Re: [patch 2/3 v2]swap: make each swap partition have one address_space

[Minchan Kim <minchan@kernel.org>]

Hi Andrew,

On Thu, Jan 31, 2013 at 01:50:42PM -0800, Andrew Morton wrote:
> On Thu, 24 Jan 2013 18:24:14 +0800
> Shaohua Li <shli@kernel.org> wrote:
> 
> > Subject: mm: add memory barrier to prevent SwapCache bit and page private out of order
> > 
> > page_mapping() checks SwapCache bit first and then read page private. Adding
> > memory barrier so page private has correct value before SwapCache bit set.
> > 
> > In some cases, page_mapping() isn't called with page locked. Without doing
> > this, we might get a wrong swap address space with SwapCache bit set. Though I
> > didn't found a problem with this so far (such code typically only checks if the
> > page has mapping or the mapping can be dirty or migrated), this is too subtle
> > and error-prone, so we want to avoid it.
> > 
> > ...
> >
> > --- linux.orig/mm/swap_state.c	2013-01-22 10:12:33.514490665 +0800
> > +++ linux/mm/swap_state.c	2013-01-24 18:08:05.149390977 +0800
> > @@ -89,6 +89,7 @@ static int __add_to_swap_cache(struct pa
> >  
> >  	page_cache_get(page);
> >  	set_page_private(page, entry.val);
> > +	smp_wmb();
> >  	SetPageSwapCache(page);
> 
> SetPageSwapCache() uses set_bit() and arch/x86/include/asm/bitops.h
> says "This function is atomic and may not be reordered".  

And says below.

 * Note: there are no guarantees that this function will not be reordered
 * on non x86 architectures, so if you are writing portable code,
 * make sure not to rely on its reordering guarantees.

-- 
Kind regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>