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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Pasha Tatashin , Russell King , Catalin Marinas , Will Deacon , Ryan Roberts , linux-arm-kernel@lists.infradead.org, Huacai Chen , loongarch@lists.linux.dev, "James E.J. Bottomley" , Helge Deller , linux-parisc@vger.kernel.org, Madhavan Srinivasan , Michael Ellerman , linuxppc-dev@lists.ozlabs.org, Paul Walmsley , Palmer Dabbelt , Albert Ou , linux-riscv@lists.infradead.org, Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Gerald Schaefer , linux-s390@vger.kernel.org, "David S. Miller" , Andreas Larsson , sparclinux@vger.kernel.org, Richard Weinberger , Anton Ivanov , Johannes Berg , linux-um@lists.infradead.org, Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Ning Sun , x86@kernel.org, tboot-devel@lists.sourceforge.net, Ard Biesheuvel , Ilias Apalodimas , linux-efi@vger.kernel.org, Vishal Moola , Alistair Popple , "Matthew Wilcox (Oracle)" , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org References: <20260714-remove_pgtable_cdtor-v1-0-44be8a7685d7@arm.com> <20260714-remove_pgtable_cdtor-v1-3-44be8a7685d7@arm.com> <01f07ea2-8bdd-4450-8a49-548f8b646cfd@kernel.org> <20260811142507.4c6c760f@pumpkin> From: Kevin Brodsky Content-Language: en-GB In-Reply-To: <20260811142507.4c6c760f@pumpkin> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: 3awx46zswhxp5dhah49yy896tnfaqmzk X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 5E48E40008 X-HE-Tag: 1786466870-152870 X-HE-Meta: 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 3LqSsU8J rrE7nRXy2+1bwayApwT8+HCU/aW2kUH0tcZlPxMLGvgSk0+kz6RbyhTZKiy5FiNuUkeDuXpYzNUxJFqJtii0N/PQUInLGqIQxxDVzIZL5cHWuKI0f5W+wFyons6se/eICsIJGOqhHDLu2HtdX2VqirKeXA7SIpnuDAKlC8KxTN0Jja00Dhp/2jjG2j+utDeUtNEDhzssgFF7soJ/YYQFwwfwcprzZpFzr5B1LB0QB9YN+huyn/jhas4mKn6rrQQavRIyvfPsqnSYH7nD0ON0LcjOpT9sG+To7L3uLlVdbwaVr5lAm+9kDSYtQOe0J1m54K8tnggqggJXQDYn26sNYNZPhY9k4lFcDSA5cGcIFu1pDx2DL4GPRc5OV1Y3fVp4Pb8ESwTdxi0lSzfmsbIXLtwt+nuA92d6jofFUHhtPC0Obzg+p4Z1AK2hRx7OhMUAfE/yE Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 11/08/2026 15:25, David Laight wrote: > On Mon, 3 Aug 2026 15:59:35 +0200 > "Christophe Leroy (CS GROUP)" wrote: > >> Le 14/07/2026 à 16:03, Kevin Brodsky a écrit : >>> mm code often needs to know whether some mm represents a kernel or >>> user address space. This is currently done by comparing the mm >>> pointer with &init_mm; besides not being particularly elegant, this >>> ignores the fact that other mm's (e.g. efi_mm) may also represent >>> parts of the kernel address space. >>> >>> Introduce a new mm flag MMF_KERNEL and set it for init_mm. >>> Subsequent patches will use this flag to replace comparisons with >>> &init_mm. No functional change is introduced for now. >> Did you consider performance impact ? This test is usually done in quite >> critical memory handling functions. >> >> init_mm is known at link time. Before your patch 08/22 there is just a >> comparison of mm (r3) with a constant (loaded in r10): >> >> c0014048 : >> c0014048: 3d 40 c1 09 lis r10,-16119 >> c001404c: 39 4a 03 98 addi r10,r10,920 >> c0014050: 7c 03 50 00 cmpw r3,r10 >> c0014054: 4d 82 00 20 beqlr >> ... >> >> After patch 08/22 we have, it first checks that mm is not 0, then it >> loads the word located at mm+528 then AND it with 0x1. This load might >> be costly. >> >> c0014048 : >> c0014048: 2c 03 00 00 cmpwi r3,0 >> c001404c: 7c 85 23 78 mr r5,r4 >> c0014050: 41 82 00 10 beq c0014060 >> c0014054: 81 23 02 10 lwz r9,528(r3) >> c0014058: 71 29 00 01 andi. r9,r9,1 >> c001405c: 4c 82 00 20 bnelr >> ... > Thought... > Could the check be replaced by one that checks for kernel static data? > That would require that other mm that represent part of the kernel > address space be static (and probably not in modules). > ISTR there is an address range check that can be used - and might be > cheaper than the explicit test for init_mm on many 64bit archs. That is actually an interesting idea. Finding those "kernel mms" boils down to looking for global mm_struct, so checking if mm points to global data would achieve essentially the same thing without a flag (and likely faster). I'm not sure we want to bake in the assumption that "kernel mm" is equivalent to "mm is global" though. - Kevin