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

[Matthew Wilcox]

On Mon, Dec 20, 2021 at 09:03:12PM -0400, Jason Gunthorpe wrote:
> That just leave the THP splitting.. I suppose we get the PTL, then
> compute the current value of the new bit based on refcount and diffuse
> it to all tail pages, then update the PMD and release the PTL. Safe
> against concurrent WP - don't need DoubleMap horrors because it isn't
> a counter.

One of the things I've been trying to figure out is how we do
can_split_huge_page().  Maybe an rmap walk to figure out how many
refcounts we would subtract if we did unmap it from everywhere it's
currently mapped?  (just to be clear, we call unmap_page() as the
next thing, so I don't mind warming up the rbtree cachelines
if it's mapped anywhere)