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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next prev parent reply other threads:[~2010-05-13 11:26 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-05-12 17:38 [PATCH 0/5] always lock the root anon_vma Rik van Riel
2010-05-12 17:39 ` [PATCH 1/5] rename anon_vma_lock to vma_lock_anon_vma Rik van Riel
2010-05-12 20:57 ` Mel Gorman
2010-05-13 0:30 ` KAMEZAWA Hiroyuki
2010-05-12 17:39 ` [PATCH 3/5] track the root (oldest) anon_vma Rik van Riel
2010-05-12 20:59 ` Mel Gorman
2010-05-12 21:01 ` Rik van Riel
2010-05-13 0:38 ` KAMEZAWA Hiroyuki
2010-05-13 2:25 ` Rik van Riel
2010-05-14 0:04 ` KAMEZAWA Hiroyuki
2010-05-12 17:40 ` [PATCH 4/5] always lock " Rik van Riel
2010-05-12 21:02 ` Mel Gorman
2010-05-12 21:08 ` Rik van Riel
2010-05-13 9:54 ` Mel Gorman
2010-05-13 14:33 ` [PATCH -v2 " Rik van Riel
2010-05-13 21:09 ` Andrew Morton
2010-05-13 22:50 ` Rik van Riel
2010-05-14 9:33 ` Mel Gorman
2010-05-26 4:00 ` Rik van Riel
2010-05-26 4:15 ` Andrew Morton
2010-05-26 5:46 ` james toy
2010-06-01 0:57 ` james toy
2010-05-26 15:24 ` [PATCH -v2 0/5] always lock the root anon_vma Rik van Riel
2010-05-26 15:25 ` [PATCH 1/5] rename anon_vma_lock to vma_lock_anon_vma Rik van Riel
2010-05-26 17:25 ` Linus Torvalds
2010-05-26 19:01 ` Rik van Riel
2010-05-26 19:25 ` Linus Torvalds
2010-05-26 19:35 ` Rik van Riel
2010-05-26 15:25 ` [PATCH 2/5] change direct call of spin_lock(anon_vma->lock) to inline function Rik van Riel
2010-05-26 15:26 ` [PATCH 3/5] track the root (oldest) anon_vma Rik van Riel
2010-05-26 15:27 ` [PATCH 4/5] always lock " Rik van Riel
2010-05-26 15:27 ` [PATCH 5/5] extend KSM refcounts to the anon_vma root Rik van Riel
2010-05-12 21:55 ` [PATCH 4/5] always lock the root (oldest) anon_vma Linus Torvalds
2010-05-12 22:18 ` Rik van Riel
2010-05-12 22:26 ` Linus Torvalds
2010-05-12 17:41 ` [PATCH 5/5] extend KSM refcounts to the anon_vma root Rik van Riel
2010-05-12 21:07 ` Mel Gorman
2010-05-12 21:09 ` Rik van Riel
2010-05-13 11:26 ` Mel Gorman [this message]
2010-05-13 13:11 ` Rik van Riel
2010-05-13 13:24 ` Mel Gorman
2010-05-13 14:34 ` [PATCH -v2 " Rik van Riel
2010-05-19 1:05 ` Andrea Arcangeli
2010-05-12 17:41 ` [PATCH 2/5] change direct call of spin_lock(anon_vma->lock) to inline function Rik van Riel
2010-05-12 20:58 ` Mel Gorman
2010-05-13 0:32 ` KAMEZAWA Hiroyuki
2010-05-20 22:42 ` [PATCH 6/5] adjust mm_take_all_locks to anon-vma-root locking Andrea Arcangeli
2010-05-20 23:07 ` Rik van Riel
-- strict thread matches above, loose matches on Subject: below --
2010-05-26 19:38 [PATCH -v3 0/5] always lock the root anon_vma Rik van Riel
2010-05-26 19:41 ` [PATCH 5/5] extend KSM refcounts to the anon_vma root Rik van Riel
2010-05-26 20:47 ` Larry Woodman
2010-05-27 14:02 ` Minchan Kim
2010-05-27 14:09 ` Rik van Riel
2010-05-27 14:31 ` Minchan Kim
2010-05-27 17:50 ` Mel Gorman
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