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mm: 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>
Tested-by: Larry Woodman <lwoodman@redhat.com>
Acked-by: Larry Woodman <lwoodman@redhat.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Tested-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Rik van Riel 2010-08-09 17:18:41 -07:00 committed by Linus Torvalds
parent 012f18004d
commit 76545066c8
4 changed files with 69 additions and 19 deletions
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15
include/linux/rmap.h
View File

@ -81,6 +81,13 @@ static inline int anonvma_external_refcount(struct anon_vma *anon_vma)
{
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_refcount(struct anon_vma *anon_vma)
{
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)

17
mm/ksm.c
View File

@ -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->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 *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) {

10
mm/migrate.c
View File

@ -639,7 +639,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
* 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();

46
mm/rmap.c
View File

@ -235,6 +235,12 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
* 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 +270,12 @@ static void anon_vma_unlink(struct anon_vma_chain *anon_vma_chain)
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 */
if (anon_vma->root != anon_vma)
drop_anon_vma(anon_vma->root);
anon_vma_free(anon_vma);
}
}
void unlink_anon_vmas(struct vm_area_struct *vma)
@ -1382,6 +1392,40 @@ 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():