Messages in this thread | | | Date | Sun, 14 Mar 2004 21:47:01 +0100 | From | Jens Axboe <> | Subject | Re: [PATCH] per-backing dev unplugging #2 |
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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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