From: "David Hildenbrand (Arm)" <david@kernel.org>
To: Yeoreum Yun <yeoreum.yun@arm.com>,
Russell King <linux@armlinux.org.uk>,
Huacai Chen <chenhuacai@kernel.org>,
WANG Xuerui <kernel@xen0n.name>,
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Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
Andrew Morton <akpm@linux-foundation.org>,
Kairui Song <kasong@tencent.com>, Qi Zheng <qi.zheng@linux.dev>,
Shakeel Butt <shakeel.butt@linux.dev>,
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Axel Rasmussen <axelrasmussen@google.com>,
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Johannes Weiner <hannes@cmpxchg.org>,
Michal Hocko <mhocko@kernel.org>,
Lorenzo Stoakes <ljs@kernel.org>,
Tianrui Zhao <zhaotianrui@loongson.cn>,
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Michal Hocko <mhocko@suse.com>, Jonas Bonn <jonas@southpole.se>,
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Stafford Horne <shorne@gmail.com>
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,
linux-mm@kvack.org, kvm@vger.kernel.org,
kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
linux-openrisc@vger.kernel.org,
Muhammad Usama Anjum <usama.anjum@arm.com>
Subject: Re: [PATCH RFC v3 00/21] mm: change behavior of pXdp_get()/pXd_page() in compile-time folded pgtable
Date: Thu, 10 Sep 2026 13:12:22 +0200 [thread overview]
Message-ID: <52562b42-1186-4e44-90ab-7fb92a6dcb1e@kernel.org> (raw)
In-Reply-To: <20260902-dummy_ptxp3-v3-0-5d8f5b17c25c@arm.com>
On 9/2/26 13:56, Yeoreum Yun wrote:
> Using ptep_get() and its counterparts in common code is suboptimal on
> kernel configurations with generic compile-time folded page tables.
> By default, ptep_get() and its friends expands to READ_ONCE(),
> forcing the compiler to emit a load even when the value is not used afterwards.
>
> This issue was recently reported by Christophe Leroy [1] for ppc32
> preventing futher code conversion to ptep_get()/pmdp_get()/... helper
> and the same behavior can also be observed on arm64 when built with
> 2- or 3-level page tables
>
> e.g) perf_get_page_size() in arm64 with CONFIG_PGTABLE_LEVEL=3:
>
> 00000000000052a0 <perf_get_page_size>:
> ...
> 52dc: d53b4234 mrs x20, DAIF
> 52e0: d50343df msr DAIFSet, #0x3
> ...
> 52fc: d35e9a69 ubfx x9, x19, #30, #9 /* pud_offset_lockless() */
> 5300: f9403508 ldr x8, [x8, #0x68]
> 5304: f869790a ldr x10, [x8, x9, lsl #3] /* pudp_get() */
> 5308: f90007ea str x10, [sp, #0x8]
> 530c: f8697908 ldr x8, [x8, x9, lsl #3] /* pudp_get() */
> ...
> 5360: 90000009 adrp x9, 0x5000 <perf_prepare_sample+0x548>
> 5364: 92746908 and x8, x8, #0x7ffffff000
> 5368: d3557675 ubfx x21, x19, #21, #9 /* pmd_offset_lockless() */
> ...
> 5394: f8757ac8 ldr x8, [x22, x21, lsl #3] /* pmdp_get() */
>
> Though PGTABLE_LEVEL=3, since the pudp_get() still remain with
> READ_ONCE(), there's redundant load for the pud which is folded.
It would be good to include what it looks like with this series.
--
Cheers,
David
prev parent reply other threads:[~2026-09-10 11:12 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-02 11:56 [PATCH RFC v3 00/21] mm: change behavior of pXdp_get()/pXd_page() in compile-time folded pgtable Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 01/21] ARM: mm: make nommu pgd_t a scalar Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 02/21] ARM: mm: make 2-level " Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 03/21] ARM: mm: remove custom pgdp_get() Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 04/21] LoongArch: mm: define pud_leaf() only when PUD exists Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 05/21] MIPS: " Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 06/21] mm/pgtable: define (pgd|p4d|pud)_leaf() for folded page tables Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 07/21] mm/pgtable: define (pgd|p4d|pud)_offset_lockless() " Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 08/21] mm: vmscan: remove stack copy address of pud/pmd pass in walk_pud/pmd_range() Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 09/21] loongarch: kvm: remove stack copy address of pXd in pXd_offset() Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 10/21] riscv: " Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 11/21] riscv: mm: use proper set_pXd() for generic compile-time folded patable in vmalloc_fault() Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 12/21] mm/pgtable: redefine PGTABLE_LEVEL enum with ascend order from PGD Yeoreum Yun
2026-09-10 11:05 ` David Hildenbrand (Arm)
2026-09-02 11:56 ` [PATCH RFC v3 13/21] x86: mm: use pgtable_level enum in effective_prot_pXd() Yeoreum Yun
2026-09-02 20:48 ` Dave Hansen
2026-09-07 8:06 ` Yeoreum Yun
2026-09-10 11:02 ` David Hildenbrand (Arm)
2026-09-10 11:03 ` David Hildenbrand (Arm)
2026-09-02 11:56 ` [PATCH RFC v3 14/21] x86: mm: carve out the generic compile-time folded pgtable case in effective_prot() Yeoreum Yun
2026-09-02 20:46 ` Dave Hansen
2026-09-10 11:06 ` David Hildenbrand (Arm)
2026-09-02 11:56 ` [PATCH RFC v3 15/21] x86: mm: skip collapse_pud_page() when CONFIG_X86_DIRECT_GBPAGES disabled Yeoreum Yun
2026-09-02 15:38 ` Dave Hansen
2026-09-02 16:48 ` Yeoreum Yun
2026-09-10 11:07 ` David Hildenbrand (Arm)
2026-09-02 11:56 ` [PATCH RFC v3 16/21] openrisc/pgtable: drop __pmd_offset() Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 17/21] mm/pgtable: optimize pmdp_get() and friends for folded pagetable levels Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 18/21] mm/pgtable: catch abuse of folded dummy pgd_t/p4d_t/pud_t Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 19/21] mm/pgtable: disallow calling (pgd|p4d|pud)_page, pgd_page_vaddr() and (p4d|pud)_pgtable with dummy Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 20/21] mm/pgtable: disallow calling folded set_pgd/set_p4d/set_pud " Yeoreum Yun
2026-09-02 11:56 ` [PATCH RFC v3 21/21] Documentation: mm: clarify behaviour of compile-time folded page tables Yeoreum Yun
2026-09-10 11:12 ` David Hildenbrand (Arm) [this message]
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