userfaultfd/selftests: create alias mappings in the shmem test

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.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Axel Rasmussen <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Brian Geffon <[email protected]>
Cc: "Dr . David Alan Gilbert" <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Lokesh Gidra <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Mina Almasry <[email protected]>
Cc: Oliver Upton <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Wang Qing <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: CmdrMoozy

tools/testing/selftests/vm/userfaultfd.c

@@ -278,13 +278,29 @@ 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 {
@@ -314,7 +330,7 @@ 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 = {