Re: [PATCH RESEND] mm/pagewalk: split walk_page_range_novma() into kernel/user parts

From: David Hildenbrand <david@redhat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: Barry Song <baohua@kernel.org>,
	"Liam R . Howlett" <Liam.Howlett@oracle.com>,
	Vlastimil Babka <vbabka@suse.cz>, Mike Rapoport <rppt@kernel.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Michal Hocko <mhocko@suse.com>,
	Muchun Song <muchun.song@linux.dev>,
	Oscar Salvador <osalvador@suse.de>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>, Jonas Bonn <jonas@southpole.se>,
	Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>,
	Stafford Horne <shorne@gmail.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>, Jann Horn <jannh@google.com>,
	loongarch@lists.linux.dev, linux-kernel@vger.kernel.org,
	linux-openrisc@vger.kernel.org, linux-riscv@lists.infradead.org,
	linux-mm@kvack.org
Subject: Re: [PATCH RESEND] mm/pagewalk: split walk_page_range_novma() into kernel/user parts
Date: Wed, 4 Jun 2025 09:39:30 +0200	[thread overview]
Message-ID: <51ec4269-b132-4163-9cb5-766042a3769d@redhat.com> (raw)
In-Reply-To: <20250603192213.182931-1-lorenzo.stoakes@oracle.com>

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

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