Re: [PATCH v3] 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 v3] mm/pagewalk: split walk_page_range_novma() into kernel/user parts
Date: Thu, 12 Jun 2025 14:45:41 +0200	[thread overview]
Message-ID: <95b46a68-3461-4555-a168-df82d53efcf3@redhat.com> (raw)
In-Reply-To: <20250605135104.90720-1-lorenzo.stoakes@oracle.com>

On 05.06.25 15:51, 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.
> 
> 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.
> 
> 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 establish two separate functions - walk_page_range_debug() for
> this single usage, and walk_kernel_page_table_range() for general kernel
> page table walking.
> 
> The walk_page_range_debug() function is currently used to traverse both
> userland and kernel mappings, so we maintain this and in the case of kernel
> mappings being traversed, we have walk_page_range_debug() invoke
> walk_kernel_page_table_range() internally.
> 
> We additionally make walk_page_range_debug() internal to mm.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Acked-by: Qi Zheng <zhengqi.arch@bytedance.com>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb


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