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    SubjectRe: [RFC] page fault retry with NOPAGE_RETRY
    On Wed, 20 Sep 2006 09:50:35 +1000
    Benjamin Herrenschmidt <> wrote:

    > On Tue, 2006-09-19 at 16:35 -0700, Mike Waychison wrote:
    > > Patch attached.
    > >
    > > As Andrew points out, the logic is a bit hacky and using a flag in
    > > current->flags to determine whether we have done the retry or not already.
    > >
    > > I too think the right approach to being able to handle these kinds of
    > > retries in a more general fashion is to introduce a struct
    > > pagefault_args along the page faulting path. Within it, we could
    > > introduce a reason for the retry so the higher levels would be able to
    > > better understand what to do.
    > .../...
    > I need to re-read your mail and Andrew as at this point, I don't quite
    > see why we need that args and/or that current->flags bit instead of
    > always returning all the way to userland and let the faulting
    > instruction happen again (which means you don't block in the kernel, can
    > take signals etc...

    That would amount to a busy wait, waiting for the disk IO to complete.

    So we need to go to sleep somewhere (in D state, because we _are_ waiting
    for disk IO). Returning all the way to userspace and immediately retaking
    the fault is unneeded extra work.

    > thus do you actually need to prevent multiple
    > retries ?)

    I expect there are livelock scenarios. For example, process A could spin
    on posix_fadvise(some libc text page, POSIX_FADV_DONTNEED), perhaps causing
    other applications to get permanently stuck in the kernel.

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      Last update: 2006-09-20 02:01    [W:0.024 / U:3.220 seconds]
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