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

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Message-ID: <ade3bdb7-7103-4ecd-bce2-7768a0d729ef@lucifer.local>
Date: Fri, 30 May 2025 09:47:58 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Ryan Roberts <ryan.roberts@....com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
        hughd@...gle.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 Fri, May 30, 2025 at 10:44:36AM +0200, David Hildenbrand wrote:
> 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).
>

I'm with David on this one.

I think it's principal of least surprise - to me 'never' is pretty
emphatic, and keep in mind the other choices are 'always' and...  'madvise'
:) which explicitly is 'hey only do this if madvise tells you to'.

I'd be rather surprised if I hadn't set madvise and a user uses madvise to
in some fashion override the never.

I mean I think we all agree this interface is to use a technical term -
crap - and we need something a lot more fine-grained and smart, but I think
given the situation we're in we should make it at least as least surprising
as possible.

> --
> Cheers,
>
> David / dhildenb
>