Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

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 ...