[PATCH 6/9] userfaultfd/selftests: create alias mappings in the shmem test

[Axel Rasmussen]

Previously, we just allocated two shm areas: area_src and area_dst. With
this commit, change this so we also allocate area_src_alias, and
area_dst_alias.

area_*_alias and area_* (respectively) point to the same underlying
physical pages, but are different VMAs. In a future commit in this
series, we'll leverage this setup to exercise minor fault handling
support for shmem, just like we do in the hugetlb_shared test.

Signed-off-by: Axel Rasmussen <axelrasmus...@google.com>
---
 tools/testing/selftests/vm/userfaultfd.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/vm/userfaultfd.c 
b/tools/testing/selftests/vm/userfaultfd.c
index b0af88b258d7..4b49b2cf9819 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -279,13 +279,29 @@ static void shmem_release_pages(char *rel_area)
 
 static void shmem_allocate_area(void **alloc_area)
 {
-       unsigned long offset =
-               alloc_area == (void **)&area_src ? 0 : nr_pages * page_size;
+       void *area_alias = NULL;
+       bool is_src = alloc_area == (void **)&area_src;
+       unsigned long offset = is_src ? 0 : nr_pages * page_size;
 
        *alloc_area = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
                           MAP_SHARED, shm_fd, offset);
        if (*alloc_area == MAP_FAILED)
                err("mmap of memfd failed");
+
+       area_alias = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
+                         MAP_SHARED, shm_fd, offset);
+       if (area_alias == MAP_FAILED)
+               err("mmap of memfd alias failed");
+
+       if (is_src)
+               area_src_alias = area_alias;
+       else
+               area_dst_alias = area_alias;
+}
+
+static void shmem_alias_mapping(__u64 *start, size_t len, unsigned long offset)
+{
+       *start = (unsigned long)area_dst_alias + offset;
 }
 
 struct uffd_test_ops {
@@ -315,7 +331,7 @@ static struct uffd_test_ops shmem_uffd_test_ops = {
        .expected_ioctls = SHMEM_EXPECTED_IOCTLS,
        .allocate_area  = shmem_allocate_area,
        .release_pages  = shmem_release_pages,
-       .alias_mapping = noop_alias_mapping,
+       .alias_mapping = shmem_alias_mapping,
 };
 
 static struct uffd_test_ops hugetlb_uffd_test_ops = {
-- 
2.31.1.295.g9ea45b61b8-goog