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    SubjectRe: [PATCH] per-backing dev unplugging #2
    On Sun, Mar 14 2004, Chris Mason wrote:
    > On Fri, 2004-03-12 at 15:51, Jens Axboe wrote:
    > > On Fri, Mar 12 2004, Chris Mason wrote:
    > > > On Fri, 2004-03-12 at 15:34, Jens Axboe wrote:
    > > >
    > > > >
    > > > > I don't see how this can make too much of a difference, aside of perhaps
    > > > > just moving the window a little. If page->mapping can disappear here,
    > > > > that's still a possibility.
    > > >
    > > > As Andrew pointed out, the mapping struct won't disappear, but
    > > > page->mapping may go null. So the idea is to use barriers to get a
    > > > trusted copy of page->mapping, and use the copy everywhere.
    > >
    > > So trusting an atomic assignment of mapping = page->mapping, it should
    > > work. It feels a bit icky, though.
    >
    > I reproduced on 2.6.4-mm1 + backing dev, but 2.6.4-mm1 alone ran fine.
    > To make a long story short, the swap address space and backing dev don't
    > define an unplug_io_fn. I was able to reproduce quickly with a swap
    > heavy workload. The patch below should fix the oops, but probably isn't
    > correct solution since no queues will get unplugged while waiting on
    > swap pages.

    Duh of course, that's pretty silly actually. So the question is if we
    want to keep assigning a dummy unplug_io_fn (default_backing_dev already
    has it), or just keep the check. I propose to check like Chris added,
    and just kill the default_unplug_io_fn() from readahead.c

    Thanks for fixing this Chris, I wonder why your back trace from this
    oops was so screwy (->unplug_io_fn() for the swap space was zero-filled,
    no?)

    --
    Jens Axboe

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