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>

include/linux/highmem-internal.h

@@ -132,7 +132,7 @@ static inline void __kunmap_atomic(const void *addr)
 }
 
 unsigned int __nr_free_highpages(void);
-extern atomic_long_t _totalhigh_pages;
+unsigned long __totalhigh_pages(void);
 
 static inline unsigned int nr_free_highpages(void)
 {
@@ -141,12 +141,7 @@ static inline unsigned int nr_free_highpages(void)
 
 static inline unsigned long totalhigh_pages(void)
 {
-	return (unsigned long)atomic_long_read(&_totalhigh_pages);
+	return __totalhigh_pages();
-}
-
-static inline void totalhigh_pages_add(long count)
-{
-	atomic_long_add(count, &_totalhigh_pages);
 }
 
 static inline bool is_kmap_addr(const void *x)

mm/highmem.c

@@ -111,9 +111,6 @@ static inline wait_queue_head_t *get_pkmap_wait_queue_head(unsigned int color)
 }
 #endif
 
-atomic_long_t _totalhigh_pages __read_mostly;
-EXPORT_SYMBOL(_totalhigh_pages);
-
 unsigned int __nr_free_highpages(void)
 {
 	struct zone *zone;
@@ -127,6 +124,20 @@ unsigned int __nr_free_highpages(void)
 	return pages;
 }
 
+unsigned long __totalhigh_pages(void)
+{
+	unsigned long pages = 0;
+	struct zone *zone;
+
+	for_each_populated_zone(zone) {
+		if (is_highmem(zone))
+			pages += zone_managed_pages(zone);
+	}
+
+	return pages;
+}
+EXPORT_SYMBOL(__totalhigh_pages);
+
 static int pkmap_count[LAST_PKMAP];
 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(kmap_lock);
 

mm/page_alloc.c

@@ -5762,10 +5762,6 @@ void adjust_managed_page_count(struct page *page, long count)
 {
 	atomic_long_add(count, &page_zone(page)->managed_pages);
 	totalram_pages_add(count);
-#ifdef CONFIG_HIGHMEM
-	if (PageHighMem(page))
-		totalhigh_pages_add(count);
-#endif
 }
 EXPORT_SYMBOL(adjust_managed_page_count);