linux-kernel - Re: [PATCH v2 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

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Message-ID: <a23c547b-d775-4d8d-a34f-b9b0f200e7f3@linux.alibaba.com>
Date: Wed, 11 Jun 2025 15:05:40 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, david@...hat.com,
 Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
 dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] fix MADV_COLLAPSE issue if THP settings are
 disabled



On 2025/6/7 20:28, Lorenzo Stoakes wrote:
> Before I get into technical criticism, to be clear - thank you very much
> for doing this :) I'm just getting into details as to the implementation,
> but am a fan of this change and consider it important.
> 
> On Thu, Jun 05, 2025 at 04:00:57PM +0800, Baolin Wang wrote:
>> As we discussed in the previous thread [1], the MADV_COLLAPSE will ignore
>> the system-wide anon/shmem THP sysfs settings, which means that even though
>> we have disabled the anon/shmem THP configuration, MADV_COLLAPSE will still
>> attempt to collapse into a anon/shmem THP. This violates the rule we have
>> agreed upon: never means never. This patch set will address this issue.
> 
> Hm this cover letter could be expanded upon quite a bit - you are doing a
> lot here and it's not only MADV_COLLAPSE, more a general change.
> 
> I'd mention that, even when TVA_ENFORCE_SYSFS is not set, callers checking
> THP order validity will not be able to specify THP orders that are either
> specifically marked as 'never' or set to 'inherit' and the global hugepage
> mode is 'never'.
> 
> Then say something like 'importantly, this changes alters the madvise(...,
> MADV_COLLAPSE) call, which previously would collapse ranges into huge pages
> even if THP was set to never. This corrects this behaviour'.
> 
> I suspect you are unable to write sensible tests here given the need to
> manipulate sysfs (though perhaps worth quickly looking at
> tools/testing/selftests/mm/khugepaged.c, transhuge-stress.c, run_vmtests.sh
> to see), but it'd be at least useful for you to give details here of how
> you have tested this and ensured it functions correctly.
> 
> It might also be worth giving a quick justification, i.e. 'system
> administrators who disabled THP everywhere must indeed very much not want
> THP to be used for whatever reason - having individual programs being able
> to quietly override this is very surprising and likely to cause headaches
> for those who desire this not to happen on their systems'.
> 

Ah, missed this comment. Good suggestion, I will update the cover 
letter. Thanks.