mm: provide more accurate estimation of pages occupied by memmap

If SPARSEMEM is enabled, it won't build page structures for non-existing
pages (holes) within a zone, so provide a more accurate estimation of
pages occupied by memmap if there are bigger holes within the zone.

And pages for highmem zones' memmap will be allocated from lowmem, so
charge nr_kernel_pages for that.

[akpm@linux-foundation.org: mark calc_memmap_size __paging_init]
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Cc: Wen Congyang <wency@cn.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Maciej Rutecki <maciej.rutecki@gmail.com>
Cc: Chris Clayton <chris2553@googlemail.com>
Cc: "Rafael J . Wysocki" <rjw@sisk.pl>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Minchan Kim <minchan@kernel.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Michal Hocko <mhocko@suse.cz>
Tested-by: Jianguo Wu <wujianguo@huawei.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -4489,6 +4489,26 @@ void __init set_pageblock_order(void)
 
 #endif /* CONFIG_HUGETLB_PAGE_SIZE_VARIABLE */
 
+static unsigned long __paginginit calc_memmap_size(unsigned long spanned_pages,
+						   unsigned long present_pages)
+{
+	unsigned long pages = spanned_pages;
+
+	/*
+	 * Provide a more accurate estimation if there are holes within
+	 * the zone and SPARSEMEM is in use. If there are holes within the
+	 * zone, each populated memory region may cost us one or two extra
+	 * memmap pages due to alignment because memmap pages for each
+	 * populated regions may not naturally algined on page boundary.
+	 * So the (present_pages >> 4) heuristic is a tradeoff for that.
+	 */
+	if (spanned_pages > present_pages + (present_pages >> 4) &&
+	    IS_ENABLED(CONFIG_SPARSEMEM))
+		pages = present_pages;
+
+	return PAGE_ALIGN(pages * sizeof(struct page)) >> PAGE_SHIFT;
+}
+
 /*
  * Set up the zone data structures:
  *   - mark all pages reserved
@@ -4523,8 +4543,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
 		 * is used by this zone for memmap. This affects the watermark
 		 * and per-cpu initialisations
 		 */
-		memmap_pages =
-			PAGE_ALIGN(size * sizeof(struct page)) >> PAGE_SHIFT;
+		memmap_pages = calc_memmap_size(size, realsize);
 		if (freesize >= memmap_pages) {
 			freesize -= memmap_pages;
 			if (memmap_pages)
@@ -4545,6 +4564,9 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
 
 		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;
 
 		zone->spanned_pages = size;