linux-kernel - Re: [PATCH 10/29] selftests/mm: Test UFFDIO

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Message-ID: <cfc0d7c8-edb5-ae5d-8edf-d4f7ee18b877@redhat.com>
Date:   Mon, 3 Apr 2023 09:55:41 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Leonardo Bras Soares Passos <lsoaresp@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH 10/29] selftests/mm: Test UFFDIO_ZEROPAGE only when
 !hugetlb

On 01.04.23 03:57, Axel Rasmussen wrote:
> On Fri, Mar 31, 2023 at 11:37 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>>
>> On 03/30/23 12:07, Peter Xu wrote:
>>> Make the check as simple as "test_type == TEST_HUGETLB" because that's the
>>> only mem that doesn't support ZEROPAGE.
>>>
>>> Signed-off-by: Peter Xu <peterx@...hat.com>
>>> ---
>>>   tools/testing/selftests/mm/userfaultfd.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/tools/testing/selftests/mm/userfaultfd.c b/tools/testing/selftests/mm/userfaultfd.c
>>> index 795fbc4d84f8..d724f1c78847 100644
>>> --- a/tools/testing/selftests/mm/userfaultfd.c
>>> +++ b/tools/testing/selftests/mm/userfaultfd.c
>>> @@ -1118,7 +1118,7 @@ static int __uffdio_zeropage(int ufd, unsigned long offset, bool retry)
>>>   {
>>>        struct uffdio_zeropage uffdio_zeropage;
>>>        int ret;
>>> -     bool has_zeropage = get_expected_ioctls(0) & (1 << _UFFDIO_ZEROPAGE);
>>> +     bool has_zeropage = !(test_type == TEST_HUGETLB);
>>
>> It is true that hugetlb is the only mem type that does not support zeropage.
>> So, the change is correct.
>>
>> However, I actually prefer the explicit check that is there today.  It seems
>> more like a test of the API.  And, is more future proof is code changes.
>>
>> Just my opinion/thoughts, not a strong objection.
> 
> I agree. The existing code is more robust to future changes where we
> might support or stop supporting this ioctl in some cases. It also
> proves that the ioctl works, any time the API reports that it is
> supported / ought to work, independent of when the *test* thinks it
> should be supported.
> 
> Then again, I think this is unlikely to change in the future, so I
> also agree with Mike that it's not the biggest deal.

As there were already discussions on eventually supporting 
UFFDIO_ZEROPAGE that doesn't place the shared zeropage but ... a fresh 
zeropage, it might make sense to keep it as is.

-- 
Thanks,

David / dhildenb