linux-kernel - Re: [PATCH v3 6/6] selftests: prctl: introduce tests for disabling THPs except for madvise

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Message-ID: <ff285199-5f29-44b8-81df-891196eeca3d@redhat.com>
Date: Tue, 5 Aug 2025 14:29:17 +0200
From: David Hildenbrand <david@...hat.com>
To: Usama Arif <usamaarif642@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc: 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, lorenzo.stoakes@...cle.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 v3 6/6] selftests: prctl: introduce tests for disabling
 THPs except for madvise

On 05.08.25 14:19, Usama Arif wrote:
> 
> 
> On 05/08/2025 11:36, David Hildenbrand wrote:
>> On 04.08.25 17:40, Usama Arif wrote:
>>> The test will set the global system THP setting to never, madvise
>>> or always depending on the fixture variant and the 2M setting to
>>> inherit before it starts (and reset to original at teardown)
>>>
>>> This tests if the process can:
>>> - successfully set and get the policy to disable THPs expect for madvise.
>>> - get hugepages only on MADV_HUGE and MADV_COLLAPSE if the global policy
>>>     is madvise/always and only with MADV_COLLAPSE if the global policy is
>>>     never.
>>> - successfully reset the policy of the process.
>>> - after reset, only get hugepages with:
>>>     - MADV_COLLAPSE when policy is set to never.
>>>     - MADV_HUGE and MADV_COLLAPSE when policy is set to madvise.
>>>     - always when policy is set to "always".
>>> - repeat the above tests in a forked process to make sure  the policy is
>>>     carried across forks.
>>>
>>> Signed-off-by: Usama Arif <usamaarif642@...il.com>
>>> ---
>>
>> [...]
>>
>>> +FIXTURE_VARIANT(prctl_thp_disable_except_madvise)
>>> +{
>>> +    enum thp_enabled thp_policy;
>>> +};
>>> +
>>> +FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, never)
>>> +{
>>> +    .thp_policy = THP_NEVER,
>>> +};
>>> +
>>> +FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, madvise)
>>> +{
>>> +    .thp_policy = THP_MADVISE,
>>> +};
>>> +
>>> +FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, always)
>>> +{
>>> +    .thp_policy = THP_ALWAYS,
>>> +};
>>> +
>>> +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");
>>
>> Should we test here if the kernel knows PR_THP_DISABLE_EXCEPT_ADVISED, and if not, skip?
>>
>> Might be as simple as trying issuing two prctl, and making sure the first disabling attempt doesn't fail. If so, SKIP.
>>
>> Nothing else jumped at me. Can you include a test run result in the patch description?
>>
> 
> Instead of 2 prctls, I think doing just the below should be enough:
> 
> diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
> index 93cedaa59854..da28bc4441ed 100644
> --- a/tools/testing/selftests/mm/prctl_thp_disable.c
> +++ b/tools/testing/selftests/mm/prctl_thp_disable.c
> @@ -236,6 +236,9 @@ FIXTURE_SETUP(prctl_thp_disable_except_madvise)
>          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");
> +
>          thp_save_settings();
>          thp_read_settings(&self->settings);
>          self->settings.thp_enabled = variant->thp_policy;

Then probably best to remove the

ASSERT_EQ(prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, 
NULL, NULL), 0);

 From both test functions?

You can consider doing the same in patch #5.

-- 
Cheers,

David / dhildenb