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smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=G62+q4R0; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="G62+q4R0" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 294991F000E9; Tue, 14 Jul 2026 10:54:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784026484; bh=BE9Tn1RAp+IvoGH4WDLW9L8P6c+reQ5yX7yO2HvT8Kk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=G62+q4R01qBeZPrRNev9vE4OIHnXH7tAR6mXURlQFZOUnZg764GFzh5Ws+CzndRp4 6H5eX5vAbkjRrtpWKwbBi9XaZvwVe+KWg4XH1UQdflJKeI5T18qjLvG9xw+YmRCgnz DpRikfkfs/jBBxY/QptbK/URo8iNoyoxP3bKNmBGo2YuJK2Y6G4uoCSNqoC/EnVykR hVtRmY3TGkSaiYI3wuePNIm9SOdqK+DlYhJJU5CDgz6DkBceaTBpNGAuSF/E7EDSVp 0uMrFO+GfdEin9HAT0QicMbToGASgZQCdGjyxVVYSyDCGYH61NzH6E07faEz3sSD4U bZb+fKYzllKhg== Message-ID: Date: Tue, 14 Jul 2026 12:54:28 +0200 Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 02/34] ARM: mm: make 2-level pgd_t a scalar To: Pedro Falcato , Yeoreum Yun Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-arch@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, x86@kernel.org, linux-mm@kvack.org, kasan-dev@googlegroups.com, linux-csky@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-openrisc@vger.kernel.org, linux@armlinux.org.uk, akpm@linux-foundation.org, ankur.a.arora@oracle.com, rppt@kernel.org, linmag7@gmail.com, chleroy@kernel.org, klarasmodin@gmail.com, chenhuacai@kernel.org, kernel@xen0n.name, kas@kernel.org, zhangtianyang@loongson.cn, wangyuli@aosc.io, tsbogend@alpha.franken.de, ljs@kernel.org, jgg@ziepe.ca, catalin.marinas@arm.com, will@kernel.org, arnd@arndb.de, ryan.roberts@arm.com, pasha.tatashin@soleen.com, rmclure@linux.ibm.com, baolin.wang@linux.alibaba.com, tj@kernel.org, kevin.brodsky@arm.com, anup@brainfault.org, atish.patra@linux.dev, pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, hannes@cmpxchg.org, mhocko@kernel.org, qi.zheng@linux.dev, shakeel.butt@linux.dev, kasong@tencent.com, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, ryabinin.a.a@gmail.com, glider@google.com, andreyknvl@gmail.com, dvyukov@google.com, vincenzo.frascino@arm.com, anshuman.khandual@arm.com, yang@os.amperecomputing.com, chaitanyas.prakash@arm.com, ardb@kernel.org, guoren@kernel.org, yang.li85200@gmail.com, viro@zeniv.linux.org.uk, dinguyen@kernel.org, schuster.simon@siemens-energy.com, wangruikang@iscas.ac.cn, junhui.liu@pigmoral.tech, muchun.song@linux.dev, vishal.moola@gmail.com, namcao@linutronix.de, pavel@kernel.org, djbw@kernel.org, yu-cheng.yu@intel.com, baolu.lu@linux.intel.com, Jonathan.Cameron@huawei.com, coxu@redhat.com, andreas@gaisler.com, liam@infradead.org, vbabka@kernel.org, surenb@google.com, mhocko@suse.com, geert@linux-m68k.org, shorne@gmail.com, jonas@southpole.se, stefan.kristiansson@saunalahti.fi References: <20260713135614.1618183-1-yeoreum.yun@arm.com> <20260713135614.1618183-3-yeoreum.yun@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/14/26 12:26, Pedro Falcato wrote: > On Mon, Jul 13, 2026 at 02:55:41PM +0100, Yeoreum Yun wrote: >> From: "David Hildenbrand (Arm)" >> >> We don't want pgd_t to be an array, as it prohibits returning it from a >> function, like pgdp_get(). >> >> So let's just use an u64, and extract the right 32bit value in >> pgd_val(). >> >> Leave the STRICT_MM_TYPECHECKS case alone for now. > > I have to ask: is there a good reason for the STRICT_MM_TYPECHECKS ifdef? > > I see the compiler has an awkward time returning a u64 struct (see > https://godbolt.org/z/qejbv6j9a), but if this doesn't work maybe we should > get rid of the STRICT_MM_TYPECHECKS stuff? I seriously doubt anyone is > purposefully toggling it on for testing from time to time. > > If STRICT_MM_TYPEDEFS's worse codegen doesn't matter then maybe we should > permanently toggle it on. >> >> As an alternative, we could use the STRICT_MM_TYPECHECKS approach here >> as well, but using an u64 looks conceptually cleaner, even though >> pgd_val() gets a bit more involved. >> >> Signed-off-by: David Hildenbrand (Arm) >> Signed-off-by: Yeoreum Yun >> --- >> arch/arm/include/asm/pgtable-2level-types.h | 13 ++++++++++--- >> 1 file changed, 10 insertions(+), 3 deletions(-) >> >> diff --git a/arch/arm/include/asm/pgtable-2level-types.h b/arch/arm/include/asm/pgtable-2level-types.h >> index 650e793f41429..02052cef9437a 100644 >> --- a/arch/arm/include/asm/pgtable-2level-types.h >> +++ b/arch/arm/include/asm/pgtable-2level-types.h >> @@ -25,7 +25,7 @@ typedef struct { pteval_t pgprot; } pgprot_t; >> >> #define pte_val(x) ((x).pte) >> #define pmd_val(x) ((x).pmd) >> -#define pgd_val(x) ((x).pgd[0]) >> +#define pgd_val(x) ((x).pgd[0]) >> #define pgprot_val(x) ((x).pgprot) >> >> #define __pte(x) ((pte_t) { (x) } ) >> @@ -36,14 +36,21 @@ typedef struct { pteval_t pgprot; } pgprot_t; >> /* >> * .. while these make it easier on the compiler >> */ >> +typedef u64 pgdval_t; >> + >> typedef pteval_t pte_t; >> typedef pmdval_t pmd_t; >> -typedef pmdval_t pgd_t[2]; >> +typedef pgdval_t pgd_t; >> typedef pteval_t pgprot_t; >> >> #define pte_val(x) (x) >> #define pmd_val(x) (x) >> -#define pgd_val(x) ((x)[0]) >> + >> +static inline pmdval_t pgd_val(pgd_t pgd) >> +{ >> + return (*(pmdval_t (*)[2])&pgd)[0]; > > Ugh. This isn't correct C code. It only works because the kernel passes > -fno-strict-aliasing. So it is correct C code? ;) > I would recommend either forcing a struct here, or > using a u64 with bitmasks/shifts. I had that, but little vs. big endian was annoying. -- Cheers, David