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

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Message-ID: <f3dad5b5-143d-4896-b315-38e1d7bb1248@redhat.com>
Date: Fri, 30 May 2025 10:44:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

On 30.05.25 10:04, Ryan Roberts wrote:
> On 29/05/2025 09:23, 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.
> 
> This is a drive-by comment from me without having the previous context, but...
> 
> Surely MADV_COLLAPSE *should* ignore the THP sysfs settings? It's a deliberate
> user-initiated, synchonous request to use huge pages for a range of memory.
> There is nothing *transparent* about it, it just happens to be implemented using
> the same logic that THP uses.
> 
> I always thought this was a deliberate design decision.

If the admin said "never", then why should a user be able to overwrite that?

The design decision I recall is that if VM_NOHUGEPAGE is set, we'll 
ignore that. Because that was set by the app itself (MADV_NOHUEPAGE).

-- 
Cheers,

David / dhildenb