Re: [Xen-devel] [PATCH v2 2/2] xen/m2p: use GNTTABOP_unmap_and_replace to reinstate the original mapping

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 23 Jul 2013, Roger Pau Monnà wrote:
> On 22/07/13 19:02, David Vrabel wrote:
> > On 22/07/13 17:28, Stefano Stabellini wrote:
> >> GNTTABOP_unmap_grant_ref unmaps a grant and replaces it with a 0
> >> mapping instead of reinstating the original mapping.
> >> Doing so separately would be racy.
> >>
> >> To unmap a grant and reinstate the original mapping atomically we use
> >> GNTTABOP_unmap_and_replace.
> >> GNTTABOP_unmap_and_replace doesn't work with GNTMAP_contains_pte, so
> >> don't use it for kmaps.  GNTTABOP_unmap_and_replace zeroes the mapping
> >> passed in new_addr so we have to reinstate it, however that is a
> >> per-cpu mapping only used for balloon trade pages, so we can be sure that
> >> it's not going to be accessed while the mapping is not valid.
> > 
> > This solves the problem of userspace accessing a disk image on an NFS
> > mount but what would blkback talking to an iSCSI LUN?  Will that need
> > similar fixes to blkback?  This series does not need to fix this now though.
> 
> I have not played much with iSCSI and blkback, but I guess the same
> issue exists there, the only difference is that we don't perform the
> grant mappings with GNTMAP_contains_pte, so less modifications are required.
> 
> Maybe the unmap and reinstate of the trade page address could be placed
> somewhere more generic, so that I don't have to also code the multicall
> in blkback?

I don't think that can be done with the current limitations of the
GNTTABOP_unmap_and_replace hypercall.


> Adding a proper unmap and replace op that reinstates the
> trade page address to grant-table.c does seem like the more logical
> option IMHO. (I can do that after this patch is committed).

That might be the only way to solve the problem for blkback.
http://marc.info/?l=xen-devel&m=137442818429129 should be a good start.

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