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    SubjectRe: [RFC PATCH] asynchronous page fault.
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    Peter Zijlstra wrote:
    > On Mon, 2009-12-28 at 18:58 +0900, KAMEZAWA Hiroyuki wrote:
    >> Peter Zijlstra wrote:
    >> > On Mon, 2009-12-28 at 09:36 +0900, KAMEZAWA Hiroyuki wrote:
    >> >>
    >> >> > The idea is to let the RCU lock span whatever length you need the
    >> vma
    >> >> > for, the easy way is to simply use PREEMPT_RCU=y for now,
    >> >>
    >> >> I tried to remove his kind of reference count trick but I can't do
    >> that
    >> >> without synchronize_rcu() somewhere in unmap code. I don't like that
    >> and
    >> >> use this refcnt.
    >> >
    >> > Why, because otherwise we can access page tables for an already
    >> unmapped
    >> > vma? Yeah that is the interesting bit ;-)
    >> >
    >> Without that
    >> vma->a_ops->fault()
    >> and
    >> vma->a_ops->unmap()
    >> can be called at the same time. and vma->vm_file can be dropped while
    >> vma->a_ops->fault() is called. etc...
    >
    > Right, so acquiring the PTE lock will either instantiate page tables for
    > a non-existing vma, leaving you with an interesting mess to clean up, or
    > you can also RCU free the page tables (in the same RCU domain as the
    > vma) which will mostly[*] avoid that issue.
    >
    > [ To make live really really interesting you could even re-use the
    > page-tables and abort the RCU free when the region gets re-mapped
    > before the RCU callbacks happen, this will avoid a free/alloc cycle
    > for fast remapping workloads. ]
    >
    > Once you hold the PTE lock, you can validate the vma you looked up,
    > since ->unmap() syncs against it. If at that time you find the
    > speculative vma is dead, you fail and re-try the fault.
    >
    My previous one did similar but still used vma->refcnt. I'll consider again.

    > [*] there still is the case of faulting on an address that didn't
    > previously have page-tables hence the unmap page table scan will have
    > skipped it -- my hacks simply leaked page tables here, but the idea was
    > to acquire the mmap_sem for reading and cleanup properly.
    >
    Hmm, thank you for hints.

    But this current version implementation has some reasons.
    - because pmd has some trobles because of quicklists..I don't wanted to
    touch free routine of them.
    - pmd can be removed asynchronously while page fault is going on.
    - I'd like to avoid modification to free_pte_range etc...

    I feel pmd/page-table-lock is a hard to handle object than expected.

    I'll consider some about per-thread approach or split vma approach
    or scalable range lock or some synchronization without heavy atomic op.

    Anyway, I think I show something can be done without mmap_sem modification.
    See you next year.

    Thanks,
    -Kame



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