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    SubjectRe: [PATCH 1 of 2] block_page_mkwrite() Implementation V2
    On Wed, May 16, 2007 at 11:19:29AM +0100, David Howells wrote:
    > However, page_mkwrite() isn't told which bit of the page is going to be
    > written to. This means it has to ask prepare_write() to make sure the whole
    > page is filled in. In other words, offset and to must be equal (in AFS I set
    > them both to 0).

    The assumption is the page is already up to date and we are writing
    the whole page unless EOF lands inside the page. AFAICT, we can't
    get called with a page that is not uptodate and so page filling is
    not something we should be doing (or want to be doing) here. All we
    want to do is to be able to change the mapping from a read to a
    write mapping (e.g. a read mapping of a hole needs to be changed on
    write) and do the relevant space reservation/allocation and buffer
    mapping needed for this change.

    > However, if someone adds a syscall to punch holes in files, this may change...

    We already have them - ioctl(XFS_IOC_UNRESVSP) and
    madvise(MADV_REMOVE) - and another - fallocate(FA_DEALLOCATE) - is
    on it's way. Racing with truncates should already be handled by the
    truncate code (i.e. partial page truncation does the zero filling).

    /me makes note to implement ->truncate_range() in XFS for MADV_REMOVE.


    Dave Chinner
    Principal Engineer
    SGI Australian Software Group
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