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[2003:cb:c710:f600:634b:35f:ffa8:475b]) by smtp.gmail.com with ESMTPSA id p17-20020a05600c469100b003fee6e170f9sm2174737wmo.45.2023.11.29.04.48.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 29 Nov 2023 04:48:52 -0800 (PST) Message-ID: Date: Wed, 29 Nov 2023 13:48:51 +0100 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: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for mprotect(PROT_MTE) Content-Language: en-US To: Alexandru Elisei Cc: catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev, maz@kernel.org, james.morse@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, arnd@arndb.de, akpm@linux-foundation.org, mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, mhiramat@kernel.org, rppt@kernel.org, hughd@google.com, pcc@google.com, steven.price@arm.com, anshuman.khandual@arm.com, vincenzo.frascino@arm.com, eugenis@google.com, kcc@google.com, hyesoo.yu@samsung.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvmarm@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org References: <20231119165721.9849-1-alexandru.elisei@arm.com> <20231119165721.9849-20-alexandru.elisei@arm.com> <1c79ad05-cb52-4820-b2aa-bbe07ff82b19@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 29.11.23 12:55, Alexandru Elisei wrote: > Hi, > > On Tue, Nov 28, 2023 at 06:55:18PM +0100, David Hildenbrand wrote: >> On 19.11.23 17:57, Alexandru Elisei wrote: >>> To enable tagging on a memory range, userspace can use mprotect() with the >>> PROT_MTE access flag. Pages already mapped in the VMA don't have the >>> associated tag storage block reserved, so mark the PTEs as >>> PAGE_FAULT_ON_ACCESS to trigger a fault next time they are accessed, and >>> reserve the tag storage on the fault path. >> >> That sounds alot like fake PROT_NONE. Would there be a way to unify hat > > Yes, arm64 basically defines PAGE_FAULT_ON_ACCESS as PAGE_NONE | > PTE_TAG_STORAGE_NONE. > >> handling and simply reuse pte_protnone()? For example, could we special case >> on VMA flags? >> >> Like, don't do NUMA hinting in these special VMAs. Then, have something >> like: >> >> if (pte_protnone(vmf->orig_pte)) >> return handle_pte_protnone(vmf); >> >> In there, special case on the VMA flags. > > Your suggestion from the follow-up reply that an arch should know if it needs to > do something was spot on, arm64 can use the software bit in the translation > table entry for that. > > So what you are proposing is this: > > * Rename do_numa_page->handle_pte_protnone > * At some point in the do_numa_page (now renamed to handle_pte_protnone) flow, > decide if pte_protnone() has been set for an arch specific reason or because > of automatic NUMA balancing. > * if pte_protnone() has been set by an architecture, then let the architecture > handle the fault. > > If I understood you correctly, that's a good idea, and should be easy to > implement. yes! :) -- Cheers, David / dhildenb