Re: [PATCH v3 07/13] selftest/mm: register existing mapping with userfaultfd in hugepage-mremap
From: Sayali Patil
Date: Fri Apr 03 2026 - 13:45:07 EST
On 02/04/26 13:01, David Hildenbrand (Arm) wrote:
On 4/1/26 16:43, Sayali Patil wrote:Thanks for the review. Yes, tested with this change and it behaves as expected now.
On 01/04/26 19:48, David Hildenbrand (Arm) wrote:
On 3/27/26 08:16, Sayali Patil wrote:Hi David,
Previously, register_region_with_uffd() created a new anonymousOkay, so we tested mremap() of something that is not even hugetlb.
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")/*
Tested-by: Venkat Rao Bagalkote <venkat88@xxxxxxxxxxxxx>
Signed-off-by: Sayali Patil <sayalip@xxxxxxxxxxxxx>
---
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 b8f7d92e5a35..e611249080d6 100644
--- a/tools/testing/selftests/mm/hugepage-mremap.c
+++ b/tools/testing/selftests/mm/hugepage-mremap.c
@@ -85,25 +85,14 @@ 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.
* ...
While at it.
+ * In mode, we request to track missing pagesYes, that code is extremely weird. I wonder if this was some
+ * (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[])
copy-and-paste from other uffd test code.
Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
Yes, the test operates on hugetlb mappings created with
|MAP_HUGETLB | MAP_POPULATE|and sets up userfaultfd. Consequently,
registering it with |UFFDIO_REGISTER_MODE_MISSING| does not result in
any userfaults.
Originally, the helper function created a separate anonymous mapping and
registered it with userfaultfd instead of the address supplied by the
caller. However, the test operates on hugetlb mappings, and the registered
anonymous mapping is never used in the |mremap()| path being exercised.
Would it be better to remove userfaultfd registration entirely from this
test, since that path is not actually being tested?
If it's tested with your change now (which I think that's what
happenes), this is fine.
It was just very weird before, because it tested something fairly unrelated.