mm/mm_init.c: remove meaningless calculation of zone->managed_pages in free_area_init_core()

Currently, in free_area_init_core(), when initialize zone's field, a rough
value is set to zone->managed_pages.  That value is calculated by
(zone->present_pages - memmap_pages).

In the meantime, add the value to nr_all_pages and nr_kernel_pages which
represent all free pages of system (only low memory or including HIGHMEM
memory separately).  Both of them are gonna be used in
alloc_large_system_hash().

However, the rough calculation and setting of zone->managed_pages is
meaningless because
  a) memmap pages are allocated on units of node in sparse_init() or
     alloc_node_mem_map(pgdat); The simple (zone->present_pages -
     memmap_pages) is too rough to make sense for zone;
  b) the set zone->managed_pages will be zeroed out and reset with
     acutal value in mem_init() via memblock_free_all(). Before the
     resetting, no buddy allocation request is issued.

Here, remove the meaningless and complicated calculation of
(zone->present_pages - memmap_pages), directly set zone->managed_pages as
zone->present_pages for now.  It will be adjusted in mem_init().

And also remove the assignment of nr_all_pages and nr_kernel_pages in
free_area_init_core().  Instead, call the newly added
calc_nr_kernel_pages() to count up all free but not reserved memory in
memblock and assign to nr_all_pages and nr_kernel_pages.  The counting
excludes memmap_pages, and other kernel used data, which is more accurate
than old way and simpler, and can also cover the ppc required
arch_reserved_kernel_pages() case.

And also clean up the outdated code comment above free_area_init_core(). 
And free_area_init_core() is easy to understand now, no need to add words
to explain.

[bhe@redhat.com: initialize zone->managed_pages as zone->present_pages for now]
  Link: https://lkml.kernel.org/r/ZgU0bsJ2FEjykvju@MiWiFi-R3L-srv
Link: https://lkml.kernel.org/r/20240325145646.1044760-5-bhe@redhat.comSigned-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mm_init.c

@@ -1566,15 +1566,6 @@ void __ref free_area_init_core_hotplug(struct pglist_data *pgdat)
 }
 #endif
 
-/*
- * Set up the zone data structures:
- *   - mark all pages reserved
- *   - mark all memory queues empty
- *   - clear the memory bitmaps
- *
- * NOTE: pgdat should get zeroed by caller.
- * NOTE: this function is only called during early init.
- */
 static void __init free_area_init_core(struct pglist_data *pgdat)
 {
 	enum zone_type j;
@@ -1585,41 +1576,13 @@ static void __init free_area_init_core(struct pglist_data *pgdat)
 
 	for (j = 0; j < MAX_NR_ZONES; j++) {
 		struct zone *zone = pgdat->node_zones + j;
-		unsigned long size, freesize, memmap_pages;
-
-		size = zone->spanned_pages;
-		freesize = zone->present_pages;
-
-		/*
-		 * Adjust freesize so that it accounts for how much memory
-		 * is used by this zone for memmap. This affects the watermark
-		 * and per-cpu initialisations
-		 */
-		memmap_pages = calc_memmap_size(size, freesize);
-		if (!is_highmem_idx(j)) {
-			if (freesize >= memmap_pages) {
-				freesize -= memmap_pages;
-				if (memmap_pages)
-					pr_debug("  %s zone: %lu pages used for memmap\n",
-						 zone_names[j], memmap_pages);
-			} else
-				pr_warn("  %s zone: %lu memmap pages exceeds freesize %lu\n",
-					zone_names[j], memmap_pages, freesize);
-		}
-
-		if (!is_highmem_idx(j))
-			nr_kernel_pages += freesize;
-		/* Charge for highmem memmap if there are enough kernel pages */
-		else if (nr_kernel_pages > memmap_pages * 2)
-			nr_kernel_pages -= memmap_pages;
-		nr_all_pages += freesize;
+		unsigned long size = zone->spanned_pages;
 
 		/*
-		 * Set an approximate value for lowmem here, it will be adjusted
-		 * when the bootmem allocator frees pages into the buddy system.
-		 * And all highmem pages will be managed by the buddy system.
+		 * Initialize zone->managed_pages as 0 , it will be reset
+		 * when memblock allocator frees pages into buddy system.
 		 */
-		zone_init_internals(zone, j, nid, freesize);
+		zone_init_internals(zone, j, nid, zone->present_pages);
 
 		if (!size)
 			continue;
@@ -1916,6 +1879,7 @@ void __init free_area_init(unsigned long *max_zone_pfn)
 		check_for_memory(pgdat);
 	}
 
+	calc_nr_kernel_pages();
 	memmap_init();
 
 	/* disable hash distribution for systems with a single node */