Re: [Xen-devel] Linux grant map/unmap improvement proposal (Draft B)

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Tue, 2014-10-14 at 13:49 +0100, David Vrabel wrote:
> On 14/10/14 11:35, Ian Campbell wrote:
> > On Tue, 2014-10-14 at 11:32 +0100, David Vrabel wrote:
> >> On 14/10/14 11:27, Ian Campbell wrote:
> >>> On Mon, 2014-10-13 at 14:41 +0100, David Vrabel wrote:
> >>>> Safe grant unmap
> >>>> ----------------
> >>>>
> >>>> Grant references will only be unmapped when they are no longer in use.
> >>>> i.e., the page reference count is one.
> >>>>
> >>>>     int gnttab_unmap_refs_async(struct gnttab_unmap_grant_ref *unmap_ops,
> >>>>         struct gnttab_unmap_grant_ref *kunmap_ops,
> >>>>         struct page **pages, unsigned int count,
> >>>>         void (*done)(void *data), void *data);
> >>>>
> >>>> The `gnttab_unmap_refs_async()` function will unmap the grant
> >>>> references using the supplied unmap operations and call `done(data)`.
> >>>> The grant unmap will only be done once all pages are no longer in use.
> >>>>
> >>>> It shall run synchronously on the first attempt (this is expected to
> >>>> be the most common case).  If any page is in use, it shall queue the
> >>>> unmap request to be tried at a later time.
> >>>>
> >>>> Only the blkback and gntdev devices need to use asynchronouse unmaps.
> >>>
> >>> What about storage over networking? Does this work for that case too? I
> >>> suppose that would just manifest as >1 reference counts when the blk op
> >>> finishes, which would be taken care of by the delay.
> >>
> >> I'm not sure I follow what use case you're talking about here.  If the
> >> guest is using NFS or iSCSI or similar, then netback just sees ethernet
> >> packets and doesn't need to distinguish between different types of
> >> network traffic from the guest.
> > 
> > I meant dom0 mounted NFS/ISCSI disks (either loopback or from driver
> > domains) going out over either physical or virtual network interfaces.
> 
> I'm still confused.  Is this not the use case I describe in the "Blkback
> and network storage" section?  Whether the retransmitted packet is sent
> via a physical NIC or a virtual one doesn't matter.

Ah yes, but that was in the "problems" not the "solutions" section.
wasn't it?

So my question is ultimately: is this safe unmap functionality intended
to address the problem introduced in "Blkback and network storage".
Sounds like the answer is yes.

> 
> David



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