[PATCH] mm: migrate: Fix THP's mapcount on isolation

[Gavin Shan <gshan@redhat.com>]

The issue is reported when removing memory through virtio_mem device.
The transparent huge page, experienced copy-on-write fault, is wrongly
regarded as pinned. The transparent huge page is escaped from being
isolated in isolate_migratepages_block(). The transparent huge page
can't be migrated and the corresponding memory block can't be put
into offline state.

Fix it by replacing page_mapcount() with total_mapcount(). With this,
the transparent huge page can be isolated and migrated, and the memory
block can be put into offline state.

Fixes: 3917c80280c9 ("thp: change CoW semantics for anon-THP")
Cc: stable@vger.kernel.org   # v5.8+
Reported-by: Zhenyu Zhang <zhenyzha@redhat.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Gavin Shan <gshan@redhat.com>
---
 mm/compaction.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index c51f7f545afe..c408b5e04c1d 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -990,7 +990,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		 * admittedly racy check.
 		 */
 		mapping = page_mapping(page);
-		if (!mapping && page_count(page) > page_mapcount(page))
+		if (!mapping && page_count(page) > total_mapcount(page))
 			goto isolate_fail;
 
 		/*
-- 
2.23.0