linux-kernel - Re: [PATCH v3] p2m: use GNTTABOP_unmap_and

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Message-ID: <527B75B4.20700@citrix.com>
Date:	Thu, 7 Nov 2013 11:12:52 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Roger Pau Monne <roger.pau@...rix.com>
CC:	<xen-devel@...ts.xen.org>, <linux-kernel@...r.kernel.org>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v3] p2m: use GNTTABOP_unmap_and_duplicate if available

On 04/11/13 15:38, Roger Pau Monne wrote:
> The new GNTTABOP_unmap_and_duplicate operation doesn't zero the
> mapping passed in new_addr, allowing us to perform batch unmaps in p2m
> code without requiring the use of a multicall.

I have recently investigated some problems that were caused by a user
space process using gntdev.  It was unmapping page that still had
outstanding I/O.  This caused a number of failures:

1. Oopses due to swiotlb_bounce() attempting to memcpy() back to a page
that now has a read-only mapping to a scratch page MFN.

2. Bad page errors due to the balloon page being freed by gntdev while
the page count > 1 and the balloon driver setting page count to 1 and
freeing the page.

I think we need to take a step back and look at the design of the gntdev
device to make it handle misbehaved or crashing programs.

In particular, I think we need to use regular (non-ballooned) pages and
restore their original direct mappings when grant unmapping.  My initial
thoughts are that this would require a GNTTABOP_unmap_and_replace
variant that takes a GFN direct instead of a finding the GFN via a
virtual address.

I think it is best to hold off on any optimization attempts here until
we get the gntdev design right.

David
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