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From: Nikita Danilov <nikita@clusterfs.com>
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Message-ID: <17359.53508.481749.294382@gargle.gargle.HOWL>
Date: Thu, 19 Jan 2006 20:48:52 +0300
To: Nick Piggin <npiggin@suse.de>
Cc: Hugh Dickins <hugh@veritas.com>, Andrew Morton <akpm@osdl.org>,
       Andrea Arcangeli <andrea@suse.de>, Linus Torvalds <torvalds@osdl.org>,
       David Miller <davem@davemloft.net>,
       Linux Kernel Mailing List <Linux-Kernel@vger.kernel.org>
Subject: Re: [patch 2/4] mm: PageLRU no testset
Newsgroups: gmane.linux.kernel,gmane.linux.kernel.mm
In-Reply-To: <20060118024128.10241.82524.sendpatchset@linux.site>
References: <20060118024106.10241.69438.sendpatchset@linux.site>
	<20060118024128.10241.82524.sendpatchset@linux.site>
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Nick Piggin writes:
 > PG_lru is protected by zone->lru_lock. It does not need TestSet/TestClear
 > operations.
 > 
 > Signed-off-by: Nick Piggin <npiggin@suse.de>
 > 
 > Index: linux-2.6/mm/vmscan.c
 > ===================================================================
 > --- linux-2.6.orig/mm/vmscan.c
 > +++ linux-2.6/mm/vmscan.c
 > @@ -775,9 +775,10 @@ int isolate_lru_page(struct page *page)
 >  	if (PageLRU(page)) {
 >  		struct zone *zone = page_zone(page);
 >  		spin_lock_irq(&zone->lru_lock);
 > -		if (TestClearPageLRU(page)) {
 > +		if (PageLRU(page)) {
 >  			ret = 1;
 >  			get_page(page);
 > +			ClearPageLRU(page);

Why is that better? ClearPageLRU() is also "atomic" operation (in the
sense of using LOCK_PREFIX on x86), so it seems this change strictly
adds cycles to the hot-path when page is on LRU.

Nikita.