linux-kernel - Re: [rfc][patch] mm: half-fix page tail zeroing on write problem

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Date:	Fri, 2 Feb 2007 17:29:06 +1100
From:	Neil Brown <neilb@...e.de>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc][patch] mm: half-fix page tail zeroing on write problem

On Friday February 2, npiggin@...e.de wrote:
> Hi,
> 
> For no important reason, I've again looked at those zeroing patches that
> Neil did a while back. I've always thought that a simple
> `write(fd, NULL, size)` would cause the same sorts of problems.

Yeh, but who in their right mind would do that???
Oh, you did :-)

> 
> Turns out it does. If you first write all 1s into a page, then do the
> `write(fd, NULL, size)` at the same position, you end up with all 0s in
> the page (test-case available on request).  Incredible; surely this
> violates the spec?

Does it?
I guess filling with zeros isn't what one would expect, but you could
make a case for it being right.
  write(fd, 0, size)
writes 'size' 0s.  Cool.   Ok, bad-cool.

> 
> The buffered-write fixes I've got actually fix this properly, but  they
> don't look like getting merged any time soon. We could do this simple
> patch which just reduces the chance of corruption from a certainty down
> to a small race.
> 
> Any thoughts?

I cannot see why you make a change to fault_in_pages_writeable.  Is it
just for symmetry?
For the rest, it certainly makes sense to return an early -EFAULT if
you cannot fault in the page.

NeilBrown
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