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xen-devel
[Xen-devel] Re: Memory sharing, is it possible a page is freed in DomU o
At 14:26 +0100 on 19 Apr (1303223214), MaoXiaoyun wrote:
> Hi:
>
> Just come up this question.
>
> Say a process in domU read IO, then blkfront driver will have a
> new page prepared to
> fill the IO data. The blkfront pass the gref to blkback in dom0, later
> passed to blktap, then
> forward to tapdisk for physical IO read, in memory sharing, the
> gref may be nominate to
> Xen for page sharing again( say this is sharing step).
>
> My question is, it is possible during the IO data comes back from
> tapdisk, the page
> referred by gerf in domU could be freed? (maybe by process termination,
> or blkfront free this page)
>
> And if it is possible, then the page is free in domU, it is also
> possible that the page be given
> back to Xen through ballloon driver, and the P2M will be invaild. This
> may make *sharing step*
> gfn points to a invalid mfn possible.
>
> So is this possible happen?
It shouldn't be. The reference counting of pages means that if the mem
sharing code has taken a reference to the page it can't be freed until
it gives it up again. Do you think you've seen this happening?
Tim.
--
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd. (Company #02937203, SL9 0BG)
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