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[Xen-devel] Re: [PATCH] mem_sharing: fix race condition of nominate and

To: Jui-Hao Chiang <juihaochiang@xxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH] mem_sharing: fix race condition of nominate and unshare
From: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Date: Thu, 13 Jan 2011 09:24:27 +0000
Cc: MaoXiaoyun <tinnycloud@xxxxxxxxxxx>, xen devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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At 02:26 +0000 on 13 Jan (1294885615), Jui-Hao Chiang wrote:
> There seems a typo:
> "if ( get_page_and_type(page, d, PGT_shared_page) )" should be changed
> to "if ( !get_page_and_type(page, d, PGT_shared_page) )"

Oops!  Yes, thanks for that. :)

> (2) The major problem is the __put_page_type() never handle the
> special case for shared pages.
> If the (1) is changed as I said, the problem still exists as the following
> /* Before nominating domain 1, gfn 0x63 */
> (XEN) Debug for domain=1, gfn=63, Debug page: MFN=4836c7 is
> ci=8000000000000002, ti=0, owner_id=1
> /* After a failed nominate  [desired: ci=8000000000000002, ti=0]*/
> (XEN) Debug for domain=1, gfn=63, Debug page: MFN=4836c7 is
> ci=8000000000000002, ti=8400000000000000, owner_id=1

Is this causing a real problem other than this printout?

One of the reasons to use get_page_and_type/put_page_and_type was that
it gets rid of the code that requires pages to have PGT_none before
they're shared. 

As I have been trying to explain, when a page has typecount 0 its type
is only relevant for the TLB flushing logic.  If there's still a place
in the page-sharing code that relies on (type == PGT_none && count == 0)
then AFAICS that's a bug. 



> 2011/1/12 MaoXiaoyun <tinnycloud@xxxxxxxxxxx>:
> > Hi Tim:
> >
> >         That's it, I am running the test, so far so good, I'll test more,
> > thanks.
> >
> >       Currently from the code of tapdisk, it indicates only *read only* IO
> > with secs 8 has the
> > chance to be shared, so does it mean only the parent image can be shared,
> > still it needs to
> > be opened read only, right?
> >
> >       So it looks like page sharing are refer to those pages contain disk
> > data been loaded
> > into Guest IO buffer, and this is the page sharing in Xen, right?
> >
> >
> Bests,
> Jui-Hao

Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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