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

[Rik van Riel <riel@redhat.com>]

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.

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