Re: 2.6.22 -mm merge plans -- vm bugfixes

[Nick Piggin]

Hugh Dickins wrote:
> On Thu, 3 May 2007, Nick Piggin wrote:
>
> > The problem is that lock/unlock_page is expensive on powerpc, and
> > if we improve that, we improve more than just the fault handler...
> >
> > The attached patch gets performance up a bit by avoiding some
> > barriers and some cachelines:
>
>
> There's a strong whiff of raciness about this...
> but I could very easily be wrong.
>
>
> > Index: linux-2.6/mm/filemap.c
> > ===================================================================
> > --- linux-2.6.orig/mm/filemap.c	2007-05-02 15:00:26.000000000 +1000
> > +++ linux-2.6/mm/filemap.c	2007-05-03 08:34:32.000000000 +1000
> > @@ -532,11 +532,13 @@
> >  */
> > void fastcall unlock_page(struct page *page)
> > {
> > +	VM_BUG_ON(!PageLocked(page));
> > 	smp_mb__before_clear_bit();
> > -	if (!TestClearPageLocked(page))
> > -		BUG();
> > -	smp_mb__after_clear_bit(); 
> > -	wake_up_page(page, PG_locked);
> > +	ClearPageLocked(page);
> > +	if (unlikely(test_bit(PG_waiters, &page->flags))) {
> > +		clear_bit(PG_waiters, &page->flags);
> > +		wake_up_page(page, PG_locked);
> > +	}
> > }
> > EXPORT_SYMBOL(unlock_page);
> >
> > @@ -568,6 +570,11 @@ __lock_page (diff -p would tell us!)
> > {
> > 	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
> >
> > +	set_bit(PG_waiters, &page->flags);
> > +	if (unlikely(!TestSetPageLocked(page))) {
>
>
> What happens if another cpu is coming through __lock_page at the
> same time, did its set_bit, now finds PageLocked, and so proceeds
> to the __wait_on_bit_lock?  But this cpu now clears PG_waiters,
> so this task's unlock_page won't wake the other?

You're right, we can't clear the bit here. Doubt it mattered much anyway?

BTW. I also forgot an smp_mb__after_clear_bit() before the wake_up_page
above... that barrier is in the slow path as well though, so it shouldn't
matter either.

> > +		clear_bit(PG_waiters, &page->flags);
> > +		return;
> > +	}
> > 	__wait_on_bit_lock(page_waitqueue(page), &wait, sync_page,
> > 							TASK_UNINTERRUPTIBLE);
> > }


--
SUSE Labs, Novell Inc.
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