linux-kernel - Re: 2.6.22 -mm merge plans -- vm bugfixes

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Message-ID: <4639DED0.2070507@yahoo.com.au>
Date:	Thu, 03 May 2007 23:08:32 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Hugh Dickins <hugh@...itas.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrea Arcangeli <andrea@...e.de>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: 2.6.22 -mm merge plans -- vm bugfixes

Hugh Dickins wrote:
> On Thu, 3 May 2007, Nick Piggin wrote:
> 
>>>>@@ -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?
> 
> 
> Ah yes, that's a good easy answer.  In fact, just remove this whole
> test and block (we already tried TestSetPageLocked outside just a
> short while ago, so this repeat won't often save anything).

Yeah, I was getting too clever for my own boots :)

I think the patch has merit though. Unfortunate that it uses another page
flag, however it seemed to have quite a bit speedup on unlock_page (probably
from both the barriers and an extra random cacheline load (from the hash)).

I guess it has to get good results from more benchmarks...


>>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.
> 
> 
> I vaguely wondered how such barriers had managed to dissolve away,
> but cranking my brain up to think about barriers takes far too long.

That barrier was one too many :)

However I believe the fastpath barrier can go away because the PG_locked
operation is depending on the same cacheline as PG_waiters.

-- 
SUSE Labs, Novell Inc.
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