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SubjectRe: [PATCH 13/13] mm: Optimize page_lock_anon_vma
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@chello.nl>
> ---
> 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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