On Wed, May 16, 2007 at 11:04:11PM +1000, Nick Piggin wrote:

Chris Mason wrote:

quoted

On Wed, May 16, 2007 at 08:09:19PM +0800, David Woodhouse wrote:

quoted

On Wed, 2007-05-16 at 11:19 +0100, David Howells wrote:

quoted

The start and end points passed to block_prepare_write() delimit the 
region of
the page that is going to be modified.  This means that prepare_write()
doesn't need to fill it in if the page is not up to date. 

Really? Is it _really_ going to be modified? Even if the pointer
userspace gave to write() is bogus, and is going to fault half-way
through the copy_from_user()?

This is why there are so many variations on copy_from_user that zero on
faults.  One way or another, the prepare_write/commit_write pair are
responsible for filling it in.

I'll add to David's question about David's comment on David's patch, yes
it will be modified but in that case it would be zero-filled as Chris
says. However I believe this is incorrect behaviour.

It is possible to easily fix that so it would only happen via a tiny race
window (where the source memory gets unmapped at just the right time)
however nobody seemed to interested (just by checking the return value of
fault_in_pages_readable).

The buffered write patches I'm working on fix that (among other things) of
course. But they do away with prepare_write and introduce new aops, and
they indeed must not expect the full range to have been written to.

I was also wrong to say prepare_write and commit_write are
responsible, they work together with their callers to make the right
things happen.  Oh well, so much for trying to give a short answer for a
chunk of code full of corner cases ;)

-chris

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>