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

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Date:	Thu, 03 May 2007 22:43:20 +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:
> 
>>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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