linux-kernel - Re: [PATCH v4 7/7] selftests: prctl: introduce tests for disabling THPs except for madvise

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Message-ID: <eb90eff6-ded8-40a3-818f-fce3331df464@redhat.com>
Date: Thu, 14 Aug 2025 14:00:27 +0200
From: David Hildenbrand <david@...hat.com>
To: Mark Brown <broonie@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Usama Arif <usamaarif642@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, corbet@....net, rppt@...nel.org,
 surenb@...gle.com, mhocko@...e.com, hannes@...xchg.org, baohua@...nel.org,
 shakeel.butt@...ux.dev, riel@...riel.com, ziy@...dia.com,
 laoar.shao@...il.com, dev.jain@....com, baolin.wang@...ux.alibaba.com,
 npache@...hat.com, Liam.Howlett@...cle.com, ryan.roberts@....com,
 vbabka@...e.cz, jannh@...gle.com, Arnd Bergmann <arnd@...db.de>,
 sj@...nel.org, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 kernel-team@...a.com
Subject: Re: [PATCH v4 7/7] selftests: prctl: introduce tests for disabling
 THPs except for madvise

On 14.08.25 13:45, Mark Brown wrote:
> On Thu, Aug 14, 2025 at 11:49:15AM +0100, Lorenzo Stoakes wrote:
>> On Thu, Aug 14, 2025 at 11:32:55AM +0200, David Hildenbrand wrote:
>>> On 13.08.25 20:52, Lorenzo Stoakes wrote:
> 
>>>> I can't see anything in the kernel to #ifdef it out so I suppose you mean
>>>> running these tests on an older kernel?
> 
> ...
> 
>>>> But this is an unsupported way of running self-tests, they are tied to the
>>>> kernel version in which they reside, and test that specific version.
> 
>>>> Unless I'm missing something here?
> 
>>> I remember we allow for a bit of flexibility when it is simple to handle.
> 
>>> Is that documented somewhere?
> 
>> Not sure if it's documented, but it'd make testing extremely egregious if
>> you had to consider all of the possible kernels and interactions and etc.
> 
>> I think it's 'if it happens to work then fine' but otherwise it is expected
>> that the tests match the kernel.
> 
>> It's also very neat that with a revision you get a set of (hopefully)
>> working tests for that revision :)
> 
> Some people do try to run the selftests with older kernels, they're
> trying to get better coverage for the stables.  For a lot of areas the
> skipping falls out natually since there's some optionality (so even with
> the same kernel version you might not have the feature in the running
> kernel) or it's a new API which has a discovery mechanism in the ABI
> anyway.  OTOH some areas have been actively hostile to the idea of
> running on older kernels so there are things that do break when you try.
> TBH so long as the tests don't crash the system or something people are
> probably just going to ignore any tests that have never passed.

Some people (hello :) ) run tests against distro kernels ... shame that 
prctl just knows one sort of "EINVAL" so we cannot distinguish :(

But yeah, maybe one has to be more careful of filtering these failures 
out then.

-- 
Cheers

David / dhildenb