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Date: Thu, 27 May 2010 23:02:12 +0900
From: Minchan Kim <minchan.kim@...il.com>
To: Rik van Riel <riel@...hat.com>
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Mel Gorman <mel@....ul.ie>,
Andrea Arcangeli <aarcange@...hat.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Lee Schermerhorn <Lee.Schermerhorn@...com>
Subject: Re: [PATCH 5/5] extend KSM refcounts to the anon_vma root
On Wed, May 26, 2010 at 03:41:24PM -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.
Hi, Rik.
Hmm, I can understand this point.
Now, rmap code always depeneds on root anon_vma's lock.
I think it doesn't depends on KSM and MIGRATION.
If we don't use KSM and MIGRATION and it is compiled out,
Can root's anon_vma disappear during rmap walking?
who prevent it?
What am I missing?
>
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
> v2:
> - merge with -mm and the compaction code
> - improve the anon_vma refcount comment in anon_vma_fork with the
> refcount lifetime
>
> include/linux/rmap.h | 15 +++++++++++++++
> mm/ksm.c | 17 ++++++-----------
> mm/migrate.c | 10 +++-------
> mm/rmap.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
> 4 files changed, 68 insertions(+), 19 deletions(-)
>
> Index: linux-2.6.34/include/linux/rmap.h
> ===================================================================
> --- linux-2.6.34.orig/include/linux/rmap.h
> +++ linux-2.6.34/include/linux/rmap.h
> @@ -81,6 +81,13 @@ static inline int anonvma_external_refco
> {
> return atomic_read(&anon_vma->external_refcount);
> }
> +
> +static inline void get_anon_vma(struct anon_vma *anon_vma)
> +{
> + atomic_inc(&anon_vma->external_refcount);
> +}
> +
> +void drop_anon_vma(struct anon_vma *);
> #else
> static inline void anonvma_external_refcount_init(struct anon_vma *anon_vma)
> {
> @@ -90,6 +97,14 @@ static inline int anonvma_external_refco
> {
> return 0;
> }
> +
> +static inline void get_anon_vma(struct anon_vma *anon_vma)
> +{
> +}
> +
> +static inline void drop_anon_vma(struct anon_vma *anon_vma)
> +{
> +}
> #endif /* CONFIG_KSM */
>
> static inline struct anon_vma *page_anon_vma(struct page *page)
> Index: linux-2.6.34/mm/ksm.c
> ===================================================================
> --- linux-2.6.34.orig/mm/ksm.c
> +++ linux-2.6.34/mm/ksm.c
> @@ -318,19 +318,14 @@ static void hold_anon_vma(struct rmap_it
> struct anon_vma *anon_vma)
> {
> rmap_item->anon_vma = anon_vma;
> - atomic_inc(&anon_vma->external_refcount);
> + get_anon_vma(anon_vma);
> }
>
> -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->external_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 *
> * 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
> 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(s
> 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) {
> Index: linux-2.6.34/mm/rmap.c
> ===================================================================
> --- linux-2.6.34.orig/mm/rmap.c
> +++ linux-2.6.34/mm/rmap.c
> @@ -235,6 +235,12 @@ int anon_vma_fork(struct vm_area_struct
> * lock any of the anon_vmas in this anon_vma tree.
> */
> anon_vma->root = pvma->anon_vma->root;
> + /*
> + * With KSM refcounts, an anon_vma can stay around longer than the
> + * process it belongs to. The root anon_vma needs to be pinned
> + * until this anon_vma is freed, because the lock lives in the root.
> + */
> + 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);
> @@ -264,8 +270,11 @@ static void anon_vma_unlink(struct anon_
> empty = list_empty(&anon_vma->head) && !anonvma_external_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)
> @@ -1389,6 +1398,40 @@ int try_to_munlock(struct page *page)
> return try_to_unmap_file(page, TTU_MUNLOCK);
> }
>
> +#if defined(CONFIG_KSM) || defined(CONFIG_MIGRATION)
> +/*
> + * 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->external_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->external_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():
> Index: linux-2.6.34/mm/migrate.c
> ===================================================================
> --- linux-2.6.34.orig/mm/migrate.c
> +++ linux-2.6.34/mm/migrate.c
> @@ -639,7 +639,7 @@ static int unmap_and_move(new_page_t get
> * exist when the page is remapped later
> */
> anon_vma = page_anon_vma(page);
> - atomic_inc(&anon_vma->external_refcount);
> + get_anon_vma(anon_vma);
> }
> }
>
> @@ -682,12 +682,8 @@ skip_unmap:
> rcu_unlock:
>
> /* Drop an anon_vma reference if we took one */
> - if (anon_vma && atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->root->lock)) {
> - int empty = list_empty(&anon_vma->head);
> - anon_vma_unlock(anon_vma);
> - if (empty)
> - anon_vma_free(anon_vma);
> - }
> + if (anon_vma)
> + drop_anon_vma(anon_vma);
>
> if (rcu_locked)
> rcu_read_unlock();
--
Kind regards,
Minchan Kim
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