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

[Rik van Riel]

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.

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