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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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  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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