Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

[Matthew Wilcox]

On Thu, Aug 10, 2023 at 04:57:11PM -0400, Peter Xu wrote:
> AFAICS if that patch was all correct (while I'm not yet sure..), you can
> actually fit your new total mapcount field into page 1 so even avoid the
> extra cacheline access.  You can have a look: the trick is refcount for
> tail page 1 is still seems to be free on 32 bits (if that was your worry
> before).  Then it'll be very nice if to keep Hugh's counter all in tail 1.

No, refcount must be 0 on all tail pages.  We rely on this in many places
in the MM.