linux-kernel - Re: [PATCH RESEND] mm/pagewalk: split walk_page_range

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Message-ID: <d0df9d25-f6c7-46ce-9808-cc92855d6e9d@lucifer.local>
Date: Wed, 4 Jun 2025 10:09:05 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Barry Song <baohua@...nel.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Oscar Salvador <osalvador@...e.de>,
        Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Stafford Horne <shorne@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
        Alexandre Ghiti <alex@...ti.fr>, Jann Horn <jannh@...gle.com>,
        loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
        linux-openrisc@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-mm@...ck.org
Subject: Re: [PATCH RESEND] mm/pagewalk: split walk_page_range_novma() into
 kernel/user parts

On Wed, Jun 04, 2025 at 11:07:05AM +0300, Mike Rapoport wrote:
> On Wed, Jun 04, 2025 at 09:39:30AM +0200, David Hildenbrand wrote:
> > On 03.06.25 21:22, Lorenzo Stoakes wrote:
> > > The walk_page_range_novma() function is rather confusing - it supports two
> > > modes, one used often, the other used only for debugging.
> > >
> > > The first mode is the common case of traversal of kernel page tables, which
> > > is what nearly all callers use this for.
> >
> > ... and what people should be using it for 🙂
> >
> > >
> > > Secondly it provides an unusual debugging interface that allows for the
> > > traversal of page tables in a userland range of memory even for that memory
> > > which is not described by a VMA.
> > >
> > > This is highly unusual and it is far from certain that such page tables
> > > should even exist, but perhaps this is precisely why it is useful as a
> > > debugging mechanism.
> > >
> > > As a result, this is utilised by ptdump only. Historically, things were
> > > reversed - ptdump was the only user, and other parts of the kernel evolved
> > > to use the kernel page table walking here.
> > >
> > > Since we have some complicated and confusing locking rules for the novma
> > > case, it makes sense to separate the two usages into their own functions.
> > >
> > > Doing this also provide self-documentation as to the intent of the caller -
> > > are they doing something rather unusual or are they simply doing a standard
> > > kernel page table walk?
> > >
> > > We therefore maintain walk_page_range_novma() for this single usage, and
> > > document the function as such.
> >
> > If we have to keep this dangerous interface, it should probably be
> >
> > walk_page_range_debug() or walk_page_range_dump()
>
> We can also move it from include/linux/pagewalk.h to mm/internal.h

Yeah I was wondering about a rename actually, but then thought 'oh novma kinda
fits blah blah' but you're right, this is better.

Nice idea to move to mm/internal.h also :) I like this...

Will fixup on respin

>
> > >
> > > Note that ptdump uses the precise same function for kernel walking as a
> > > convenience, so we permit this but make it very explicit by having
> > > walk_page_range_novma() invoke walk_page_range_kernel() in this case.
> > >
> > > We introduce walk_page_range_kernel() for the far more common case of
> > > kernel page table traversal.
> >
> > I wonder if we should give it a completely different name scheme to
> > highlight that this is something completely different.
> >
> > walk_kernel_page_table_range()
> >
> > etc.
> >
> >
> > --
> > Cheers,
> >
> > David / dhildenb
> >
>
> --
> Sincerely yours,
> Mike.