linux-kernel - Re: [PATCH] virtio: 9p: correctly pass physical address to userspace for high pages

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Date:	Fri, 19 Oct 2012 10:09:51 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Will Deacon <will.deacon@....com>
Cc:	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Sasha Levin <levinsasha928@...il.com>,
	Marc Zyngier <Marc.Zyngier@....com>,
	lf-virt <virtualization@...ts.linux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Van Hensbergen <ericvh@...il.com>
Subject: Re: [PATCH] virtio: 9p: correctly pass physical address to userspace for high pages

Will Deacon <will.deacon@....com> writes:
> On Thu, Oct 18, 2012 at 03:19:06AM +0100, Rusty Russell wrote:
>> We do have an alternate solution: masking out __GFP_HIGHMEM from the
>> kmalloc of desc.  If it fails, we will fall back to laying out the
>> virtio request directly inside the ring; if it doesn't fit, we'll wait
>> for the device to consume more buffers.
>
> Hmm, that will probably work for the vring but the zero-copy code for 9p may
> just give us an address from userspace if I'm understanding it correctly. In
> that case, we really have to do the translation as below (which is actually
> much cleaner because everything is page-aligned).
>
>> > @@ -325,7 +326,7 @@ static int p9_get_mapped_pages(struct virtio_chan *chan,
>> >  		int count = nr_pages;
>> >  		while (nr_pages) {
>> >  			s = rest_of_page(data);
>> > -			pages[index++] = virt_to_page(data);
>> > +			pages[index++] = kmap_to_page(data);
>> >  			data += s;
>> >  			nr_pages--;
>> >  		}
>
> So what do you reckon? How about I leave this hunk as a separate patch and
> have a play masking out __GFP_HIGHMEM for the vring descriptor?

Yes, I think so.  A scathing comment would be nice, too...

Thanks,
Rusty.
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