linux-kernel - Re: [PATCH v2] mm/pagewalk: split walk_page_range

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Message-ID: <CAG48ez3hvPbfc2dapQQu9TKrjdi5mhZ4tAWi+m0tNZeEtSZBrQ@mail.gmail.com>
Date: Thu, 5 Jun 2025 21:19:02 +0200
From: Jann Horn <jannh@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Barry Song <baohua@...nel.org>, 
	David Hildenbrand <david@...hat.com>, "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>, 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 v2] mm/pagewalk: split walk_page_range_novma() into
 kernel/user parts

On Wed, Jun 4, 2025 at 4:21 PM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> 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.
>
> 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.
>
> 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.

Just for the record, copy-pasting my comment on v1 that was
accidentally sent off-list:
```
Sort of a tangential comment: I wonder if it would make sense to give
ptdump a different page table walker that uses roughly the same safety
contract as gup_fast() - turn off IRQs and then walk the page tables
locklessly. We'd need basically no locking and no special cases
(regarding userspace mappings at least), at the cost of having to
write the walker code such that we periodically restart the walk from
scratch and not being able to inspect referenced pages. (That might
also be nicer for debugging, since it wouldn't block on locks...)
```