[PATCH 6/8] mm: Use refcounts for page_lock_anon_vma()

Peter Zijlstra
2011-03-02T18:00:11Z

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Convert page_lock_anon_vma() over to use refcounts. This is done to
prepare for the conversion of anon_vma from spinlock to mutex.

Sadly this inceases the cost of page_lock_anon_vma() from one to two
atomics, a follow up patch addresses this, lets keep that simple for
now.

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
mm/migrate.c | 17 ++++-------------
mm/rmap.c | 42 +++++++++++++++++++++++++++---------------
2 files changed, 31 insertions(+), 28 deletions(-)

Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -336,9 +336,9 @@ void __init anon_vma_init(void)
* that the anon_vma pointer from page->mapping is valid if there is a
* mapcount, we can dereference the anon_vma after observing those.
*/
-struct anon_vma *page_lock_anon_vma(struct page *page)
+struct anon_vma *page_get_anon_vma(struct page *page)
{
- struct anon_vma *anon_vma, *root_anon_vma;
+ struct anon_vma *anon_vma = NULL;
unsigned long anon_mapping;

rcu_read_lock();
@@ -349,30 +349,42 @@ struct anon_vma *page_lock_anon_vma(stru
goto out;

anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
- root_anon_vma = ACCESS_ONCE(anon_vma->root);
- spin_lock(&root_anon_vma->lock);
+ if (!atomic_inc_not_zero(&anon_vma->refcount)) {
+ anon_vma = NULL;
+ goto out;
+ }

/*
* If this page is still mapped, then its anon_vma cannot have been
- * freed. But if it has been unmapped, we have no security against
- * the anon_vma structure being freed and reused (for another anon_vma:
- * SLAB_DESTROY_BY_RCU guarantees that - so the spin_lock above cannot
- * corrupt): with anon_vma_prepare() or anon_vma_fork() redirecting
- * anon_vma->root before page_unlock_anon_vma() is called to unlock.
+ * freed. But if it has been unmapped, we have no security against the
+ * anon_vma structure being freed and reused (for another anon_vma:
+ * SLAB_DESTROY_BY_RCU guarantees that - so the atomic_inc_not_zero()
+ * above cannot corrupt).
*/
- if (page_mapped(page))
- return anon_vma;
-
- spin_unlock(&root_anon_vma->lock);
+ if (!page_mapped(page)) {
+ put_anon_vma(anon_vma);
+ anon_vma = NULL;
+ }
out:
rcu_read_unlock();
- return NULL;
+
+ return anon_vma;
+}
+
+struct anon_vma *page_lock_anon_vma(struct page *page)
+{
+ struct anon_vma *anon_vma = page_get_anon_vma(page);
+
+ if (anon_vma)
+ anon_vma_lock(anon_vma);
+
+ return anon_vma;
}

void page_unlock_anon_vma(struct anon_vma *anon_vma)
{
anon_vma_unlock(anon_vma);
- rcu_read_unlock();
+ put_anon_vma(anon_vma);
}

/*
Index: linux-2.6/mm/migrate.c
===================================================================
--- linux-2.6.orig/mm/migrate.c
+++ linux-2.6/mm/migrate.c
@@ -703,15 +703,11 @@ static int unmap_and_move(new_page_t get
* Only page_lock_anon_vma() understands the subtleties of
* getting a hold on an anon_vma from outside one of its mms.
*/
- anon_vma = page_lock_anon_vma(page);
+ anon_vma = page_get_anon_vma(page);
if (anon_vma) {
/*
- * Take a reference count on the anon_vma if the
- * page is mapped so that it is guaranteed to
- * exist when the page is remapped later
+ * Anon page
*/
- get_anon_vma(anon_vma);
- page_unlock_anon_vma(anon_vma);
} else if (PageSwapCache(page)) {
/*
* We cannot be sure that the anon_vma of an unmapped
@@ -840,13 +836,8 @@ static int unmap_and_move_huge_page(new_
lock_page(hpage);
}

- if (PageAnon(hpage)) {
- anon_vma = page_lock_anon_vma(hpage);
- if (anon_vma) {
- get_anon_vma(anon_vma);
- page_unlock_anon_vma(anon_vma);
- }
- }
+ if (PageAnon(hpage))
+ anon_vma = page_get_anon_vma(hpage);

try_to_unmap(hpage, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);