Re: [PATCH v6] xen/grant-table: Avoid m2p_override during mapping

From: Zoltan Kiss
Date: Fri Jan 24 2014 - 12:20:34 EST


On 24/01/14 05:48, Matt Wilson wrote:
On Thu, Jan 23, 2014 at 09:23:44PM +0000, Zoltan Kiss wrote:
Apologies for coming in late on this thread. I'm quite behind on
xen-devel mail that isn't CC: to me.

It seems to have been forgotten that Anthony and I proposed a similar
change last November.

https://lkml.kernel.org/r/1384307336-5328-1-git-send-email-anthony@xxxxxxxxxxxxx

Or am I misunderstanding the change?

I didn't know about this patch, but yes, both of them do basically the same. One subtle difference that you store the old mfn in page->private, while my patch keeps the original behaviour, and store it in page->index. page->private is used instead to store the new mfn we got from Xen, however I haven't checked where do we use that.
Your approach might be better, we also talked with David that we should stop using page->index, as e.g. with the netback grant mapping patches I spent a lot of time to figure out a packet drop issue, which eventually boiled down to the fact that index is in union with pfmemalloc, and if you don't set mapping, the local IP stack will think it is a pfmemalloc page. (see the comment in my second patch, xenvif_fill_frags)
However I think that should be a separate patch, I tried to keep the original behaviour as much as possible, and focus just on avoiding m2p_override when possible.

Regards,

Zoli
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