linux-kernel - Re: [PATCH 1/6] prctl: introduce PR_THP_POLICY_DEFAULT

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Message-ID: <8ea288f2-5196-41f9-bd65-e29f22bb29e8@lucifer.local>
Date: Thu, 15 May 2025 19:08:48 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Usama Arif <usamaarif642@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        hannes@...xchg.org, shakeel.butt@...ux.dev, riel@...riel.com,
        ziy@...dia.com, laoar.shao@...il.com, baolin.wang@...ux.alibaba.com,
        Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...a.com
Subject: Re: [PATCH 1/6] prctl: introduce PR_THP_POLICY_DEFAULT_HUGE for the
 process

On Thu, May 15, 2025 at 06:11:55PM +0200, David Hildenbrand wrote:
> > > > So if you're not overriding VM_NOHUGEPAGE, the whole point of this exercise
> > > > is to override global 'never'?
> > > >
> > >
> > > Again, I am not overriding never.
> > >
> > > hugepage_global_always and hugepage_global_enabled will evaluate to false
> > > and you will not get a hugepage.
> >
> > Yeah, again ack, but I kind of hate that we set VM_HUGEPAGE everywhere even
> > if the policy is never.
>
> I think it should behave just as if someone does manually an madvise(). So
> whatever we do here during an madvise, we should try to do the same thing
> here.

Ack I agree with this.

It actually simplifies things a LOT to view it this way - we're saying 'by
default apply madvise(...) to new VMAs'.

Hm I wonder if we could have a more generic version of this...

Note though that we're not _quite_ doing this.

So in hugepage_madvise():

int hugepage_madvise(struct vm_area_struct *vma,
		     unsigned long *vm_flags, int advice)
{
	...

	switch (advice) {
	case MADV_HUGEPAGE:
		*vm_flags &= ~VM_NOHUGEPAGE;
		*vm_flags |= VM_HUGEPAGE;

		...

		break;

		...
	}

	...
}

So here we're actually clearing VM_NOHUGEPAGE and overriding it, but in the
proposed code we're not.

So we're back into confusing territory again :)

I wonder if we could...

1. Add an MADV_xxx that mimics the desired behaviour here.

2. Add a generic 'madvise() by default' thing at a process level?

Is this crazy?

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