linux-kernel - Re: [PATCH v4 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

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Message-ID: <8691d74b-67ee-4e26-81ac-f6bf1725361e@redhat.com>
Date: Wed, 25 Jun 2025 09:36:53 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Hugh Dickins <hughd@...gle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 ziy@...dia.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, baohua@...nel.org,
 zokeefe@...gle.com, shy828301@...il.com, usamaarif642@...il.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/2] fix MADV_COLLAPSE issue if THP settings are
 disabled

On 25.06.25 09:30, Lorenzo Stoakes wrote:
> On Wed, Jun 25, 2025 at 09:23:31AM +0200, David Hildenbrand wrote:
>> Well, one could argue we broke user space (admin settings) when we converted
>> "never" to no longer mean "never", but "never by page faults + khugepaged".
>> And we did so without updating the documentation.
>>
>> I finally went back and checked the original discussions and, yes, this was
>> deliberate [1].
>>
>> As so often, we created a mess with THP toggles.
> 
> I mean... !!!
> 
>>
>> Probably best to fixup the "never" documentation, and state that there is no
>> way to disable MADV_COLLAPSE anymore.
> 
> I disagree on the basis that system administrators will absolutely expect:
> 
> # echo never > /sys/kernel/mm/transparent-hugepage/enabled
> 
> To disable THP.
> 
> I _guarantee_ you that's what nearly everybody except a handful of people will
> expect.

I know, See my other mail, the problem is rather if there is no somebody 
relying on never+MADV_COLLAPSE from doing the MADV_COLLAPSE-documented 
thing.

It's a mess.

(I have the suspicion that Hugh might know of such a user :) )

> 
> If we do decide to not do this series, _please_ can we seriously update the
> documentation to be _absolutely crystal clear_ about this.
> 
> I will volunteer to do this in this case :)
> 
>>
>> I agree that if we want a way to disable all of them, we better have a
>> "deny" now. ... until someone else breaks that, then we can have a
>> "really_never_deny_all" etc. ;)
> 
> I really really dislike this. 'Deny' is weaker than 'never'. And now we have to
> add even more complexity to the thing.

See my other mail, for shmem_enabled "deny" is stronger than "never", lol.

-- 
Cheers,

David / dhildenb