[PATCH mmotm] mm/thp: ClearPageDoubleMap in first page_add_file_rmap()

[Hugh Dickins]

PageDoubleMap is maintained differently for anon and for shmem+file:
the shmem+file one was never cleared, because a safe place to do so
could not be found; so it would blight future use of the cached
hugepage until evicted.

See https://lore.kernel.org/lkml/1571938066-29031-1-git-send-email-yang.shi@xxxxxxxxxxxxxxxxx/

But page_add_file_rmap() does provide a safe place to do so (though
later than one might wish): allowing testing to return to an initial
state without a damaging drop_caches.

Fixes: 9a73f61bdb8a ("thp, mlock: do not mlock PTE-mapped file huge pages")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 mm/rmap.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1251,6 +1251,17 @@ void page_add_file_rmap(struct page *page,
 		}
 		if (!atomic_inc_and_test(compound_mapcount_ptr(page)))
 			goto out;
+
+		/*
+		 * It is racy to ClearPageDoubleMap in page_remove_file_rmap();
+		 * but page lock is held by all page_add_file_rmap() compound
+		 * callers, and SetPageDoubleMap below warns if !PageLocked:
+		 * so here is a place that DoubleMap can be safely cleared.
+		 */
+		VM_WARN_ON_ONCE(!PageLocked(page));
+		if (nr == nr_pages && PageDoubleMap(page))
+			ClearPageDoubleMap(page);
+
 		if (PageSwapBacked(page))
 			__mod_lruvec_page_state(page, NR_SHMEM_PMDMAPPED,
 						nr_pages);