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

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Message-ID: <1fd82421-90ad-4952-8fc0-77859f861570@bytedance.com>
Date: Thu, 5 Jun 2025 14:13:27 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: 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>, 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, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] mm/pagewalk: split walk_page_range_novma() into
 kernel/user parts



On 6/4/25 10:19 PM, 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.
> 
> We additionally make walk_page_range_debug() internal to mm.
> 
> 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_kernel_page_table_range() in this case.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Acked-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> ---
> v2:
> * Renamed walk_page_range_novma() to walk_page_range_debug() as per David.
> * Moved walk_page_range_debug() definition to mm/internal.h as per Mike.
> * Renamed walk_page_range_kernel() to walk_kernel_page_table_range() as
>    per David.
> 
> v1 resend:
> * Actually cc'd lists...
> * Fixed mistake in walk_page_range_novma() not handling kernel mappings and
>    update commit message to referene.
> * Added Mike's off-list Acked-by.
> * Fixed up comments as per Mike.
> * Add some historic flavour to the commit message as per Mike.
> https://lore.kernel.org/all/20250603192213.182931-1-lorenzo.stoakes@oracle.com/
> 
> v1:
> (accidentally sent off-list due to error in scripting)
> 
>   arch/loongarch/mm/pageattr.c |  2 +-
>   arch/openrisc/kernel/dma.c   |  4 +-
>   arch/riscv/mm/pageattr.c     |  8 +--
>   include/linux/pagewalk.h     |  7 ++-
>   mm/hugetlb_vmemmap.c         |  2 +-
>   mm/internal.h                |  4 ++
>   mm/pagewalk.c                | 98 ++++++++++++++++++++++++------------
>   mm/ptdump.c                  |  3 +-
>   8 files changed, 82 insertions(+), 46 deletions(-)
> 

Acked-by: Qi Zheng <zhengqi.arch@...edance.com>

Thanks.