linux-kernel - Re: [PATCH v3 03/10] selftests/mm: Skip uffd-wp-mremap if userfaultfd not available

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Message-ID: <ca8d7011-5f5a-4270-af8e-44b37aff2bb4@arm.com>
Date: Mon, 3 Mar 2025 16:30:45 +0530
From: Dev Jain <dev.jain@....com>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
 linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 Muhammad Usama Anjum <usama.anjum@...labora.com>
Subject: Re: [PATCH v3 03/10] selftests/mm: Skip uffd-wp-mremap if userfaultfd
 not available

+ Muhammad, I guess he has been working on selftests, maybe he can chime in.

On 03/03/25 4:18 pm, Brendan Jackman wrote:
> On Fri, Feb 28, 2025 at 10:55:00PM +0530, Dev Jain wrote:
>>
>>
>> On 28/02/25 10:24 pm, Brendan Jackman wrote:
>>> It's obvious that this should fail in that case, but still, save the
>>> reader the effort of figuring out that they've run into this by just
>>> SKIPping
>>>
>>> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
>>> ---
>>>    tools/testing/selftests/mm/uffd-wp-mremap.c | 5 ++++-
>>>    1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/tools/testing/selftests/mm/uffd-wp-mremap.c b/tools/testing/selftests/mm/uffd-wp-mremap.c
>>> index 2c4f984bd73caa17e12b9f4a5bb71e7fdf5d8554..c2ba7d46c7b4581a3c32a6b6acd148e3e89c2172 100644
>>> --- a/tools/testing/selftests/mm/uffd-wp-mremap.c
>>> +++ b/tools/testing/selftests/mm/uffd-wp-mremap.c
>>> @@ -182,7 +182,10 @@ static void test_one_folio(size_t size, bool private, bool swapout, bool hugetlb
>>>    	/* Register range for uffd-wp. */
>>>    	if (userfaultfd_open(&features)) {
>>> -		ksft_test_result_fail("userfaultfd_open() failed\n");
>>> +		if (errno == ENOENT)
>>> +			ksft_test_result_skip("userfaultfd not available\n");
>>> +		else
>>> +			ksft_test_result_fail("userfaultfd_open() failed\n");
>>>    		goto out;
>>>    	}
>>>    	if (uffd_register(uffd, mem, size, false, true, false)) {
>>>
>>
>> I think you are correct, just want to confirm whether "uffd not available"
>> if and only if "errno == ENOENT" is true. That is,
>> is it possible that errno can be something else and uffd is still not
>> available,
> 
> Yeah, I strongly suspect this can happen. This is an attempt to
> improve things but I don't think it's a full solution.
> 
> I've been pondering this a bit and I think it's impractical to solve
> problems like this in the code of individual testst. I think the right
> thing to do is either:
> 
> 1. Have a centralised facility for detecting conditions like
>     "userfaultfd not available" that tests can just query it, so they
>     say something like:
> 
>     ksft_test_requires("userfaultfd");

Agreed, there should be a single point of reporting whether the facility 
is available.

> 
>     Which would do some sort of actual principled check for presence
>     and then skip the test with an informative message when it's not
>     there. There would be a list of these "system requirements" in the
>     code so you can easily see in one place what things might be needed
>     to successfully run all the tests.
> 
> or
> 
> 2. Specify out of band that there's a fixed set of requirements for
>     running the tests and document that you shouldn't run them without
>     satisfying them. Then just don't bother with SKIPs and call it user
>     error.
> 
>     This would require some reasonably usable tooling for actually
>     getting a system that satisfies the requirements.
> 
> But both of them require a deeper investment. I would quite like to
> explore option 1 a bit but that's for a future Brendan.
> 
> In the meantime I'm just trying to get these tests running on
> virtme-ng. (I'm not even gonna add all of them, because e.g. once I
> noticed this one I added a `scripts/config -e USERFAULTFD` to my
> script, so I won't notice if anything else is missing the check).
> 
>> or errno can be ENOENT even if uffd is available.
> 
> I think it's probably posible for this to happen too, e.g. if the
> system has a perverted /dev or something. But again I think that can
> only be solved with the kinda stuff I mentioned above.
> 
> Sorry for the essay :D