Re: [Xen-devel] Re: mapping problems in xenpaging

[Tim Deegan <tim@xxxxxxx>]

At 17:02 -0400 on 30 Sep (1317402151), Adin Scannell wrote:
> I believe I have recreated this problem a few times, resulting in
> various crashes... unfortunately, there is somewhat of an implicit
> assumption throughout the code that when you grab an mfn via
> gfn_to_mfn, that mfn won't disappear underneath you (for example, see
> vmx_load_pdptrs).  Really, you want something like gfn_to_mfn_getpage,
> where the underlying page has its refcount bumped so that it won't be
> nominated/evicted while you map and use the page, then you must put it
> back when you're done.  

Quite right - there are a lot of places that assume that a p2m mapping
won't change underfoot, and the right answer will involve reference
counting of some kind - either better integration with the underlying
refcount/typecount system or some reference count in the p2m. 

The tricky part will be what to do when a p2m update can't be made
because of one of those refcounts. 

Tim.

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