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    SubjectRe: [patch 1/6] mmu_notifier: Core code
    On Wed, Jan 30, 2008 at 04:20:35PM -0600, Robin Holt wrote:
    > On Wed, Jan 30, 2008 at 11:19:28AM -0800, Christoph Lameter wrote:
    > > On Wed, 30 Jan 2008, Jack Steiner wrote:
    > >
    > > > Moving to a different lock solves the problem.
    > >
    > > Well it gets us back to the issue why we removed the lock. As Robin said
    > > before: If its global then we can have a huge number of tasks contending
    > > for the lock on startup of a process with a large number of ranks. The
    > > reason to go to mmap_sem was that it was placed in the mm_struct and so we
    > > would just have a couple of contentions per mm_struct.
    > >
    > > I'll be looking for some other way to do this.
    > I think Andrea's original concept of the lock in the mmu_notifier_head
    > structure was the best. I agree with him that it should be a spinlock
    > instead of the rw_lock.

    BTW, I don't see the scalability concern with huge number of tasks:
    the lock is still in the mm, down_write(mm->mmap_sem); oneinstruction;
    up_write(mm->mmap_sem) is always going to scale worse than
    spin_lock(mm->somethingelse); oneinstruction;

    Furthermore if we go this route and we don't relay on implicit
    serialization of all the mmu notifier users against exit_mmap
    (i.e. the mmu notifier user must agree to stop calling
    mmu_notifier_register on a mm after the last mmput) the autodisarming
    feature will likely have to be removed or it can't possibly be safe to
    run mmu_notifier_unregister while mmu_notifier_release runs. With the
    auto-disarming feature, there is no way to safely know if
    mmu_notifier_unregister has to be called or not. I'm ok with removing
    the auto-disarming feature and to have as self-contained-as-possible
    locking. Then mmu_notifier_release can just become the
    invalidate_all_after and invalidate_all, invalidate_all_before.

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