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    SubjectRe: [PATCH 1 of 2] block_page_mkwrite() Implementation V2
    On Wed, May 16, 2007 at 11:04:11PM +1000, Nick Piggin wrote:
    > Chris Mason wrote:
    > >On Wed, May 16, 2007 at 08:09:19PM +0800, David Woodhouse wrote:
    > >
    > >>On Wed, 2007-05-16 at 11:19 +0100, David Howells wrote:
    > >>
    > >>>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

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