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

From: Ryan Roberts <ryan.roberts@arm.com>
To: Kevin Brodsky <kevin.brodsky@arm.com>, linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org, Albert Ou <aou@eecs.berkeley.edu>,
	Andreas Larsson <andreas@gaisler.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Linus Walleij <linus.walleij@linaro.org>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Matthew Wilcox <willy@infradead.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	"Mike Rapoport (IBM)" <rppt@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Qi Zheng <zhengqi.arch@bytedance.com>,
	Will Deacon <will@kernel.org>,
	Yang Shi <yang@os.amperecomputing.com>,
	linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-csky@vger.kernel.org, linux-m68k@lists.linux-m68k.org,
	linux-openrisc@vger.kernel.org, linux-riscv@lists.infradead.org,
	linux-s390@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	sparclinux@vger.kernel.org
Subject: Re: [PATCH 00/11] Always call constructor for kernel page tables
Date: Mon, 17 Mar 2025 15:30:47 +0000	[thread overview]
Message-ID: <70349335-84ee-4bca-a3d6-d7cf3c05b92b@arm.com> (raw)
In-Reply-To: <20250317141700.3701581-1-kevin.brodsky@arm.com>

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