Re: [PATCH 5/5] extend KSM refcounts to the anon_vma root

[Mel Gorman]

On Thu, May 13, 2010 at 09:11:30AM -0400, Rik van Riel wrote:
> On 05/13/2010 07:26 AM, Mel Gorman wrote:
>> 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@xxxxxxxxxx>
>>>>> ---
>>>>>    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?
>
> It is kept around all the way from fork until exit.
>

Ok, now I get it. I was thinking in terms of a reference count being for
the duration of a specific operation. In this case, the root anon_vma
has an elevated reference count for the lifetime of the anon_vma forest.
Thanks.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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