[RFT PATCH v2 4/5] mm: provide more accurate estimation of pages occupied by memmap

[Jiang Liu <liuj97@gmail.com>]

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.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
---
 mm/page_alloc.c |   26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fc10071..9bbac97 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4442,6 +4442,26 @@ void __init set_pageblock_order(void)
 
 #endif /* CONFIG_HUGETLB_PAGE_SIZE_VARIABLE */
 
+static unsigned long 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
@@ -4476,8 +4496,7 @@ 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)
@@ -4498,6 +4517,9 @@ 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;
-- 
1.7.9.5

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