[Xen-devel] RE: Memory sharing, is it possible a page is freed in DomU on its way to be shared?

[pengfei zhang <zpfalpc23@xxxxxxxxxxx>]

	After thinking of this problem for some time, I think this situation can not happen for my point of view.

After the block disk driver has get the request from I/O dispatcher, if the buffer memory is freed for other use, 

no one will notify the driver about this, and this will cause data reading conflict. So the kernel must do something(protect the buffer until response from driver?) to avoid this. 

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Date: Tue, 19 Apr 2011 06:27:46 -0700
From: [hidden email]
To: [hidden email]
Subject: Memory sharing, is it possible a page is freed in DomU on its way to be shared?

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?
 
     thanks.
    
 
     
 
 
     

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