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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
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> 
> Btw, the extra bit doesn't really have to be in the page tables. It
> could be a bit in the page itself. We could add another page bit that
> we just clear when we do the "add ref to page as you make a virtual
> copy during fork() etc".

^ I'm playing with the idea if using a page bit to express: "This page
is exclusive". On a CoW fault, if that bit is set, I can simply reuse
the page.

The semantics under which semantics to set the bit are slightly
different than what you describe, and I'm playing with additional
unsharing (on GUP R/O) that avoids mapping the copied page similarly R/O
and simply sets the bit.

But the general idea could fly I think, devil's in the detail ...

-- 
Thanks,

David / dhildenb