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    SubjectRe: Race between vmtruncate and mapped areas?
    On Tue, May 13, 2003 at 05:26:24PM -0500, Dave McCracken wrote:
    > Hmm... Yep, it is. I did some more investigating. My initial scenario
    > required that the task mapping the page extend the file after the truncate,
    > which must be done via some kind of write(). The write() would trip over
    > i_sem and therefore hang waiting for vmtruncate() to complete. So I was
    > wrong about that one.
    > Hoever, vmtruncate() does get to truncate_complete_page() with a page
    > that's mapped...
    > After some though it occurred to me there is a simple alternative scenario
    > that's not protected. If a task is *already* in a page fault mapping the
    > page in, then vmtruncate() could call zap_page_range() before the page
    > fault completes. When the page fault does complete the page will be mapped
    > into the area previously cleared by vmtruncate().
    > We could make vmtruncate() take mmap_sem for write, but that seems somewhat
    > drastic. Does anyone have any alternative ideas?

    That doesn't sound like it's going to help, there isn't a unique
    mmap_sem to be taken and so we just get caught between acquisitions
    with the same problem.


    -- wli
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