Re: [PATCH] floppy: switch to one queue per drive instead of sharing a queue (Was: Re: cgq vs bdi names, was "cfq-iosched: fix a kernel OOPs when usb key is inserted")

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Sep 20, 2010 at 06:30:01PM -0400, Vivek Goyal wrote:
> On Mon, Sep 20, 2010 at 03:12:44PM +0200, Jens Axboe wrote:
> > On 2010-09-20 15:03, Christoph Hellwig wrote:
> > > Hi Vivek, hi Jens,
> > > 
> > > where was http://git.kernel.dk/?p=linux-2.6-block.git;a=commitdiff_plain;h=44c74d6292e97f8bd9adfa6b0df3cb4f3c42a6dc posted  on the mailinglist?
> > > 
> > > I can't find it in my lkml or fsdevel inboxes.  Either way I don't think
> > > just papering over the underlying issue like this is a good idea.  
> > > 
> > > The big issue is that cfq tries to scanf the textual representation of
> > > the dev_t from the request_queue by abusing the bdi.  But the reason why
> > > we don't have a dev_t in the request_queue is that it's still not
> > > unique.  If it was we could easily add a dev_t into the request_queue
> > > and be done with it.
> > > 
> > > So the fix is either to get rid of the last remaining users of shared
> > > request_queues (IIRC the various floppy drivers) and just add a dev_t
> > > in the request_queue for the bdi, tracing and cfq, or add a dev_t into
> > > the request_queue and add a flag for shared request queues that the
> > > floppy driver and whoever needs it set and let the bdi sysfs code, cfq
> > > and blocktrace ignore theis request_queue.  This will also allow to
> > > get rid of the crap about ignoring failures due to already register
> > > or prematurely unregistered bdis and actually add real error handling
> > > to that code.
> > 
> > I did this one 15 months ago (according to git), but I never got it
> > tested:
> > 
> > http://git.kernel.dk/?p=linux-2.6-block.git;a=commitdiff;h=7a0ebc7ea1db42a71841df6a15be9fd420fae980
> > 
> > IIRC, the mtd stuff also uses a shared queue. But I think that is it.
> > Would indeed be VERY nice to finally get rid of that crap, it has
> > technically been outlawed since 2.5.1.
> 
> Hi Jens,
> 
> I have refreshed your patch to apply on latest tree. I did testing with
> one floppy controller and it was hanging because we were taking
> floppy_lock both inside and outside of function set_next_request(). I got
> rid of that and now atleast with one flopply controller this patch is
> working fine.
> 
> I tried connecting another floppy controller to see if I see two request
> queues being registered, but somehow my BIOS does not recognize two 
> floppy controllers and only makes one of these operational.
> 

Christoph suggested to use qemu and test with multiple flopply disk
controller and it worked. Thanks Christoph.

I exported two floppy disks and they show up as /dev/fd0 and /dev/fd1 in
the guest.

With the patch I also verified that changing IO scheduler on fd0 does
not change it automatically for fd1. That means they both are using 
separate requests queues after the patch. Without the patch changing
IO scheduler on one changed it automatically for the other controller
meaning they were sharing same request queue.

I did basic testing of being able to mount the floppy disks and being
able to read/write simple text files.

Jens, do let me know if you are curious to know about test results in
some other configuration.

Thanks
Vivek