linux-kernel - Re: [RFC V1 PATCH mm-hotfixes 1/3] mm: introduce and use {pgd,p4d}_populate

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Message-ID: <aHS7hK-BDC3miisN@hyeyoo>
Date: Mon, 14 Jul 2025 17:10:44 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: David Hildenbrand <david@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...two.org>, "H . Peter Anvin" <hpa@...or.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Juergen Gross <jgross@...e.com>, Kevin Brodsky <kevin.brodsky@....com>,
        Muchun Song <muchun.song@...ux.dev>,
        Oscar Salvador <osalvador@...e.de>,
        Joao Martins <joao.m.martins@...cle.com>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Jane Chu <jane.chu@...cle.com>, Alistair Popple <apopple@...dia.com>,
        Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, stable@...r.kernel.org
Subject: Re: [RFC V1 PATCH mm-hotfixes 1/3] mm: introduce and use
 {pgd,p4d}_populate_kernel()

On Sun, Jul 13, 2025 at 08:56:10PM +0300, Mike Rapoport wrote:
> On Sun, Jul 13, 2025 at 08:39:53PM +0900, Harry Yoo wrote:
> > On Fri, Jul 11, 2025 at 06:18:44PM +0200, David Hildenbrand wrote:
> > > > population helpers that invoke architecture-specific hooks to properly
> > > > synchronize the page tables.
> > > 
> > > I was expecting to see the sync be done in common code -- such that it
> > > cannot be missed :)
> > 
> > You mean something like an arch-independent implementation of
> > sync_global_pgds()?
> >
> > That would be a "much more robust" approach ;)
> > 
> > To do that, the kernel would need to maintain a list of page tables that
> > have kernel portion mapped and perform the sync in the common code.
> > 
> > But determining which page tables to add to the list would be highly
> > architecture-specific. For example, I think some architectures use separate
> > page tables for kernel space, unlike x86 (e.g., arm64 TTBR1, SPARC) and
> > user page tables should not be affected.
> 
> sync_global_pgds() can be still implemented per architecture, but it can be
> called from the common code.

A good point, and that can be done!

Actually, that was the initial plan and I somehow thought that
you can't determine if the architecture is using 5-level or 4-level paging
and decide whether to call arch_sync_kernel_pagetables(). But looking at
how it's done in vmalloc, I think it can be done in a similar way.

> We already have something like that for vmalloc that calls
> arch_sync_kernel_mappings(). It's implemented only by x86-32 and arm, other
> architectures do not define it.

It is indeed a good example and was helpful.
Thank you for the comment, Mike!

> -- 
> Sincerely yours,
> Mike.

-- 
Cheers,
Harry / Hyeonggon