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[35.189.240.49]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-354de2d1ae0sm1576149f8f.44.2024.05.22.07.27.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 22 May 2024 07:27:19 -0700 (PDT) Date: Wed, 22 May 2024 14:27:15 +0000 From: Brendan Jackman To: David Hildenbrand Cc: Oscar Salvador , Andrew Morton , Mike Rapoport , Michal Hocko , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] mm,memory_hotplug: Remove un-taken lock Message-ID: 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <78e646af-e8b5-4596-8fbf-17b139cfdddd@redhat.com> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: B0FEF14000A X-Stat-Signature: eb4k6y4x5xuqpso1iaug45dx19yog3d5 X-Rspam-User: X-HE-Tag: 1716388041-679214 X-HE-Meta: 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 jwUtXnNq d39VttbwInuOQR2eGTGIML6z7tw9JtzPWLG+iL8/IyU0OrXOwNIdp5NjuEMCIs8cX1gzY/6JcAQ4feIj5EDK35Z2k2pj/Kt65J9UmfoXS4cR9kZQVdG71XLNrludWzbNYrEez4Pzq7JkPM8HJt78FTQ5H/wnkHFN+4yNlFj8AmQhMuBej1JLc7Img9uGB4ZwOMutq0PfF4U7Bczn1uEwoY1Vt7P7vUb/ZIlzbwt2AAIw0/3b68BYIMAdVL92DoyqceRyMirOTrVMufA+AwItukfwepRPxapK9yA2rU6MvODMye0MJkEPeBTRPfGC1nYnHbdgce6La/w6WFK1zBrIGmSyfnVTFZtCoAf3tZjsZur978kfn25mmKG63kmSa2k6Y3T69eu0qVDdZWvRvkENHoXAyBc9LoS028+62 X-Bogosity: Ham, tests=bogofilter, spamicity=0.359111, 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 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. 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? Also, is it ever possible for pages to get removed and then added back and end up in a different zone than before?