linux-kernel - Re: [RFC][PATCH] putback_lru_page()/unevictable page handling rework v3

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Date:	Tue, 24 Jun 2008 13:15:41 -0400
From:	Lee Schermerhorn <Lee.Schermerhorn@...com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [RFC][PATCH] putback_lru_page()/unevictable page handling
	rework v3

On Tue, 2008-06-24 at 18:49 +0900, KOSAKI Motohiro wrote:
> I found one bug ;)
> 
> > -int putback_lru_page(struct page *page)
> > +#ifdef CONFIG_UNEVICTABLE_LRU
> > +void putback_lru_page(struct page *page)
> >  {
> >  	int lru;
> > -	int ret = 1;
> >  	int was_unevictable;
> >  
> > -	VM_BUG_ON(!PageLocked(page));
> >  	VM_BUG_ON(PageLRU(page));
> >  
> > +	was_unevictable = TestClearPageUnevictable(page);
> > +
> > +redo:
> >  	lru = !!TestClearPageActive(page);
> > -	was_unevictable = TestClearPageUnevictable(page); /* for page_evictable() */
> 
> (snip)
> 
> > +	mem_cgroup_move_lists(page, lru);
> > +
> > +	/*
> > +	 * page's status can change while we move it among lru. If an evictable
> > +	 * page is on unevictable list, it never be freed. To avoid that,
> > +	 * check after we added it to the list, again.
> > +	 */
> > +	if (lru == LRU_UNEVICTABLE && page_evictable(page, NULL)) {
> > +		if (!isolate_lru_page(page)) {
> > +			put_page(page);
> > +			goto redo;
> 
> at this point, We should call ClearPageUnevictable().
> otherwise, BUG() is called on isolate_lru_pages().
> 

To which BUG() are you referring here?  There used to be a
BUG_ON(PageUnevictable(page)) in page_evictable(), but Kame-san removed
that.

By the wah, we'll never take the retry because 'lru' never ==
LRU_UNEVICTABLE in this version of putback_lru_page().  Patch to follow.

Lee

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