While maintaining virtual 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 virtual 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 virtual page offsets. Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]> --- 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..c093e27d0aea 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, virt_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("virt_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 fault in, 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 virtual 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 virtual 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; -- 2.55.0
