linux-kernel - Re: [PATCH v3 5/6] selftests: prctl: introduce tests for disabling THPs completely

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Message-ID: <be784521-5507-4d6a-8028-2816c1a18325@redhat.com>
Date: Tue, 5 Aug 2025 16:32:43 +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 5/6] selftests: prctl: introduce tests for disabling
 THPs completely

On 05.08.25 16:26, Usama Arif wrote:
> 
> 
> On 05/08/2025 13:55, David Hildenbrand wrote:
>> On 05.08.25 14:46, Usama Arif wrote:
>>>
>>>
>>> On 05/08/2025 13:39, David Hildenbrand wrote:
>>>>> +FIXTURE_SETUP(prctl_thp_disable_completely)
>>>>> +{
>>>>> +    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");
>>>>> +
>>>>> +    thp_save_settings();
>>>>> +    thp_read_settings(&self->settings);
>>>>> +    self->settings.thp_enabled = variant->thp_policy;
>>>>> +    self->settings.hugepages[sz2ord(self->pmdsize, getpagesize())].enabled = THP_INHERIT;
>>>>
>>>> Oh, one more thing: should we set all other sizes also to THP_INHERIT or (for simplicity) THP_NEVER?
>>>>
>>>
>>> hmm do we need to? I am hoping that we should always get the PMD size THP no matter what the policy
>>> for others is in the scenario we have?
>>
>> Assuming 64K is set to "always", couldn't it mislead us in the "madvise"/"never" test cases in some scenarios?
>>
> 
> I tried it with 64K to always and seems to be ok. check_huge_anon checks AnonHugepages from smaps which
> only indicates the pmd mapped THPs only. So I think should be ok?

Ah, yes. If that's the case, then we can focus on PMD only and not care 
about how the others are configured.

-- 
Cheers,

David / dhildenb