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[70.30.145.63]) by smtp.gmail.com with ESMTPSA id g186-20020a37b6c3000000b007430494ab92sm9730302qkf.67.2023.03.07.09.04.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Mar 2023 09:04:37 -0800 (PST) Date: Tue, 7 Mar 2023 12:04:36 -0500 From: Peter Xu To: Chuang Xu Cc: qemu-devel@nongnu.org, dgilbert@redhat.com, quintela@redhat.com, pbonzini@redhat.com, david@redhat.com, philmd@linaro.org, zhouyibo@bytedance.com Subject: Re: [PATCH RESEND v6 0/5] migration: reduce time of loading non-iterable vmstate Message-ID: References: <20230303105655.396446-1-xuchuangxclwt@bytedance.com> <1ea4db90-7535-1dc3-a2ae-d44113a24e29@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Mar 07, 2023 at 09:24:31PM +0800, Chuang Xu wrote: > > Why do we need address_space_get_flatview_rcu()? I'm not sure whether you > > address_space_cahce_init() uses address_space_get_flatview() to acquire > a ref-ed flatview. If we want to use address_space_to_flatview_rcu() and > make the flatview ref-ed, maybe we need to add address_space_get_flatview_rcu()? > Or if we use address_space_to_flatview_rcu() directly, then we'll get a > unref-ed flatview, I'm not sure wheather it's safe in other cases.. At least > I don't want the assertion to be triggered one day. No we can't touch that, afaict. It was a real fix (447b0d0b9e) to a real bug. What I meant is we make address_space_get_flatview() always use address_space_to_flatview_rcu(). I think it's safe, if you see the current callers of it (after my patch 1 fixed version applied): memory_region_sync_dirty_bitmap[2249] view = address_space_get_flatview(as); memory_region_update_coalesced_range[2457] view = address_space_get_flatview(as); memory_region_clear_dirty_bitmap[2285] view = address_space_get_flatview(as); mtree_info_flatview[3418] view = address_space_get_flatview(as); address_space_cache_init[3350] cache->fv = address_space_get_flatview(as); Case 5 is what we're targeting for which I think it's fine. Logically any commit() hook should just use address_space_get_flatview_raw() to reference the flatview, but at least address_space_cache_init() is also called in non-BQL sections, so _rcu is the only thing we can use here, iiuc.. Case 1-3 is never called during vm load, so I think it's safe. Case 4 can be accessing stalled flatview but I assume fine since it's just for debugging. E.g. if someone tries "info mtree -f" during vm loading after your patch applied, then one can see stalled info. I don't think it can even happen because so far "info mtree" should also take BQL.. so it'll be blocked until vm load completes. The whole thing is still tricky. I didn't yet make my mind through on how to make it very clean, I think it's slightly beyond what this series does and I need some more thinkings (or suggestions from others). -- Peter Xu