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

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Message-ID: <51ec4269-b132-4163-9cb5-766042a3769d@redhat.com>
Date: Wed, 4 Jun 2025 09:39:30 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Barry Song <baohua@...nel.org>, "Liam R . Howlett"
 <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>,
 Mike Rapoport <rppt@...nel.org>, 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 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()

> 
> 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