[PATCH v1 1/2] KVM: arm64: Do not transfer page refcount for THP adjustment

[Matthew Wilcox]

On Tue, Sep 26, 2023 at 07:19:31PM +0100, Vincent Donnefort wrote:
> With folios, GUP affects a refcount common to all pages forming the THP.
> There is therefore no need to move the refcount from the tail to the
> head page. Under the hood it decrements and increments
> the same counter.

FYI, this isn't new to folios.  get_page() has _always_ operated on the
head page of a compound page.  Here's how it worked in 2.6.12-rc2:

+static inline void get_page(struct page *page)
+{
+       if (unlikely(PageCompound(page)))
+               page = (struct page *)page->private;
+       atomic_inc(&page->_count);
+}

I can't be bothered to switch over to the linux-fullhistory tree and
show how it evolved, because i think this sufficiently makes the point --
you can backport this patch as far as you like.

Folios just make it obvious that this is what's going on.  And that's a
good thing!  Making the MM less inscrutable is a win for everybody.