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[79.242.57.171]) by smtp.gmail.com with ESMTPSA id vr3sm4406332ejb.124.2020.10.17.00.39.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 17 Oct 2020 00:39:39 -0700 (PDT) From: David Hildenbrand Mime-Version: 1.0 (1.0) Subject: Re: [PATCH v1 05/29] virtio-mem: generalize check for added memory Date: Sat, 17 Oct 2020 09:39:38 +0200 Message-Id: <2E12AC3C-872A-4B30-8FD7-12420FA1D14E@redhat.com> References: <20201016223811.GJ44269@L-31X9LVDL-1304.local> Cc: David Hildenbrand , linux-kernel@vger.kernel.org, linux-mm@kvack.org, virtualization@lists.linux-foundation.org, Andrew Morton , "Michael S . Tsirkin" , Jason Wang , Pankaj Gupta In-Reply-To: <20201016223811.GJ44269@L-31X9LVDL-1304.local> To: Wei Yang X-Mailer: iPhone Mail (18A393) Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: > Am 17.10.2020 um 00:38 schrieb Wei Yang : >=20 > =EF=BB=BFOn Fri, Oct 16, 2020 at 12:32:50PM +0200, David Hildenbrand wrot= e: >>>>> Ok, I seems to understand the logic now. >>>>>=20 >>>>> But how we prevent ONLINE_PARTIAL memory block get offlined? There ar= e three >>>>> calls in virtio_mem_set_fake_offline(), while all of them adjust page= 's flag. >>>>> How they hold reference to struct page? >>>>=20 >>>> Sorry, I should have given you the right pointer. (similar to my other >>>> reply) >>>>=20 >>>> We hold a reference either via >>>>=20 >>>> 1. alloc_contig_range() >>>=20 >>> I am not familiar with this one, need to spend some time to look into. >>=20 >> Each individual page will have a pagecount of 1. >>=20 >>>=20 >>>> 2. memmap init code, when not calling generic_online_page(). >>>=20 >>> I may miss some code here. Before online pages, memmaps are allocated i= n >>> section_activate(). They are supposed to be zero-ed. (I don't get the e= xact >>> code line.) I am not sure when we grab a refcount here. >>=20 >> Best to refer to __init_single_page() -> init_page_count(). >>=20 >> Each page that wasn't onlined via generic_online_page() has a refcount >> of 1 and looks like allocated. >>=20 >=20 > Thanks, I see the logic. >=20 > online_pages() > move_pfn_range_to_zone() --- 1) > online_pages_range() --- 2) >=20 > At 1), __init_single_page() would set page count to 1. At 2), > generic_online_page() would clear page count, while the call back would n= ot. >=20 > Then I am trying to search the place where un-zero page count prevent off= line. > scan_movable_pages() would fail, since this is a PageOffline() and has 1 = page > count. >=20 > So the GUARD we prevent offline partial-onlined pages is >=20 > (PageOffline && page_count) >=20 > And your commit aa218795cb5fd583c94f >=20 > mm: Allow to offline unmovable PageOffline() pages via MEM_GOING_OFFLINE >=20 > is introduced to handle this case. >=20 > That's pretty clear now. >=20 I=E2=80=98m happy to see that I am no longer the only person that understan= ds all this magic :) Thanks for having a look / reviewing! >> --=20 >> Thanks, >>=20 >> David / dhildenb >=20 > --=20 > Wei Yang > Help you, Help me >=20