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    SubjectRe: [RFC v1][PATCH]page_fault retry with NOPAGE_RETRY
    On 2008-11-29 01:02, Mike Waychison wrote:
    > Nick Piggin wrote:
    >> On Thu, Nov 27, 2008 at 11:03:40AM -0800, Mike Waychison wrote:
    >>> Nick Piggin wrote:
    >>>> On Thu, Nov 27, 2008 at 01:28:41AM -0800, Mike Waychison wrote:
    >>>>> Török however identified mmap taking on the order of several
    >>>>> milliseconds due to this exact problem:
    >>>>>
    >>>>> http://lkml.org/lkml/2008/9/12/185
    >>>> Turns out to be a different problem.
    >>>>
    >>> What do you mean?
    >>
    >> His is just contending on the write side. The retry patch doesn't help.
    >>
    >
    > I disagree. How do you get 'write contention' from the following
    > paragraph:
    >
    > "Just to confirm that the problem is with pagefaults and mmap, I dropped
    > the mmap_sem in filemap_fault, and then
    > I got same performance in my testprogram for mmap and read. Of course
    > this is totally unsafe, because the mapping could change at any time."
    >
    > It reads to me that the writers were held off by the readers sleeping
    > in IO.

    It is true that I have a write/write contention too, but do_page_fault
    shows up too on lock_stat.

    This is my guess at what happens:
    * filemap_fault used to sleep with mmap_sem held while waiting for the
    page lock.
    * the google patch avoids that, which is fine: if page lock can't be
    taken, it drops mmap_sem, waits, then retries the fault once
    * however after we acquired the page lock, mapping->a_ops->readpage is
    invoked, mmap_sem is NOT dropped here:

    error = mapping->a_ops->readpage(file, page);
    if (!error) {
    wait_on_page_locked(page);

    If my understanding is correct ->readpage does the actual disk I/O, and
    it keeps the page locked, when the lock is released we know it has finished.
    So wait_on_page_locked(page) holds mmap_sem locked for read during the
    disk I/O, preventing sys_mmap/sys_munmap from making progress.

    I don't know how to prove/disprove my guess above, suggestions welcome.

    Could the patch be changed to also release the mmap_sem after readpage,
    and before wait_on_page_locked?

    Best regards,
    --Edwin
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