linux-kernel - Re: [PATCH v2 1/2] mm: Allow lockless kernel pagetable walking

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Message-ID: <359668fb-df61-4135-89ee-2675a06a5c1b@lucifer.local>
Date: Tue, 10 Jun 2025 14:27:01 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org,
        catalin.marinas@....com, will@...nel.org, Liam.Howlett@...cle.com,
        vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        suzuki.poulose@....com, steven.price@....com, gshan@...hat.com,
        linux-arm-kernel@...ts.infradead.org, yang@...amperecomputing.com,
        ryan.roberts@....com, anshuman.khandual@....com
Subject: Re: [PATCH v2 1/2] mm: Allow lockless kernel pagetable walking

On Tue, Jun 10, 2025 at 03:24:16PM +0200, David Hildenbrand wrote:
> On 10.06.25 14:07, Lorenzo Stoakes wrote:
> > OK so I think the best solution here is to just update check_ops_valid(), which
> > was kind of sucky anyway (we check everywhere but walk_page_range_mm() to
> > enforce the install pte thing).
> >
> > Let's do something like:
> >
> > #define OPS_MAY_INSTALL_PTE	(1<<0)
> > #define OPS_MAY_AVOID_LOCK	(1<<1)
> >
> > and update check_ops_valid() to take a flags or maybe 'capabilities' field.
> >
> > Then check based on this e.g.:
> >
> > if (ops->install_pte && !(capabilities & OPS_MAY_INSTALL_PTE))
> > 	return false;
> >
> > if (ops->walk_lock == PGWALK_NOLOCK && !(capabilities & OPS_MAY_AVOID_LOCK))
> > 	return false;
> >
>
> Hm. I mean, we really only want to allow this lockless check for
> walk_kernel_page_table_range(), right?
>
> Having a walk_kernel_page_table_range_lockeless() might (or might not) be
> better, to really only special-case this specific path.

Agree completely, Dev - let's definitely do this.

>
> So, I am wondering if we should further start splitting the
> kernel-page-table walker up from the mm walker, at least on the "entry"
> function for now.

How do you mean?

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