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

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Message-ID: <aD_-qdg2OvKQIyRg@kernel.org>
Date: Wed, 4 Jun 2025 11:07:05 +0300
From: Mike Rapoport <rppt@...nel.org>
To: David Hildenbrand <david@...hat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 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
 
> > 
> > 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.