lkml.org 
[lkml]   [2010]   [Apr]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH 13/13] mm: Optimize page_lock_anon_vma
    From
    Date
    On Thu, 2010-04-08 at 15:18 -0700, Paul E. McKenney wrote:
    > 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?

    The thing we gain is that when the holder of the lock finds a !0
    refcount it knows it can't go away because any free will first wait to
    acquire 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? ;-)

    No, synchronize_rcu_expedited() will not work here, there is no RCU read
    side that covers the full usage of the anon_vma (there can't be, it
    needs to sleep).

    > > 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.


    Right so the only thing rcu_read_lock() does here is create the
    guarantee that anon_vma is safe to dereference (it lives on a
    SLAB_DESTROY_BY_RCU slab).

    But yes, I suppose that page->mapping read that now uses ACCESS_ONCE()
    would actually want to be an rcu_dereference(), since that both provides
    the ACCESS_ONCE() as the read-depend barrier that I thing would be
    needed.

    > > + 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?

    Yes, it is:

    atomic_dec_and_test(&anon_vma->ref) /* implies mb */

    smp_rmb();
    atomic_read(&anon_vma->ref);

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

    Yes, that lives in one of the other patches (does not exist in
    mainline).




    \
     
     \ /
      Last update: 2010-04-09 10:37    [W:7.548 / U:0.336 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site