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[Xen-devel] Memory sharing, is it possible a page is freed in DomU on it

To: xen devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Memory sharing, is it possible a page is freed in DomU on its way to be shared?
From: MaoXiaoyun <tinnycloud@xxxxxxxxxxx>
Date: Tue, 19 Apr 2011 21:26:54 +0800
Cc: george.dunlap@xxxxxxxxxxxxx, tim.deegan@xxxxxxxxxx, juihaochiang@xxxxxxxxx
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      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?
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