Re: [PATCH 13/13] mm: Optimize page_lock_anon_vma

[Paul E. McKenney]

On Thu, Apr 08, 2010 at 09:17:50PM +0200, Peter Zijlstra wrote:
> 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.

Some questions and a disclaimer below.

> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ---
>  mm/rmap.c |   71 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 67 insertions(+), 4 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);

On some systems, the CPU is permitted to pull references into the critical
section from either side.  So, do we also need an smp_mb() here?

> +	mutex_unlock(&anon_vma->lock);

So, a question...

Can the above mutex be contended?  If yes, what happens when the
competing mutex_lock() acquires the lock at this point?  Or, worse yet,
after the kmem_cache_free()?

If no, what do we accomplish by acquiring the lock?

If the above mutex can be contended, can we fix by substituting
synchronize_rcu_expedited()?  Which will soon require some scalability
attention if it gets used here, but what else is new?  ;-)

>  	kmem_cache_free(anon_vma_cachep, anon_vma);
>  }
> 
> @@ -291,7 +297,7 @@ 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)
>  {
> @@ -320,12 +326,70 @@ 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;
> +
> +again:
> +	rcu_read_lock();

This is interesting.  You have an RCU read-side critical section with
no rcu_dereference().

This strange state of affairs is actually legal (assuming that
anon_mapping is the RCU-protected structure) because all dereferences
of the anon_vma variable are atomic operations that guarantee ordering
(the mutex_trylock() and the atomic_inc_not_zero().

The other dereferences (the atomic_read()s) are under the lock, so
are also OK assuming that the lock is held when initializing and
updating these fields, and even more OK due to the smp_rmb() below.

But see below.

> +	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();
> 
> -	if (anon_vma)
>  		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() */

This is ordering the fetch into anon_vma against the atomic_read() below?
If so, smp_read_barrier_depends() will cover it more cheaply.  Alternatively,
use rcu_dereference() when fetching into anon_vma.

Or am I misunderstanding the purpose of this barrier?

(Disclaimer: I have not yet found anon_vma_put(), so I am assuming that
anon_vma_free() plays the role of a grace period.)

> +	if (!atomic_read(&anon_vma->ref)) {
> +		mutex_unlock(&anon_vma->lock);
> +		anon_vma = NULL;
> +	}
> +
> +unlock:
> +	rcu_read_unlock();
> +out:
> +	if (anon_vma && page_rmapping(page) != anon_vma) {
> +		mutex_unlock(&anon_vma->lock);
> +		goto again;
> +	}
> 
>  	return anon_vma;
>  }
> @@ -333,7 +397,6 @@ struct anon_vma *page_lock_anon_vma(stru
>  void page_unlock_anon_vma(struct anon_vma *anon_vma)
>  {
>  	mutex_unlock(&anon_vma->lock);
> -	anon_vma_put(anon_vma);
>  }
> 
>  /*
> 
> 
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