On 08/07/13 21:41, Konrad Rzeszutek Wilk wrote: > On Mon, Jul 08, 2013 at 03:03:27PM +0200, Roger Pau Monne wrote: >> Right now blkfront has no way to unmap grant refs, if using persistent >> grants once a grant is used blkfront cannot assure if blkback will >> have this grant mapped or not. To solve this problem, a new request >> type (BLKIF_OP_UNMAP) that allows requesting blkback to unmap certain >> grants is introduced. > > I don't think this is the right way of doing it. It is a new operation > (BLKIF_OP_UNMAP) that has nothing to do with READ/WRITE. All it is > is just some way for the frontend to say: unmap this grant if you can. > > As such I would think a better mechanism would be to have a new > grant mechanism that can say: 'I am done with this grant you can > remove it' - that is called to the hypervisor. The hypervisor > can then figure out whether it is free or not and lazily delete it. > (And the guest would be notified when it is freed).
I would prefer not to involve the hypervisor in persistent grants, this is something between the frontends and the backends. The hypervisor already provides the basic operations (map/unmap), IMHO there's no need to add more logic to the hypervisor itself. I agree that it would be better to have a generic way to request a backend to unmap certain grants, but so far this seems like the best solution. > > I would presume that this problem would also exist with netback/netfront > if it started using persisten grants, right? I'm not sure of that, it depends on the number of persistent grants netfront/netback use, in the block case we need this operation because of indirect descriptors, but netfront/netback might not suffer from this problem if the maximum number of grants they use is relatively small. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
