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Re: [Xen-devel] Seamlessly sharing identical memory pages among domains

> Yep. The only slight subtlety is that that the page in question
> may appear in multiple page tables and at multiple VAs: if the
> page is truly read-write shared in the guest (e.g. sysv shared
> memory [*]) then it it will be necessary to find all the ptes and
> patch them to the newly allocated machine frame. I think such
> pages are rare, and tracking them down by scanning the list of
> vma's that are mapping the object (e.g. a file) in question is
> pretty easy and quick. However, this would most sensibly be done
> by making minor modifications to architecture independent code,

Just adding an upcall wouldn't be too terrible. Iterating through the
list of vtop mappings for a page should be fairly light weight.
Politically, the most realistic thing to do is put the arch-independent
code changes in the xen arch-dependent tree wherever possible. I don't
think anyone wants to see xen specific changes going on outside of the
arch subtree.

> [*] Hmm, do memory mapped files behave like this too? I can't
> remember.

If multiple processes share the page read-write, the OS has to have some
way to know about it.


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