linux-kernel - Re: [PATCH 00/11] Always call constructor for kernel page tables

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Message-ID: <70349335-84ee-4bca-a3d6-d7cf3c05b92b@arm.com>
Date: Mon, 17 Mar 2025 15:30:47 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Kevin Brodsky <kevin.brodsky@....com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Albert Ou <aou@...s.berkeley.edu>,
 Andreas Larsson <andreas@...sler.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Catalin Marinas <catalin.marinas@....com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 "David S. Miller" <davem@...emloft.net>,
 Geert Uytterhoeven <geert@...ux-m68k.org>,
 Linus Walleij <linus.walleij@...aro.org>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Mark Rutland <mark.rutland@....com>, Matthew Wilcox <willy@...radead.org>,
 Michael Ellerman <mpe@...erman.id.au>, "Mike Rapoport (IBM)"
 <rppt@...nel.org>, Palmer Dabbelt <palmer@...belt.com>,
 Paul Walmsley <paul.walmsley@...ive.com>,
 Peter Zijlstra <peterz@...radead.org>, Qi Zheng
 <zhengqi.arch@...edance.com>, Will Deacon <will@...nel.org>,
 Yang Shi <yang@...amperecomputing.com>, linux-arch@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-csky@...r.kernel.org,
 linux-m68k@...ts.linux-m68k.org, linux-openrisc@...r.kernel.org,
 linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH 00/11] Always call constructor for kernel page tables

On 17/03/2025 14:16, Kevin Brodsky wrote:
> The complications in those special pgtable allocators beg the question:
> does it really make sense to treat efi_mm and init_mm differently in
> e.g. apply_to_pte_range()? Maybe what we really need is a way to tell if
> an mm corresponds to user memory or not, and never use split locks for
> non-user mm's. Feedback and suggestions welcome!

The difference in treatment is whether or not the ptl is taken, right? So the
real question is when calling apply_to_pte_range() for efi_mm, is there already
a higher level serialization mechanism that prevents racy accesses? For init_mm,
I think this is handled implicitly because there is no way for user space to
cause apply_to_pte_range() for an arbitrary piece of kernel memory. Although I
can't even see where apply_to_page_range() is called for efi_mm.

FWIW, contpte.c has mm_is_user() which is used by arm64.

Thanks,
Ryan