From fb580e196497738d98dbad5b8459913435b376e7 Mon Sep 17 00:00:00 2001 From: "Lorenzo Stoakes (ARM)" Date: Thu, 13 Aug 2026 18:32:33 +0100 Subject: [PATCH] tools/testing/selftests/mm: test anonymous page offset merge behaviour MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit While maintaining anonymous page offsets for VMAs has no impact for most merge cases, it does impact MAP_PRIVATE-mapped file-backed mappings which happen to have matching page offset but not matching anonymous page offset. Assert this behaviour by attempting to map an unfaulted MAP_PRIVATE-memfd region with a faulted one with compatible file page offsets but incompatible anonymous page offsets. Link: https://lore.kernel.org/20260813-b4-scalable-cow-virt-pgoff-v5-16-c21581c0c3c8@kernel.org Signed-off-by: Lorenzo Stoakes (ARM) Acked-by: David Hildenbrand (Arm) Cc: Adrian Hunter Cc: Alexander Deucher Cc: Alexander Gordeev Cc: Alexander Shishkin Cc: Alistair Popple Cc: Arnaldo Carvalho de Melo Cc: Arnd Bergmann Cc: Baolin Wang Cc: Baoquan He Cc: Barry Song Cc: Boris Brezillon Cc: Byungchul Park Cc: Chengming Zhou Cc: Chris Li Cc: Christan König Cc: Christian Borntraeger Cc: Claudio Imbrenda Cc: Dave Airlie Cc: Dev Jain Cc: Gerald Schaefer Cc: Greg Kroah-Hartman Cc: Gregory Price (Meta) Cc: Harry Yoo Cc: Heiko Carstens Cc: Huang Ray Cc: "Huang, Ying" Cc: Ian Rogers Cc: Ingo Molnar Cc: James Clark Cc: Jan Kara Cc: Jann Horn Cc: Janosch Frank Cc: Jason Gunthorpe Cc: Jiri Olsa Cc: John Hubbard Cc: Joshua Hahn Cc: Kairui Song Cc: Kees Cook Cc: Kemeng Shi Cc: Lance Yang Cc: Liam R. Howlett Cc: Liviu Dudau Cc: Maarten Lankhorst Cc: Marc Rutland Cc: "Masami Hiramatsu (Google)" Cc: Matthew Auld Cc: Matthew Brost Cc: Matthew Wilcox (Oracle) Cc: Maxime Ripard Cc: Miaohe Lin Cc: Michal Hocko Cc: Mike Rapoport Cc: Muchun Song Cc: Namhyung kim Cc: Naoya Horiguchi Cc: Nhat Pham Cc: Nico Pache Cc: Oleg Nesterov Cc: Oscar Salvador Cc: Pedro Falcato Cc: Peter Xu Cc: Peter Zijlstra Cc: Rakie Kim Cc: Rik van Riel Cc: Rodrigo Vivi Cc: Ryan Roberts Cc: Steven Price Cc: Suren Baghdasaryan Cc: Sven Schnelle Cc: Thomas Hellström Cc: Thomas Zimemrmann Cc: Vasily Gorbik Cc: Vlastimil Babka Cc: xu xin Cc: Zi Yan Signed-off-by: Andrew Morton --- tools/testing/selftests/mm/merge.c | 57 ++++++++++++++++++++++++++++++ 1 file changed, 57 insertions(+) diff --git a/tools/testing/selftests/mm/merge.c b/tools/testing/selftests/mm/merge.c index 519e5ac02db7..52b8727b6628 100644 --- a/tools/testing/selftests/mm/merge.c +++ b/tools/testing/selftests/mm/merge.c @@ -1305,6 +1305,63 @@ TEST_F(merge, merge_vmas_with_mseal) ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 2 * page_size); } +TEST_F(merge, anon_and_page_offset_mismatch_memfd) +{ + struct procmap_fd *procmap = &self->procmap; + unsigned int page_size = self->page_size; + char *carveout = self->carveout; + char *ptr, *ptr2; + int fd; + + /* Create a 10 page memfd descriptor. */ + fd = memfd_create("anon_page_offset_test", MFD_CLOEXEC); + ASSERT_NE(fd, -1); + ASSERT_EQ(ftruncate(fd, 10 * page_size), 0); + + /* Map a region using the memfd at page offset 0. */ + ptr = mmap(carveout, 5 * page_size, PROT_READ | PROT_WRITE, + MAP_FIXED | MAP_PRIVATE, fd, 0); + ASSERT_NE(ptr, MAP_FAILED); + + /* + * Map another separately and trigger a CoW fault at page offset 5: + * + * |-----------| |---------| + * | unfaulted | | faulted | + * |-----------| |---------| + */ + ptr2 = mmap(&carveout[10 * page_size], 5 * page_size, + PROT_READ | PROT_WRITE, MAP_FIXED | MAP_PRIVATE, + fd, 5 * page_size); + ASSERT_NE(ptr2, MAP_FAILED); + ptr2[0] = 'x'; + + /* + * Now move it in place: + * + * |----------| + * | | + * v | + * |-----------| |---------| + * | unfaulted | | faulted | + * |-----------| |---------| + * + * Because the anonymous page offset of the faulted region is now + * &carveout[10 * page_size], despite the two regions being mergeable + * due to file page offset, they are NOT mergeable due to anonymous + * page offset. + */ + ptr2 = sys_mremap(ptr2, 5 * page_size, 5 * page_size, + MREMAP_MAYMOVE | MREMAP_FIXED, + &carveout[5 * page_size]); + ASSERT_NE(ptr2, MAP_FAILED); + + /* Assert that they did not merge. */ + ASSERT_TRUE(find_vma_procmap(procmap, ptr)); + ASSERT_EQ(procmap->query.vma_start, (unsigned long)ptr); + ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 5 * page_size); +} + TEST_F(merge_with_fork, mremap_faulted_to_unfaulted_prev) { struct procmap_fd *procmap = &self->procmap;