Re: [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath

[Mel Gorman]

On Wed, May 14, 2014 at 09:29:45PM +0200, Oleg Nesterov wrote:
> On 05/14, Oleg Nesterov wrote:
> >
> > The subsequent discussion was "off-topic", and it seems that the patch
> > itself needs a bit more discussion,
> >
> > On 05/13, Peter Zijlstra wrote:
> > >
> > > On Tue, May 13, 2014 at 01:53:13PM +0100, Mel Gorman wrote:
> > > > On Tue, May 13, 2014 at 10:45:50AM +0100, Mel Gorman wrote:
> > > > >  void unlock_page(struct page *page)
> > > > >  {
> > > > > +	wait_queue_head_t *wqh = clear_page_waiters(page);
> > > > > +
> > > > >  	VM_BUG_ON_PAGE(!PageLocked(page), page);
> > > > > +
> > > > > +	/*
> > > > > +	 * No additional barrier needed due to clear_bit_unlock barriering all updates
> > > > > +	 * before waking waiters
> > > > > +	 */
> > > > >  	clear_bit_unlock(PG_locked, &page->flags);
> > > > > -	smp_mb__after_clear_bit();
> > > > > -	wake_up_page(page, PG_locked);
> > > >
> > > > This is wrong.
> >
> > Yes,
> >
> > > > The smp_mb__after_clear_bit() is still required to ensure
> > > > that the cleared bit is visible before the wakeup on all architectures.
> >
> > But note that "the cleared bit is visible before the wakeup" is confusing.
> > I mean, we do not need mb() before __wake_up(). We need it only because
> > __wake_up_bit() checks waitqueue_active().
> 
> OOPS. Sorry Mel, I wrote this looking at the chunk above.  But when I found
> the whole patch http://marc.info/?l=linux-mm&m=139997442008267 I see that
> it removes waitqueue_active(), so this can be correct. I do not really know,
> so far I can't say I fully understand this PageWaiters() trick.
> 

The intent is to use a page bit to determine if looking up the waitqueue is
worthwhile. However, it is currently race-prone and while barriers can be
used to reduce the race, I did not see how it could be eliminated without
using a lock which would defeat the purpose.

> Hmm. But at least prepare_to_wait_exclusive() doesn't look right ;)
> 
> If nothing else, this needs abort_exclusive_wait() if killed.

Yes, I'll fix that.

> And while
> "exclusive" is probably fine for __lock_page.*(), I am not sure that
> __wait_on_page_locked_*() should be exclusive.
> 

Indeed it shouldn't. Exclusive waits should only be if the lock is being
acquired. Thanks for pointing that out.

-- 
Mel Gorman
SUSE Labs
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