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(p200300cbc7349600af274326a2162bfb.dip0.t-ipconnect.de. [2003:cb:c734:9600:af27:4326:a216:2bfb]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43b8d6bc3fcsm122643735e9.0.2025.03.03.00.52.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 03 Mar 2025 00:52:50 -0800 (PST) Message-ID: Date: Mon, 3 Mar 2025 09:52:49 +0100 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 v1 1/4] mm: Fix lazy mmu docs and usage From: David Hildenbrand To: Ryan Roberts , Andrew Morton , "David S. Miller" , Andreas Larsson , Juergen Gross , Boris Ostrovsky , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , "Matthew Wilcox (Oracle)" , Catalin Marinas Cc: linux-mm@kvack.org, sparclinux@vger.kernel.org, xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org References: <20250302145555.3236789-1-ryan.roberts@arm.com> <20250302145555.3236789-2-ryan.roberts@arm.com> <5418a661-dbd0-46e9-8ef7-b1c5a34acce3@redhat.com> Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <5418a661-dbd0-46e9-8ef7-b1c5a34acce3@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 03.03.25 09:49, David Hildenbrand wrote: > On 02.03.25 15:55, Ryan Roberts wrote: >> The docs, implementations and use of arch_[enter|leave]_lazy_mmu_mode() >> is a bit of a mess (to put it politely). There are a number of issues >> related to nesting of lazy mmu regions and confusion over whether the >> task, when in a lazy mmu region, is preemptible or not. Fix all the >> issues relating to the core-mm. Follow up commits will fix the >> arch-specific implementations. 3 arches implement lazy mmu; powerpc, >> sparc and x86. >> >> When arch_[enter|leave]_lazy_mmu_mode() was first introduced by commit >> 6606c3e0da53 ("[PATCH] paravirt: lazy mmu mode hooks.patch"), it was >> expected that lazy mmu regions would never nest and that the appropriate >> page table lock(s) would be held while in the region, thus ensuring the >> region is non-preemptible. Additionally lazy mmu regions were only used >> during manipulation of user mappings. >> >> Commit 38e0edb15bd0 ("mm/apply_to_range: call pte function with lazy >> updates") started invoking the lazy mmu mode in apply_to_pte_range(), >> which is used for both user and kernel mappings. For kernel mappings the >> region is no longer protected by any lock so there is no longer any >> guarantee about non-preemptibility. Additionally, for RT configs, the >> holding the PTL only implies no CPU migration, it doesn't prevent >> preemption. >> >> Commit bcc6cc832573 ("mm: add default definition of set_ptes()") added >> arch_[enter|leave]_lazy_mmu_mode() to the default implementation of >> set_ptes(), used by x86. So after this commit, lazy mmu regions can be >> nested. Additionally commit 1a10a44dfc1d ("sparc64: implement the new >> page table range API") and commit 9fee28baa601 ("powerpc: implement the >> new page table range API") did the same for the sparc and powerpc >> set_ptes() overrides. >> >> powerpc couldn't deal with preemption so avoids it in commit >> b9ef323ea168 ("powerpc/64s: Disable preemption in hash lazy mmu mode"), >> which explicitly disables preemption for the whole region in its >> implementation. x86 can support preemption (or at least it could until >> it tried to add support nesting; more on this below). Sparc looks to be >> totally broken in the face of preemption, as far as I can tell. >> >> powewrpc can't deal with nesting, so avoids it in commit 47b8def9358c >> ("powerpc/mm: Avoid calling arch_enter/leave_lazy_mmu() in set_ptes"), >> which removes the lazy mmu calls from its implementation of set_ptes(). >> x86 attempted to support nesting in commit 49147beb0ccb ("x86/xen: allow >> nesting of same lazy mode") but as far as I can tell, this breaks its >> support for preemption. >> >> In short, it's all a mess; the semantics for >> arch_[enter|leave]_lazy_mmu_mode() are not clearly defined and as a >> result the implementations all have different expectations, sticking >> plasters and bugs. >> >> arm64 is aiming to start using these hooks, so let's clean everything up >> before adding an arm64 implementation. Update the documentation to state >> that lazy mmu regions can never be nested, must not be called in >> interrupt context and preemption may or may not be enabled for the >> duration of the region. >> >> Additionally, update the way arch_[enter|leave]_lazy_mmu_mode() is >> called in pagemap_scan_pmd_entry() to follow the normal pattern of >> holding the ptl for user space mappings. As a result the scope is >> reduced to only the pte table, but that's where most of the performance >> win is. While I believe there wasn't technically a bug here, the >> original scope made it easier to accidentally nest or, worse, >> accidentally call something like kmap() which would expect an immediate >> mode pte modification but it would end up deferred. >> >> arch-specific fixes to conform to the new spec will proceed this one. >> >> These issues were spotted by code review and I have no evidence of >> issues being reported in the wild. >> > > All looking good to me! > > Acked-by: David Hildenbrand > ... but I do wonder if the set_ptes change should be split from the pagemap change. -- Cheers, David / dhildenb