Subject: flush_dcache_page does too much?

From: [email-masked] (anfei)
Date: Tue, 19 Jan 2010 08:16:36 +0800

On Mon, Jan 18, 2010 at 03:01:52PM +0000, Russell King - ARM Linux wrote:
> On Mon, Jan 18, 2010 at 10:57:31PM +0800, anfei wrote:
> > On Mon, Jan 18, 2010 at 02:44:18PM +0000, Russell King - ARM Linux wrote:
> > > On Mon, Jan 18, 2010 at 10:15:30PM +0800, anfei wrote:
> > > > On Mon, Jan 18, 2010 at 02:00:05PM +0000, Russell King - ARM Linux wrote:
> > > > > On Mon, Jan 18, 2010 at 09:54:31PM +0800, anfei wrote:
> > > > > > Do you mean this implementation can ensure the coherence between write
> > > > > > and shared mmapings?  But it's easy to reproduce the alias problem by
> > > > > > this simple testcase (w/o error handler) on omap2430 with VIPT cache:
> > > > > 
> > > > > Your program doesn't do anything to identify any problem.  You don't
> > > > > even say _what_ problem you see with this program.
> > > > > 
> > > > Sorry for that.
> > > > 
> > > > > If you have a specific case which fails, please show the problem, please
> > > > > describe exactly the behaviour that you see, and what you expect to see.
> > > 
> > > Are you using a write allocate cache?
> > 
> > I guess not, because this line is neccessary to reproduce the issue:
> > 	tmp = *(addr+0);
> > If it's write allocate, this line may not be neccessary, since it's just
> > a read (and cache the data).
> 
> It makes no sense then - without write allocate, writes will go straight
> through to the underlying page, bypassing the cache.
> 
Because of the read, the write is cache hitted too even on read allocate:
       *(addr+0) = 0x44444444;	<- bypass the cache
       tmp = *(addr+0);		<- read allocate
       *(addr+1) = 0x77777777;	<- same cacheline, cache hitted

So the two write values are cached, then the sequence in sys_write
cannot guarantee the coherence:
	kmap_atomic(page);
	copy to page;
	kunmap_atomic(page);
	flush_dcache_page(page);

It should call flush_dcache_page()@the beginning too in order to
flush the shared mapping.  Actually, I think it's better to split this
function into two, such as:
	flush_dcache_user_page(page);
	copy to page;
	kunmap_atomic(page);
	flush_dcache_kern_page(page);

And this patch seems to fix it, any other fs doesn't call it need to add
that too.

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();