Re: [PATCH v3 07/13] selftest/mm: register existing mapping with userfaultfd in hugepage-mremap

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/27/26 08:16, Sayali Patil wrote:
> 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

Okay, so we tested mremap() of something that is not even hugetlb.

> 
> Fixes: 12b613206474 ("mm, hugepages: add hugetlb vma mremap() test")
> 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 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 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[])

Yes, that code is extremely weird. I wonder if this was some
copy-and-paste from other uffd test code.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David