Re: [PATCH 08/12] xen/grant-table: add a mechanism to safely unmap pages that are in use

[Ian Campbell <Ian.Campbell@citrix.com>]

On Tue, 2015-01-06 at 18:57 +0000, David Vrabel wrote:
> From: Jenny Herbert <jennifer.herbert@citrix.com>
> 
> Introduce gnttab_unmap_refs_async() that can be used to safely unmap
> pages that may be in use (ref count > 1).  If the pages are in use the
> unmap is deferred and retried later.  This polling is not very clever
> but it should be good enough if the cases where the delay is necessary
> are rare.
> 
> This is needed to allow block backends using grant mapping to safely
> use network storage (block or filesystem based such as iSCSI or NFS).
> 
> The network storage driver may complete a block request whilst there
> is a queued network packet retry (because the ack from the remote end
> races with deciding to queue the retry).  The pages for the retried
> packet would be grant unmapped and the network driver (or hardware)
> would access the unmapped page.

I thought this had been solved a little while ago by mapping a scratch
page on unmap even for kernel space grant mappings, but both the design
doc and here imply not (i.e. the scratch is for user grant mappings
only), so I must be misremembering.

Regardless, this approach seems likely to be far better...