Re: [Xen-devel] Re: [PATCH]Add a flag for shadow pages

[Gianluca Guida <gianluca.guida@xxxxxxxxxxxxx>]

Jiang, Yunhong wrote:
> Thanks for your really quick-hand implementation. I will update my another 
> patch accordingly tomorrow.
>
> So still one question to the two assertion (or to Tim??) in 
> sh_rm_write_access_from_sl1p()/sh_put_ref(). What's the potential error to be 
> protected by this checking? If page is a shadow, it's count_info will always be 
> 0, right? Or it is just a sanity checking?
They're sanity checking. Checks are there to be sure that the page is a 
shadow page. It used to be "->mbz == 0", which was clearer, in the old 
page_info model.
In the sh_rm_write_access_from_sl1p(), the check is there to ensure that 
the page is *still* a shadow page. As for current code, though, that 
check should be useless.
I actually think that having a more precise way of knowing when a page 
is a shadow page is definitely needed, for debugging or even in case the 
shadow allocation becomes more dynamic in the future.
> I need change this is because, if we mark a page offline, then the count_info 
> is not 0, even for shadow page. Can I just checking the count_mask here?
Ehr, I am not following the discussions about your patch, but I suppose 
you set the page off-line after all references and uses of the page, 
even inside shadows, are removed. Or the count_info is != 0 to signal 
that the page is going offline?
Thanks,
Gianluca

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