linux-kernel - Re: [patch 1/2] mm: speculative get

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Message-Id: <1154447729.10401.16.camel@kleikamp.austin.ibm.com>
Date:	Tue, 01 Aug 2006 10:55:29 -0500
From:	Dave Kleikamp <shaggy@...tin.ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Nick Piggin <npiggin@...e.de>, Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...l.org>,
	Andy Whitcroft <apw@...dowen.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/2] mm: speculative get_page

On Tue, 2006-08-01 at 23:32 +0400, Oleg Nesterov wrote:
> Nick Piggin wrote:
> >
> > --- linux-2.6.orig/mm/vmscan.c
> > +++ linux-2.6/mm/vmscan.c
> > @@ -380,6 +380,8 @@ int remove_mapping(struct address_space 
> >  	if (!mapping)
> >  		return 0;		/* truncate got there first */
> >
> > +	SetPageNoNewRefs(page);
> > +	smp_wmb();
> >  	write_lock_irq(&mapping->tree_lock);
> >
> 
> Is it enough?
> 
> PG_nonewrefs could be already set by another add_to_page_cache()/remove_mapping(),
> and it will be cleared when we take ->tree_lock.

Isn't the page locked when calling remove_mapping()?  It looks like
SetPageNoNewRefs & ClearPageNoNewRefs are called in safe places.  Either
the page is locked, or it's newly allocated.  I could have missed
something, though.

>  For example:
> 
> CPU_0					CPU_1					CPU_3
> 
> add_to_page_cache:
> 
>     SetPageNoNewRefs();
>     write_lock_irq(->tree_lock);

      SetPageLocked(page);

>     ...
>     write_unlock_irq(->tree_lock);
> 
> 					remove_mapping:
> 	
> 					    SetPageNoNewRefs();
> 
>     ClearPageNoNewRefs();
> 					    write_lock_irq(->tree_lock);
> 
> 					    check page_count()
> 
> 										page_cache_get_speculative:
> 
> 										    increment page_count()
> 
> 										    no PG_nonewrefs => return
> 
> Oleg.

Shaggy
-- 
David Kleikamp
IBM Linux Technology Center

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