Re: [PATCH] per-backing dev unplugging #2

[Andrew Morton <akpm@osdl.org>]

Jens Axboe <axboe@suse.de> wrote:
>
> > 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
> 

I'd be inclined to leave that as-is actually.  I'll run with Chris's patch
temporarily, but we need a real unplug function for swapper_space.  Which
will leave default_backing_dev_info unique.

I'll do swap_unplug_io_fn().  swap implements a poor-man's raid0.  What are
the locking rules for the unplug function btw?  It can sleep, yes?