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    SubjectRe: [RFC] Page table sharing


    On Tue, 19 Feb 2002, Daniel Phillips wrote:
    >
    > Somebody might read fault, changing an entry when we're in the middle of
    > copying it and might might do a duplicated read fault.

    You're confusing the mm->mmap_sem with the page_table_lock.

    The mm semaphore is really a read-write semaphore, and yes, there can be
    multiple faulters active at the same time readin gin pages.

    But the page_table_lock is 100% exclusive, and while you hold the
    page_table_lock there is absolutely _not_ going to be any concurrent page
    faulting.

    (NOTE! Sure, there might be another mm that has the same pmd shared, but
    that one is going to do an unshare before it actually touches anything in
    the pmd, so it's NOT going to change the values in the original pmd).

    So I'm personally convinced that the locking shouldn't be needed at all,
    if you just make sure that you do things in the right order (that, of
    course, might need some memory barriers, which had better be implied by
    the atomic dec-and-test anyway).

    Linus

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