linux-kernel - Re: [PATCH] Xen: Fix retry calls into PRIVCMD

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Date:	Thu, 1 Aug 2013 12:23:51 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Andres Lagar-Cavilla <andreslc@...dcentric.ca>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Andres Lagar-Cavilla <andres@...arcavilla.com>,
	<boris.ostrovsky@...cle.com>
Subject: Re: [PATCH] Xen: Fix retry calls into PRIVCMD_MMAPBATCH*.

On 01/08/13 04:30, Andres Lagar-Cavilla wrote:
> -- Resend as I haven't seen this hit the lists. Maybe some smtp misconfig. Apologies. Also expanded cc --
> 
> When a foreign mapper attempts to map guest frames that are paged out,
> the mapper receives an ENOENT response and will have to try again
> while a helper process pages the target frame back in.
> 
> Gating checks on PRIVCMD_MMAPBATCH* ioctl args were preventing retries
> of mapping calls.

This breaks the auto_translated_physmap case as will allocate another
set of empty pages and leak the previous set.

This privcmd_enforce_singleshot_mapping() stuff seems very odd anyway.
Does anyone know what it was for originally?  It would be preferrable if
we could update the mappings with a new set of foreign MFNs without
having to tear down the VMA and recreate a new VMA.

> --- a/drivers/xen/privcmd.c
> +++ b/drivers/xen/privcmd.c
[...]
> +/* For MMAPBATCH*. This allows asserting the singleshot mapping
> + * on a per pfn/pte basis. Mapping calls that fail with ENOENT
> + * can be then retried until success. */

Comment coding style.

David
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