[PATCH v4 09/15] selftest/mm: register existing mapping with userfaultfd in hugepage-mremap

Sayali Patil
2026-04-06T09:21:12Z

Previously, register_region_with_uffd() created a new anonymous
mapping and overwrote the address supplied by the caller before
registering the range with userfaultfd.

As a result, userfaultfd was applied to an unrelated anonymous mapping
instead of the hugetlb region used by the test.

Remove the extra mmap() and register the caller-provided address range
directly using UFFDIO_REGISTER_MODE_MISSING, so that faults are
generated for the hugetlb mapping used by the test.

This ensures userfaultfd operates on the actual hugetlb test region and
validates the expected fault handling.

Before patch:
running ./hugepage-mremap
-------------------------
TAP version 13
1..1
Map haddr: Returned address is 0x7eaa40000000
Map daddr: Returned address is 0x7daa40000000
Map vaddr: Returned address is 0x7faa40000000
Address returned by mmap() = 0x7fff9d000000
Mremap: Returned address is 0x7faa40000000
First hex is 0
First hex is 3020100
ok 1 Read same data
Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
[PASS]
ok 1 hugepage-mremap

After patch:
running ./hugepage-mremap
-------------------------
TAP version 13
1..1
Map haddr: Returned address is 0x7eaa40000000
Map daddr: Returned address is 0x7daa40000000
Map vaddr: Returned address is 0x7faa40000000
Registered memory at address 0x7eaa40000000 with userfaultfd
Mremap: Returned address is 0x7faa40000000
First hex is 0
First hex is 3020100
ok 1 Read same data
Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
[PASS]
ok 1 hugepage-mremap

Fixes: 12b613206474 ("mm, hugepages: add hugetlb vma mremap() test")
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Sayali Patil <sayalip@linux.ibm.com>
---
tools/testing/selftests/mm/hugepage-mremap.c | 21 +++++---------------
1 file changed, 5 insertions(+), 16 deletions(-)

diff --git a/tools/testing/selftests/mm/hugepage-mremap.c b/tools/testing/selftests/mm/hugepage-mremap.c
index f66e4d806477..68c35d1d8a5f 100644
--- a/tools/testing/selftests/mm/hugepage-mremap.c
+++ b/tools/testing/selftests/mm/hugepage-mremap.c
@@ -86,25 +86,14 @@ static void register_region_with_uffd(char *addr, size_t len)
if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1)
ksft_exit_fail_msg("ioctl-UFFDIO_API: %s\n", strerror(errno));

- /* Create a private anonymous mapping. The memory will be
- * demand-zero paged--that is, not yet allocated. When we
- * actually touch the memory, it will be allocated via
- * the userfaultfd.
- */
-
- addr = mmap(NULL, len, PROT_READ | PROT_WRITE,
- MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
- if (addr == MAP_FAILED)
- ksft_exit_fail_msg("mmap: %s\n", strerror(errno));
-
- ksft_print_msg("Address returned by mmap() = %p\n", addr);
-
- /* Register the memory range of the mapping we just created for
- * handling by the userfaultfd object. In mode, we request to track
- * missing pages (i.e., pages that have not yet been faulted in).
+ /* Register the passed memory range for handling by the userfaultfd object.
+ * In mode, we request to track missing pages
+ * (i.e., pages that have not yet been faulted in).
*/
if (uffd_register(uffd, addr, len, true, false, false))
ksft_exit_fail_msg("ioctl-UFFDIO_REGISTER: %s\n", strerror(errno));
+
+ ksft_print_msg("Registered memory at address %p with userfaultfd\n", addr);
}

int main(int argc, char *argv[])
--
2.52.0