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[34.79.165.247]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-35dd064b5e9sm2203000f8f.96.2024.05.31.09.41.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 31 May 2024 09:41:10 -0700 (PDT) Date: Fri, 31 May 2024 16:41:05 +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 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> <9c23d824-f2c7-4f9a-ade2-e8dd3a0d30af@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 5EA231C0025 X-Stat-Signature: h97y9cxqeanhnnj7x3qah5rwex9ds1xz X-HE-Tag: 1717173672-870377 X-HE-Meta: U2FsdGVkX1/Gh0JfWzFTBUBhtWakaGXbl/teowUzXyw0AtYL3mWTWYx5dQVSbUWPkzc3J/on3DqPCnzuLby73x+30hg3QEgaHWeNHLFbHB53tV5ZftKDfTrFNnAUmJJByEOJ3AJiNtOXkCdPlEFzAJsyZ7sSAsq9zU6tkAf85Uosx2e/OPxbr5q9oOpEeYGWBF4kKyxHC/fpPgNfsrw+SRblI/rxbVHzKKr9JLdBe3QvMZrXPC73hKZ94f4BkY9IOZrLPrucEsVQ277PvLGJ36Z32ZHXkXHUsWFenqaSDYgkDBPkgvWTBAzS8JDhkqEuypvVbt0z9OJVXQV7E6NQMSyGclcd5t/wPpC4mYoSP52lX9z6ql4uw50q1WDvJygCBOG8/mjHYD0RlGUgeQOwm4Y+j4tLlab/2hLB3rxqyf0PB53JmzUXOkzlxELhaGN0UKChC07ukYzJsIKUBcJALTUlIi+Nnaq96HhZr7OuCFOvBxS4eh0tCwcl4ki1xrpv3IoWqXVsulfP6jJlhdIkR1b+fB2gsfF1APL6G5Q7GNVF4u97shj+h9NQ7HAybwojLzRJO8h6KPdvaCr4f89lQH9VkmpmewcZjA3xP6E4l2QlGcD0GZgz1eA/EoyctRcFnk/Hd8ZQN+YegDT5QXqtwtpaGIdEboTiMJQveDAUapE6hYoo4eY16nFmo8zEA8HN0Bn6Ta6ZbGv77AlRyt62CmZ6jtV97BSoy5l/qpT8oK0OPebWU+0/8yOKpS0LvwVS97hxNrVe8jeZICgunsQzR2IwzEcM7cB5GVaG1X4E55ATJSXrZKlfP7A3sOKP8HgDbLBOVLEss8EiqEhOwWZabR40ph7BAHDoOH6lob41aMryyaG2+trPl8nB15wSfZqG/CqG1gCvWBTI8BhMIYNusulNdhJw+WaId6RR2WlHq7o4QaoHkYhiAO7SpBo0+BDBPE2VCJEk3aEjz31JmCV N9lo3sZm hAKEWA53ULQFQVCV2mlNENRfEHSakz0J+MQoOePX8TgC/0aWPmODuonNanztHYyOf5tywDP9V91d6yhSMsRq0rnk/NhRGnsex13dZnyQEpvog7pNtVpD0f2ANVBsd3zIvNu9MWAPWkXE03nM+d8sdcnwKe1OxZvszrxfuF4acuTBDtPHzLMyDfZaJ7DW8fxDRv0JIct1MXL6SrXMCOSx5+UOGkySAXJI1uzk7w39Mu0DHDoP+qGWROugQJrJExqMUz+eGOGlcN08FGK3HCc8cNPwSrIBAvooYuKZ1VEGKgozhz9wdx9m8kKpCsSBHqhaPGuYZ24gpW5Z8ieWMqewMQ+xfKjQVu6XYCqnFkJl7kBW1cLuyfeRDOtH4wXlljSZrbDX1im3+cLSwTjggz34lhB4YHg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.001675, 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 02:11:16PM +0000, Brendan Jackman wrote: > On Wed, May 22, 2024 at 04:05:12PM +0200, David Hildenbrand wrote: > > On 21.05.24 14:57, Brendan Jackman wrote: > > > + return zone->zone_start_pfn + READ_ONCE(zone->spanned_pages); > > > > It's weird to apply that logic only to spanned_pages, whereby zone_start_pfn > > can (and will) similarly change when onlining/offlining memory. > > > Oh, yep. For some reason I had decided that zone_start_pfn was fixed > but that is (actually very obviously) not true! > > Will take a closer look and extend v2 to cover that too, unless > someone finds a reason this whole patch is nonsense. > > Thanks for the review. Hmm so while poking around during spare moments this week I learned that compaction.c also stores a bunch of data in struct zone that is unsynchronized. It seems pretty unlikely that you can corrupt any memory there (unless there's some race possible with pfn_to_online_page, which is an orthogonal question), but it does seem like if the compiler gets smart with us we could maybe have a compaction run that takes quasi-forever or something weird like that. It seems easy enough to just spam READ_ONCE/WRITE_ONCE everywhere there too, this would remove that risk, make KCSAN happy and serve as a kinda "this is unsynchronized, take care" comment. (There's also at least one place where we could put data_race()). On the other hand it's a bit verbose & visually ugly. Personally I think it's a pretty minor downside, but anyone feel differently?