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    SubjectRe: [PATCH 2] Little fixes to previous futex patch
    Hugh Dickins wrote:
    > In sys_remap_file_pages, you set the VM_NONLINEAR flag, then clear
    > it if this particular population matches the vma. No, you cannot
    > clear that flag once set, without checking every page and pte_file
    > already set within the vma. Check if population matches vma first,
    > and if it doesn't match just set the VM_NONLINEAR flag in that case.
    > (Andrew already mentioned locking: I'd have said page_table_lock,
    > but his mmap_sem is also appropriate: it's an odd case.)

    I don't see why you can't clear the flag: the call to ->populate will
    change every page and pte_file to correspond with the linear page
    offsets, which is all that !VM_NONLINEAR indicates.

    However, it _is_ wrong to clear VM_NONLINEAR before the call to
    ->populate() has finished, with Andrew's patch which uses
    downgrade_write(). Instead, the clear must come after ->populate()
    has finished.

    > I think rip out the FIXADDR_USER_START bit, it's rather over-the-top,
    > ugly: and that area is readonly, so not a useful place for a futex.

    Agreed. I put it because the old futex has it as a side effect of
    get_user_pages(). It can go.

    > The units of keys[1]: bytes if private but pages if shared.
    > That's okay for now I think, but if a hashing expert comes along
    > later s/he'll probably want to change it. The current hash does
    > add key1 to offset, which is okay: if it xor'ed you'd lose the
    > the offset bits in the private case.

    Feel free to think up a better hash that isn't slow. Two iterations
    of hash_long() would be a good hash, but slower.

    > Those keys[1] pages: in units of PAGE_SIZE in the linear case,
    > of PAGE_CACHE_SIZE in the nonlinear case. Oh well, this is far
    > from the only place with such an inconsistency, let's worry
    > about that when never comes.


    > The err at the end of __get_page_keys would be 1 from successful
    > get_user_pages, treated as error by the callers: need to make it 0.

    Well spotted.

    > futex_wait: I didn't get around to it in my version, so haven't
    > thought through the issues, but I'm a bit worried that you get
    > curval for -EWOULDBLOCK check without holding the futex_lock.
    > That looks suspicious to me, but I'm going to be lazy and not
    > try to think about it, because Rusty is sure to understand the
    > races there. If that code is insufficient as you have it, may
    > need __pin_page reinstated for just that case (hmm, was that
    > get_user right before? I'd expect it to kmap_atomic pinned page.)

    The important things are that the futex is queued prior to checking
    curval, the requested page won't change (it's protected by mmap_sem),
    and any parallel waker changes the word prior to waking us.

    You made me notice a rather subtle memory ordering condition, though.

    We must issue the read after queuing the futex. There needs to be a
    smp_rmb() after queuing and before the read, because the spin_unlock()
    barrier only constrains earlier reads, not later ones.

    Thanks for all your great insights,
    -- Jamie
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