linux-kernel - Re: [PATCH v3] mm/pagewalk: split walk_page_range

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Message-ID: <31d04f63-5595-48c6-adf1-675648a0c130@suse.cz>
Date: Thu, 5 Jun 2025 15:54:42 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Barry Song <baohua@...nel.org>, David Hildenbrand <david@...hat.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, 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
Subject: Re: [PATCH v3] mm/pagewalk: split walk_page_range_novma() into
 kernel/user parts

On 6/5/25 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@...cle.com>
> Acked-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> Acked-by: Qi Zheng <zhengqi.arch@...edance.com>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!