linux-kernel - Re: cache alias in mmap + write

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Date:	Wed, 20 Jan 2010 17:52:42 +0800
From:	anfei <anfei.zhou@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux@....linux.org.uk, jamie@...reable.org
Subject: Re: cache alias in mmap + write

On Wed, Jan 20, 2010 at 06:10:11PM +0900, KOSAKI Motohiro wrote:
> Hello,
> 
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 96ac6b0..07056fb 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -2196,6 +2196,9 @@ again:
> >  		if (unlikely(status))
> >  			break;
> >  
> > +		if (mapping_writably_mapped(mapping))
> > +			flush_dcache_page(page);
> > +
> >  		pagefault_disable();
> >  		copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
> >  		pagefault_enable();
> 
> I'm not sure ARM cache coherency model. but I guess correct patch is here.
> 
> +		if (mapping_writably_mapped(mapping))
> +			flush_dcache_page(page);
> +
>  		pagefault_disable();
>  		copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
>  		pagefault_enable();
> -		flush_dcache_page(page);
> 
> 
> Why do we need to call flush_dcache_page() twice?
> 
The latter flush_dcache_page is used to flush the kernel changes
(iov_iter_copy_from_user_atomic), which makes the userspace to see the
write,  and the one I added is used to flush the userspace changes.
And I think it's better to split this function into two:
	flush_dcache_user_page(page);
	kmap_atomic(page);
	write to  page;
	kunmap_atomic(page);
	flush_dcache_kern_page(page);
But currently there is no such API.
> 
> 
> 
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