Commit 76545066 authored by Rik van Riel's avatar Rik van Riel Committed by Linus Torvalds
Browse files

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: default avatarRik van Riel <riel@redhat.com>
Tested-by: default avatarLarry Woodman <lwoodman@redhat.com>
Acked-by: default avatarLarry Woodman <lwoodman@redhat.com>
Reviewed-by: default avatarMinchan Kim <minchan.kim@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: default avatarMel Gorman <mel@csn.ul.ie>
Acked-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Tested-by: default avatarDave Young <hidave.darkstar@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 012f1800

include/linux/rmap.h

+15 −0
Original line number Diff line number Diff line
@@ -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)

mm/ksm.c

+6 −11
Original line number Diff line number Diff line
@@ -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) {

mm/migrate.c

+3 −7
Original line number Diff line number Diff line
@@ -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();

mm/rmap.c

+45 −1
Original line number Diff line number Diff line
@@ -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);
@@ -264,9 +270,13 @@ 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():

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