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Subject[PATCH 7/7] mm: Optimize page_lock_anon_vma
Optimize page_lock_anon_vma() by removing the atomic ref count
ops from the fast path.

Rather complicates the code a lot, but might be worth it.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
mm/rmap.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 61 insertions(+), 5 deletions(-)

Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -78,6 +78,12 @@ static inline struct anon_vma *anon_vma_
void anon_vma_free(struct anon_vma *anon_vma)
{
VM_BUG_ON(atomic_read(&anon_vma->ref));
+ /*
+ * Sync against the anon_vma->lock, so that we can hold the
+ * lock without requiring a reference. See page_lock_anon_vma().
+ */
+ mutex_lock(&anon_vma->lock);
+ mutex_unlock(&anon_vma->lock);
kmem_cache_free(anon_vma_cachep, anon_vma);
}

@@ -290,7 +296,7 @@ void __init anon_vma_init(void)

/*
* Getting a lock on a stable anon_vma from a page off the LRU is
- * tricky: page_lock_anon_vma relies on RCU to guard against the races.
+ * tricky: anon_vma_get relies on RCU to guard against the races.
*/
struct anon_vma *anon_vma_get(struct page *page)
{
@@ -312,20 +318,70 @@ out:
return anon_vma;
}

+/*
+ * Similar to anon_vma_get(), however it relies on the anon_vma->lock
+ * to pin the object. However since we cannot wait for the mutex
+ * acquisition inside the RCU read lock, we use the ref count
+ * in the slow path.
+ */
struct anon_vma *page_lock_anon_vma(struct page *page)
{
- struct anon_vma *anon_vma = anon_vma_get(page);
+ struct anon_vma *anon_vma = NULL;
+ unsigned long anon_mapping;

- if (anon_vma)
- mutex_lock(&anon_vma->lock);
+ rcu_read_lock();
+ anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
+ if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
+ goto unlock;
+ if (!page_mapped(page))
+ goto unlock;
+
+ anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
+ if (!mutex_trylock(&anon_vma->lock)) {
+ /*
+ * We failed to acquire the lock, take a ref so we can
+ * drop the RCU read lock and sleep on it.
+ */
+ if (!atomic_inc_not_zero(&anon_vma->ref)) {
+ /*
+ * Failed to get a ref, we're dead, bail.
+ */
+ anon_vma = NULL;
+ goto unlock;
+ }
+ rcu_read_unlock();

+ mutex_lock(&anon_vma->lock);
+ /*
+ * We got the lock, drop the temp. ref, if it was the last
+ * one free it and bail.
+ */
+ if (atomic_dec_and_test(&anon_vma->ref)) {
+ mutex_unlock(&anon_vma->lock);
+ anon_vma_free(anon_vma);
+ anon_vma = NULL;
+ }
+ goto out;
+ }
+ /*
+ * Got the lock, check we're still alive. Seeing a ref
+ * here guarantees the object will stay alive due to
+ * anon_vma_free() syncing against the lock we now hold.
+ */
+ smp_rmb(); /* Order against anon_vma_put() */
+ if (!atomic_read(&anon_vma->ref)) {
+ mutex_unlock(&anon_vma->lock);
+ anon_vma = NULL;
+ }
+unlock:
+ rcu_read_unlock();
+out:
return anon_vma;
}

void page_unlock_anon_vma(struct anon_vma *anon_vma)
{
mutex_unlock(&anon_vma->lock);
- anon_vma_put(anon_vma);
}

/*



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