linux-kernel - Re: [PATCH] mm:D-cache aliasing issue in cow_user

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Message-Id: <20061019.002237.130236131.davem@davemloft.net>
Date:	Thu, 19 Oct 2006 00:22:37 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	akpm@...l.org
Cc:	dmonakhov@...nvz.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm:D-cache aliasing issue in cow_user_page

From: Andrew Morton <akpm@...l.org>
Date: Thu, 19 Oct 2006 00:17:47 -0700

> On Thu, 19 Oct 2006 00:00:27 -0700 (PDT)
> David Miller <davem@...emloft.net> wrote:
> 
> > Unfortunately, the kernel has just touched the page and thus there are
> > active cache lines for the kernel side mapping.  When we map this into
> > user space, userspace might see stale cachelines instead of the
> > memset() stores.
> 
> hm.  Has it always been that way or did something change?

Always.

> > Architectures typically take care of this in copy_user_page() and
> > clear_user_page().  The absolutely depend upon those two routines
> > being used for anonymous pages, and handle the D-cache issues there.
> 
> Only anonymous pages?  There are zillions of places where we modify
> pagecache without a flush, especially against the blockdev mapping (fs
> metadata).

It's cpu stores that matter, not device DMA and the like, and we have
flush_dcache_page() calls in the correct spots.  You can see that
we take care of this even in places such as the loop driver :-)
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