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

>
> -static void drop_anon_vma(struct rmap_item *rmap_item)
> +static void ksm_drop_anon_vma(struct rmap_item *rmap_item)
> {
> struct anon_vma *anon_vma = rmap_item->anon_vma;
>
> - if (atomic_dec_and_lock(&anon_vma->ksm_refcount, &anon_vma->root->lock)) {
> - int empty = list_empty(&anon_vma->head);
> - anon_vma_unlock(anon_vma);
> - if (empty)
> - anon_vma_free(anon_vma);
> - }
> + drop_anon_vma(anon_vma);
> }
>
> /*
> @@ -415,7 +410,7 @@ static void break_cow(struct rmap_item *rmap_item)
> * It is not an accident that whenever we want to break COW
> * to undo, we also need to drop a reference to the anon_vma.
> */
> - drop_anon_vma(rmap_item);
> + ksm_drop_anon_vma(rmap_item);
>
> down_read(&mm->mmap_sem);
> if (ksm_test_exit(mm))
> @@ -470,7 +465,7 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node)
> ksm_pages_sharing--;
> else
> ksm_pages_shared--;
> - drop_anon_vma(rmap_item);
> + ksm_drop_anon_vma(rmap_item);
> rmap_item->address &= PAGE_MASK;
> cond_resched();
> }
> @@ -558,7 +553,7 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
> else
> ksm_pages_shared--;
>
> - drop_anon_vma(rmap_item);
> + ksm_drop_anon_vma(rmap_item);
> rmap_item->address &= PAGE_MASK;
>
> } else if (rmap_item->address & UNSTABLE_FLAG) {
> diff --git a/mm/rmap.c b/mm/rmap.c
> index f0ba648..d63cd91 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -238,6 +238,12 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
> */
> root_avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
> anon_vma->root = root_avc->anon_vma;
> + /*
> + * With KSM refcounts, an anon_vma can stay around longer than the
> + * process it belongs to. The root anon_vma needs to be pinned,
> + * because that is where the lock lives.
> + */
> + get_anon_vma(anon_vma->root);
> /* Mark this anon_vma as the one where our new (COWed) pages go. */
> vma->anon_vma = anon_vma;
> anon_vma_chain_link(vma, avc, anon_vma);
> @@ -267,8 +273,11 @@ static void anon_vma_unlink(struct anon_vma_chain *anon_vma_chain)
> empty = list_empty(&anon_vma->head) && !ksm_refcount(anon_vma);
> anon_vma_unlock(anon_vma);
>
> - if (empty)
> + if (empty) {
> + /* We no longer need the root anon_vma */
> + drop_anon_vma(anon_vma->root);
> anon_vma_free(anon_vma);
> + }
> }
>
> void unlink_anon_vmas(struct vm_area_struct *vma)
> @@ -1355,6 +1364,40 @@ int try_to_munlock(struct page *page)
> return try_to_unmap_file(page, TTU_MUNLOCK);
> }
>
> +#ifdef CONFIG_KSM
> +/*
> + * Drop an anon_vma refcount, freeing the anon_vma and anon_vma->root
> + * if necessary. Be careful to do all the tests under the lock. Once
> + * we know we are the last user, nobody else can get a reference and we
> + * can do the freeing without the lock.
> + */
> +void drop_anon_vma(struct anon_vma *anon_vma)
> +{
> + if (atomic_dec_and_lock(&anon_vma->ksm_refcount, &anon_vma->root->lock)) {
> + struct anon_vma *root = anon_vma->root;
> + int empty list_empty(&anon_vma->head);
> + int last_root_user = 0;
> + int root_empty = 0;
> +
> + /*
> + * The refcount on a non-root anon_vma got dropped. Drop
> + * the refcount on the root and check if we need to free it.
> + */
> + if (empty && anon_vma != root) {
> + last_root_user = atomic_dec_and_test(&root->ksm_refcount);
> + root_empty = list_empty(&root->head);
> + }
> + anon_vma_unlock(anon_vma);
> +
> + if (empty) {
> + anon_vma_free(anon_vma);
> + if (root_empty && last_root_user)
> + anon_vma_free(root);
> + }
> + }
> +}
> +#endif
> +
> #ifdef CONFIG_MIGRATION
> /*
> * rmap_walk() and its helpers rmap_walk_anon() and rmap_walk_file():
>

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


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