linux-kernel - [PATCH v1 0/2] mm/highmem: don't track highmem pages manually

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Date: Fri,  7 Jun 2024 10:37:09 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wei Yang <richard.weiyang@...il.com>
Subject: [PATCH v1 0/2] 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

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Wei Yang <richard.weiyang@...il.com>

David Hildenbrand (2):
  mm/highmem: reimplement totalhigh_pages() by walking zones
  mm/highmem: make nr_free_highpages() return "unsigned long"

 include/linux/highmem-internal.h | 17 ++++++-----------
 include/linux/highmem.h          |  2 +-
 mm/highmem.c                     | 20 +++++++++++++++-----
 mm/page_alloc.c                  |  4 ----
 4 files changed, 22 insertions(+), 21 deletions(-)

base-commit: 19b8422c5bd56fb5e7085995801c6543a98bda1f
-- 
2.45.1