Messages in this thread | | | Date | Wed, 12 May 2010 17:09:18 -0400 | From | Rik van Riel <> | Subject | Re: [PATCH 5/5] extend KSM refcounts to the anon_vma root |
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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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