[RFC PATCH] mm: dump_page: print total mapcount for compound page

[Yang Shi <shy828301@gmail.com>]

The total mapcount is a useful information for debugging, but we can't
call total_mapcount() directly since it calls some assertions which may
be triggered as commit 6dc5ea16c86f ("mm,
dump_page: do not crash with bad compound_mapcount()") met.

We could implement yet another implementation for dump_page() but
it has the limitation when individual mapcount of subpages is corrupted.

Actually the total mapcount could be decoded from refcount, pincount and
compound mapcount although it may be not very precise due to some
transient references.

Signed-off-by: Yang Shi <shy828301@gmail.com>
---
I think we are on the same page that the total mapcount is useful
information and it would be ideal to print this information when dumpping
page if possible.  But how to implement it safely seems controversial.
Some ideas and potential problems have been discussed by
https://lore.kernel.org/linux-mm/alpine.LSU.2.11.2105261733110.16920@eggly.anvils/.

So I prepared this patch to show a possible approach to get some
feedback.  The same thing could be decoded by the reader of page dump
as well by using the same formula used by this patch.  However it sounds
more convenient to have kernel do the math.
 
 mm/debug.c | 35 +++++++++++++++++++++++++++++------
 1 file changed, 29 insertions(+), 6 deletions(-)

diff --git a/mm/debug.c b/mm/debug.c
index e73fe0a8ec3d..129efcfcaf79 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -54,8 +54,13 @@ static void __dump_page(struct page *page)
 	 * inaccuracy here due to racing.
 	 */
 	bool page_cma = is_migrate_cma_page(page);
-	int mapcount;
+	int mapcount, total_mapcount;
+	int nr;
+	int refcount;
+	int pincount = 0;
+	int comp_mapcnt;
 	char *type = "";
+	bool is_slab = PageSlab(head);
 
 	if (page < head || (page >= head + MAX_ORDER_NR_PAGES)) {
 		/*
@@ -82,22 +87,40 @@ static void __dump_page(struct page *page)
 	 * page->_mapcount space in struct page is used by sl[aou]b pages to
 	 * encode own info.
 	 */
-	mapcount = PageSlab(head) ? 0 : page_mapcount(page);
+	mapcount = is_slab ? 0 : page_mapcount(page);
+
+	refcount = page_ref_count(head);
 
 	pr_warn("page:%p refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n",
-			page, page_ref_count(head), mapcount, mapping,
+			page, refcount, mapcount, mapping,
 			page_to_pgoff(page), page_to_pfn(page));
 	if (compound) {
+		comp_mapcnt = head_compound_mapcount(head);
 		if (hpage_pincount_available(page)) {
+			pincount = head_compound_pincount(head);
 			pr_warn("head:%p order:%u compound_mapcount:%d compound_pincount:%d\n",
 					head, compound_order(head),
-					head_compound_mapcount(head),
-					head_compound_pincount(head));
+					comp_mapcnt, pincount);
 		} else {
 			pr_warn("head:%p order:%u compound_mapcount:%d\n",
 					head, compound_order(head),
-					head_compound_mapcount(head));
+					comp_mapcnt);
+		}
+
+		nr = compound_nr(head);
+		if (is_slab)
+			total_mapcount = 0;
+		else if (PageHuge(head))
+			total_mapcount = comp_mapcnt;
+		else {
+			if (mapping) {
+				if (!PageAnon(head))
+					nr = nr * (comp_mapcnt + 1) - comp_mapcnt;
+			} else
+				nr = 0;
+			total_mapcount = refcount - pincount - nr;
 		}
+		pr_warn("total_mapcount(estimated):%d\n", total_mapcount);
 	}
 
 #ifdef CONFIG_MEMCG
-- 
2.26.2