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[213.151.95.130]) by smtp.gmail.com with ESMTPSA id m21sm3763917wmi.27.2020.03.10.04.59.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 10 Mar 2020 04:59:55 -0700 (PDT) Date: Tue, 10 Mar 2020 12:59:55 +0100 From: Michal Hocko To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, virtio-dev@lists.oasis-open.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, Andrew Morton , "Michael S . Tsirkin" , Jason Wang , Oscar Salvador , Igor Mammedov , Dave Young , Dan Williams , Pavel Tatashin , Stefan Hajnoczi , Vlastimil Babka Subject: Re: [PATCH v1 07/11] virtio-mem: Allow to offline partially unplugged memory blocks Message-ID: <20200310115955.GI8447@dhcp22.suse.cz> References: <20200302134941.315212-1-david@redhat.com> <20200302134941.315212-8-david@redhat.com> <20200310114312.GG8447@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Bogosity: Ham, tests=bogofilter, spamicity=0.000627, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue 10-03-20 12:46:05, David Hildenbrand wrote: > On 10.03.20 12:43, Michal Hocko wrote: > > On Mon 02-03-20 14:49:37, David Hildenbrand wrote: > > [...] > >> +static void virtio_mem_notify_going_offline(struct virtio_mem *vm, > >> + unsigned long mb_id) > >> +{ > >> + const unsigned long nr_pages = PFN_DOWN(vm->subblock_size); > >> + unsigned long pfn; > >> + int sb_id, i; > >> + > >> + for (sb_id = 0; sb_id < vm->nb_sb_per_mb; sb_id++) { > >> + if (virtio_mem_mb_test_sb_plugged(vm, mb_id, sb_id, 1)) > >> + continue; > >> + /* > >> + * Drop our reference to the pages so the memory can get > >> + * offlined and add the unplugged pages to the managed > >> + * page counters (so offlining code can correctly subtract > >> + * them again). > >> + */ > >> + pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) + > >> + sb_id * vm->subblock_size); > >> + adjust_managed_page_count(pfn_to_page(pfn), nr_pages); > >> + for (i = 0; i < nr_pages; i++) > >> + page_ref_dec(pfn_to_page(pfn + i)); > > > > Is there ever situation this might be a different than 1->0 transition? > > Only if some other code would be taking a reference. At least not from > virtio-mem perspective. OK, so that is essentially an error condition. I think it shouldn't go silent and you want something like if (WARN_ON(!page_ref_sub_and_test(page))) dump_page(pfn_to_page(pfn + i), "YOUR REASON"); -- Michal Hocko SUSE Labs