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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45320b16657sm14394555e9.8.2025.06.10.06.44.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 10 Jun 2025 06:44:28 -0700 (PDT) Message-ID: <881baf6e-8118-4d09-86dc-09dcf3a3cd02@redhat.com> Date: Tue, 10 Jun 2025 15:44:27 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm: Allow lockless kernel pagetable walking To: Lorenzo Stoakes Cc: Dev Jain , akpm@linux-foundation.org, catalin.marinas@arm.com, will@kernel.org, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, suzuki.poulose@arm.com, steven.price@arm.com, gshan@redhat.com, linux-arm-kernel@lists.infradead.org, yang@os.amperecomputing.com, ryan.roberts@arm.com, anshuman.khandual@arm.com References: <20250610114401.7097-1-dev.jain@arm.com> <20250610114401.7097-2-dev.jain@arm.com> <970e1b84-7f47-4a51-ad99-4df9026b8f7d@redhat.com> <359668fb-df61-4135-89ee-2675a06a5c1b@lucifer.local> <1c4714bb-97f3-4261-92b7-34223a32e016@lucifer.local> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10.06.25 15:35, Lorenzo Stoakes wrote: > On Tue, Jun 10, 2025 at 03:31:56PM +0200, David Hildenbrand wrote: >> On 10.06.25 15:27, Lorenzo Stoakes wrote: >>> On Tue, Jun 10, 2025 at 03:24:16PM +0200, David Hildenbrand wrote: >>>> On 10.06.25 14:07, Lorenzo Stoakes wrote: >>>>> OK so I think the best solution here is to just update check_ops_valid(), which >>>>> was kind of sucky anyway (we check everywhere but walk_page_range_mm() to >>>>> enforce the install pte thing). >>>>> >>>>> Let's do something like: >>>>> >>>>> #define OPS_MAY_INSTALL_PTE (1<<0) >>>>> #define OPS_MAY_AVOID_LOCK (1<<1) >>>>> >>>>> and update check_ops_valid() to take a flags or maybe 'capabilities' field. >>>>> >>>>> Then check based on this e.g.: >>>>> >>>>> if (ops->install_pte && !(capabilities & OPS_MAY_INSTALL_PTE)) >>>>> return false; >>>>> >>>>> if (ops->walk_lock == PGWALK_NOLOCK && !(capabilities & OPS_MAY_AVOID_LOCK)) >>>>> return false; >>>>> >>>> >>>> Hm. I mean, we really only want to allow this lockless check for >>>> walk_kernel_page_table_range(), right? >>>> >>>> Having a walk_kernel_page_table_range_lockeless() might (or might not) be >>>> better, to really only special-case this specific path. >>> >>> Agree completely, Dev - let's definitely do this. >>> >>>> >>>> So, I am wondering if we should further start splitting the >>>> kernel-page-table walker up from the mm walker, at least on the "entry" >>>> function for now. >>> >>> How do you mean? >> >> In particular, "struct mm_walk_ops" >> >> does not quite make sense when not actually walking a "real" mm . >> >> So maybe we should start having a separate structure where *vma, >> install_pte, walk_lock, hugetlb* does not even exist. >> >> It might be a bit of churn, though ... not sure if there could be an easy >> translation layer for now. > > But you know... I looove churn right? <3 <3 <3 :))) > > That's a nice idea, but I think something that should be a follow up. Yes, absolutely, just wanted to raise it :) -- Cheers, David / dhildenb