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[35.189.240.49]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3502baacfb9sm33786732f8f.68.2024.05.22.03.10.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 22 May 2024 03:10:43 -0700 (PDT) Date: Wed, 22 May 2024 10:10:40 +0000 From: Brendan Jackman To: Lance Yang Cc: David Hildenbrand , 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 2/2] mm,memory_hotplug: {READ,WRITE}_ONCE unsynchronized zone data Message-ID: References: <20240521-mm-hotplug-sync-v1-0-6d53706c1ba8@google.com> <20240521-mm-hotplug-sync-v1-2-6d53706c1ba8@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Queue-Id: 3E37240015 X-Stat-Signature: oe8cjzfuc4j4tnckx9tmzcxuybcck5sx X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1716372646-932456 X-HE-Meta: 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 1/lGWq+F LAPHcNm/NQmSeQB7I/agso8lfRacRLLSiosycZ2rsdhEsYrr1goEA5Or4OYvyCYDM8WZICZrK6aFxyxtQ5PB/WF6uk9zJiEKwXWdpf43YgAkW0YTTCAU4as7a7jP5BZ21MJYlSH3tYcZs37i9vucPiwNj7ApP21JSBrFO91iS0X1g0JhklmoxCsALZ37DdzfUzYDnQr7cobbQPxWBvKpTE7RaP2j/2Ex5DJKCf+TSjYCTKMNwq4mmvcwjBq9Qdgi9yJ+QgfJwELq2dYHehzcPZWsYTfmfuzgE0zRIB9erDIZNKDqdFhAVGkRxw0zaA4+apK6emKVdjv01794hhyQfdvBfd9x3DUSLRj0x9E6avHCQUU8raO4UlXjhenYBZiX/VLr9/Fab3IQXq/+uZMXEHP5u/KSO0P2L5HUBZEXysIodlWg= X-Bogosity: Ham, tests=bogofilter, spamicity=0.019180, 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 05:20:08PM +0800, Lance Yang wrote: > On Wed, May 22, 2024 at 4:38 PM Brendan Jackman wrote: > > > > Hi Lance, thanks for taking a look. > > > > On Wed, May 22, 2024 at 12:25:30PM +0800, Lance Yang wrote: > > > Hi Brendan, > > > > > > On Tue, May 21, 2024 at 8:57 PM Brendan Jackman wrote: > > > > @@ -1077,7 +1081,7 @@ void adjust_present_page_count(struct page *page, struct memory_group *group, > > > > */ > > > > if (early_section(__pfn_to_section(page_to_pfn(page)))) > > > > zone->present_early_pages += nr_pages; > > > > - zone->present_pages += nr_pages; > > > > + WRITE_ONCE(zone->present_pages, zone->present_pages + nr_pages); > > > > > > I'm not sure that using the WRITE_ONCE() wrapper would prevent load tearing > > > on 'zone->present_pages', but it's probably just me overthinking it :) > > > > Hmm.. this isn't for load-tearing, it's for store-tearing. I have a > > feeling I might be missing your pont here though, can you elaborate? > > Sorry, my explanation wasn't clear :( > > I'm a bit confused about whether 'WRITE_ONCE(zone->present_pages, > zone->present_pages + nr_pages);' > is equivalent to the following: > > 1 a = zone->present_pages + nr_pages; > 2 WRITE_ONCE(zone->present_pages, a); > > If so, is there any possibility of load tearing on > 'zone->present_pages' in line 1? Ah gotcha, thanks for clarifying. Loads are protected by mem_hotplug_lock here, so it's fine for them to get split up (because the value can't change between loads). This is what I was referring to in the bit of the commit message about not needing READ_ONCE.