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

[Sayali Patil <sayalip@linux.ibm.com>]

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On 01/04/26 19:48, David Hildenbrand (Arm) wrote:
> 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>
>
>
Hi David,

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?

Thanks,
Sayali


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