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

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

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@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?

It is kept around all the way from fork until exit.

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