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

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Message-ID: <0b7543dd-4621-432c-9185-874963e8a6af@redhat.com>
Date: Thu, 14 Aug 2025 11:32:55 +0200
From: David Hildenbrand <david@...hat.com>
To: 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 13.08.25 20:52, Lorenzo Stoakes wrote:
> On Wed, Aug 13, 2025 at 06:24:11PM +0200, David Hildenbrand wrote:
>>>> +
>>>> +FIXTURE_SETUP(prctl_thp_disable_except_madvise)
>>>> +{
>>>> +	if (!thp_available())
>>>> +		SKIP(return, "Transparent Hugepages not available\n");
>>>> +
>>>> +	self->pmdsize = read_pmd_pagesize();
>>>> +	if (!self->pmdsize)
>>>> +		SKIP(return, "Unable to read PMD size\n");
>>>> +
>>>> +	if (prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, NULL, NULL))
>>>> +		SKIP(return, "Unable to set PR_THP_DISABLE_EXCEPT_ADVISED\n");
>>>
>>> This should be a test fail I think, as the only ways this could fail are
>>> invalid flags, or failure to obtain an mmap write lock.
>>
>> Running a kernel that does not support it?
> 
> I can't see anything in the kernel to #ifdef it out so I suppose you mean
> running these tests on an older kernel?

Yes.

> 
> 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?

> 
>>
>> We could check the errno to distinguish I guess.
> 
> Which one? manpage says -EINVAL, but can also be due to incorrect invocation,
> which would mean a typo could mean tests pass but your tests do nothing :)

Right, no ENOSYS in that case to distinguish :(

-- 
Cheers

David / dhildenb