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/1571938066-29031-1-git-send-email-yang.shi@linux.alibaba.com/

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/61c5cf99-a962-9a25-597a-53ab1bd8fbc0@google.com
Fixes: 97bfeea9c9fa ("thp, mlock: do not mlock PTE-mapped file huge pages")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/rmap.c b/mm/rmap.c
index 09b08888120e115e0bc2a4680dea78ca24e223ef..66cb69c1c5dde0ed17e8a9f9b74f846b2bed2d19 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1252,6 +1252,17 @@ void page_add_file_rmap(struct page *page, bool compound)
                }
                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);