Re: [rfc][patch] mm: half-fix page tail zeroing on write problem
- From: Neil Brown <neilb@xxxxxxx>
- Date: Fri, 2 Feb 2007 17:29:06 +1100
On Friday February 2, npiggin@xxxxxxx 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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