Re: [Xen-devel] blktap2 and CONFIG_XEN_BLKBACK_PAGEMAP

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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?

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?

Ian.


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