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xen-devel
RE: [Xen-devel] blktap2 and CONFIG_XEN_BLKBACK_PAGEMAP
> -----Original Message-----
> From: Daniel Stodden
> Sent: Monday, July 19, 2010 9:53 AM
> To: Ian Campbell
> Cc: Shriram Rajagopalan; Kaushik Kumar Ram; xen-devel@xxxxxxxxxxxxxxxxxxx;
> Jake Wires
> Subject: Re: [Xen-devel] blktap2 and CONFIG_XEN_BLKBACK_PAGEMAP
>
> On Mon, 2010-07-19 at 09:36 -0400, Ian Campbell wrote:
> > On Fri, 2010-07-16 at 21:16 +0100, Daniel Stodden wrote:
> > >
> > > The reason for the duplicate mapping is that userspace has to re-queue
> > > those frames at the physical device layer, and -- iirc -- the problem
> > > was that queuing pages twice, once on the blktap2 bdev and once on the
> > > underlying disk, will deadlock.
> >
> > I was wondering what the duplicate mappings were for just last week.
> >
> > So is this need to play tricks with the p2m to avoid a deadlock the only
> > dependency blktap2 has on Xen? IOW if we could find another way around
> > the deadlock would a) blktap2 be esable on native and/or b) would all
> > the Xen specific bits (grant mappings etc) be confined to blkback only?
>
> [cc Jake. Did most of the mapping code, and still the one who knows best
> what prevents that path from getting simpler.]
>
> Both the xen and native datapaths are presently inlined in the same disk
> type. The solution to that would be an ops struct to separate the
> handling. But that's certainly not a hard problem.
>
> Apart from that, I believe native was more of a problem than blkback.
>
> Only out my memory: Consider non-foreign r/w in dom0. There's going to
> be a page lock foregoing queuing on the tapdev. And a second lock
> attempt on the path from tapdisk to the physical device, because what
> userland is sending down the native I/O path is sold as normal user
> memory.
>
> So it's probably rather tribute to zero-copy than anything else. The
> problem might evaporate if the physical I/O were bounced off anon
> memory. That might be one possible alternative.
Daniel is correct -- in the non-xen case, the blktap mapping is used merely
to give us a new (unlocked) page struct that tapdisk can send back down
through the IO stack. we could do away with this in the non-xen case if we
give up zero-copy. blktap would still need to use xen to map foreign pages
to tapdisk.
> Note that the blkback path is different, because it directly goes for
> the disk queue, not through the filemap. I'd expect that to just work.
>
>
> > I guess the difference between blktap and e.g. device mapper is that in
> > the later case the requeuing is done in the kernel and in the former the
> > page goes via userspace and hence the association with the original I/O
> > is lost?
>
> Yep.
>
> I think another difference was that dm nodes only do request
> translation, then just pass them on the the physical layer. So dm nodes
> are rather thin compared to a tapdev. But that might not matter here.
>
> Daniel
>
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