[180/227] mm/thp: ClearPageDoubleMap in first page_add_file_rmap()

Commit Message

Andrew Morton March 22, 2022, 9:47 p.m. UTC

From: Hugh Dickins <[e-mail]>
Subject: mm/thp: ClearPageDoubleMap in first page_add_file_rmap()

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/[e-mail]/

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.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 9a73f61bdb8a ("thp, mlock: do not mlock PTE-mapped file huge pages")
Signed-off-by: Hugh Dickins <[e-mail]>
Reviewed-by: Yang Shi <[e-mail]>
Cc: "Kirill A. Shutemov" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

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

Patch

--- a/mm/rmap.c~mm-thp-clearpagedoublemap-in-first-page_add_file_rmap
+++ a/mm/rmap.c
@@ -1252,6 +1252,17 @@  void page_add_file_rmap(struct page *pag
 		}
 		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);