mm/highmem: reimplement totalhigh_pages() by walking zones

mm/highmem: reimplement totalhigh_pages() by walking zones

Patch series "mm/highmem: don't track highmem pages manually".

Let's remove highmem special-casing from adjust_managed_page_count(), to
result in less confusion why memblock manually adjusts totalram_pages, and
__free_pages_core() only adjusts the zone's managed pages -- what about
the highmem pages that adjust_managed_page_count() updates?

Now, we only maintain totalram_pages and a zone's managed pages
independent of highmem support.  We can derive the number of highmem pages
simply by looking at the relevant zone's managed pages.  I don't think
there is any particular fast path that needs a maximum-efficient
totalhigh_pages() implementation.

Note that highmem memory is currently initialized using
free_highmem_page()->free_reserved_page(), not __free_pages_core().  In
the future we might want to also use __free_pages_core() to initialize
highmem memory, to make that less special, and consider moving
totalram_pages updates into __free_pages_core() [1], so we can just use
adjust_managed_page_count() in there as well.

Booting a simple kernel in QEMU reveals no highmem accounting change:

Before:
  Memory: 3095448K/3145208K available (14802K kernel code, 2073K rwdata,
  5000K rodata, 740K init, 556K bss, 49760K reserved, 0K cma-reserved,
  2244488K highmem)

After:
  Memory: 3095276K/3145208K available (14802K kernel code, 2073K rwdata,
  5000K rodata, 740K init, 556K bss, 49932K reserved, 0K cma-reserved,
  2244488K highmem)

[1] https://lkml.kernel.org/r/20240601133402.2675-1-richard.weiyang@gmail.com

This patch (of 2):

Can we get rid of the highmem ifdef in adjust_managed_page_count()?
Likely yes: we don't have that many totalhigh_pages() users, and they all
don't seem to be very performance critical.

So let's implement totalhigh_pages() like nr_free_highpages(), collecting
information from all zones.  This is now similar to what we do in
si_meminfo_node() to collect the per-node highmem page count.

In the common case (single node, 3-4 zones), we really shouldn't care.  We
could optimize a bit further (only walk ZONE_HIGHMEM and ZONE_MOVABLE if
required), but there doesn't seem a real need for that.

[david@redhat.com: fix build bot complaint]
Link: https://lkml.kernel.org/r/b57e5bc4-eb72-40e3-add4-57dfa6e03df6@redhat.com
Link: https://lkml.kernel.org/r/20240607083711.62833-1-david@redhat.com
Link: https://lkml.kernel.org/r/20240607083711.62833-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>