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[2003:cb:c709:d500:4044:6d4c:f060:f677]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3502b79bdc7sm34502308f8f.22.2024.05.22.08.24.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 22 May 2024 08:24:18 -0700 (PDT) Message-ID: <0506ae4e-e17d-4c3c-aa3e-1cea04909e5a@redhat.com> Date: Wed, 22 May 2024 17:24:17 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] mm,memory_hotplug: Remove un-taken lock To: Brendan Jackman Cc: Oscar Salvador , Andrew Morton , Mike Rapoport , Michal Hocko , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240521-mm-hotplug-sync-v1-0-6d53706c1ba8@google.com> <20240521-mm-hotplug-sync-v1-1-6d53706c1ba8@google.com> <78e646af-e8b5-4596-8fbf-17b139cfdddd@redhat.com> From: David Hildenbrand 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: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 0A67D4001B X-Stat-Signature: wfgkbtbhqceb38kooprxhoh6gsp6n4i9 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1716391462-240958 X-HE-Meta: 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 1TyPuUb+ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.001399, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 22.05.24 16:27, Brendan Jackman wrote: > On Wed, May 22, 2024 at 04:09:41PM +0200, David Hildenbrand wrote: >> On 21.05.24 14:57, Brendan Jackman wrote: >> The old seqlock guaranteed that we would have obtained consistent values >> here. start + spanned_pages defines a range. For example, growing a zone to >> the beginning implies that both ranges must be changed. >> >> I do wonder if it might be better to instead have zone->zone_start_pfn and >> zone->zone_end_pfn. That way, both can be changed individually, not >> requiring adjustment of both to grow/shrink a zone at the beginning. > > Thanks this is a good point. > > So basically the fact that spanned_pages is "once or eventually" > correct is certainly not enough because it only has meaning with > reference to zone_start_pfn. I didn't realise this because of my > spontaneous inspiration to believe that zone_start_pfn was fixed. Right, it isn't. > > By the way, some noob questions: am I OK with my assumption that it's > fine for reader code to operate on zone spans that are both stale and > "from the future"? thinking abstractly I guess that seeing a stale > value when racing with offline_pages is roughly the same as seeing a > value "from the future" when racing with online_pages? Right. PFN walkers should be using pfn_to_online_page(), where races are possible but barely seen in practice. zone handlers like mm/compaction.c can likely deal with races, although it might all be cleaner (and safer?) when using start+end. I recall it also recalls on pfn_to_online_page(). Regarding page_outside_zone_boundaries(), it should be fine if we can read start+end atomically, that way we would not accidentally report "page outside ..." when changing the start address. I think with your current patch that might happen (although likely extremely hard to trigger) when growing the zone at the start, reducing zone_start_pfn. > > Also, is it ever possible for pages to get removed and then added back > and end up in a different zone than before? Yes. Changing between MOVABLE and NORMAL is possible and can easily be triggered by offlining+re-onlining memory blocks. -- Cheers, David / dhildenb