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[76.70.75.133]) by smtp.gmail.com with ESMTPSA id u11sm2205441qkk.72.2021.08.04.17.04.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Aug 2021 17:04:50 -0700 (PDT) Date: Wed, 4 Aug 2021 20:04:49 -0400 From: Peter Xu To: David Hildenbrand Subject: Re: [PATCH v3 6/7] migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination Message-ID: References: <20210730085249.8246-1-david@redhat.com> <20210730085249.8246-7-david@redhat.com> MIME-Version: 1.0 In-Reply-To: <20210730085249.8246-7-david@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline 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: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.699, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Eduardo Habkost , Juan Quintela , Pankaj Gupta , "Michael S. Tsirkin" , teawater , qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Alex Williamson , Marek Kedzierski , Paolo Bonzini , Andrey Gruzdev , Wei Yang Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Jul 30, 2021 at 10:52:48AM +0200, David Hildenbrand wrote: > Currently, when someone (i.e., the VM) accesses discarded parts inside a > RAMBlock with a RamDiscardManager managing the corresponding mapped memory > region, postcopy will request migration of the corresponding page from the > source. The source, however, will never answer, because it refuses to > migrate such pages with undefined content ("logically unplugged"): the > pages are never dirty, and get_queued_page() will consequently skip > processing these postcopy requests. > > Especially reading discarded ("logically unplugged") ranges is supposed to > work in some setups (for example with current virtio-mem), although it > barely ever happens: still, not placing a page would currently stall the > VM, as it cannot make forward progress. > > Let's check the state via the RamDiscardManager (the state e.g., > of virtio-mem is migrated during precopy) and avoid sending a request > that will never get answered. Place a fresh zero page instead to keep > the VM working. This is the same behavior that would happen > automatically without userfaultfd being active, when accessing virtual > memory regions without populated pages -- "populate on demand". > > For now, there are valid cases (as documented in the virtio-mem spec) where > a VM might read discarded memory; in the future, we will disallow that. > Then, we might want to handle that case differently, e.g., warning the > user that the VM seems to be mis-behaving. > > Signed-off-by: David Hildenbrand > --- > migration/postcopy-ram.c | 31 +++++++++++++++++++++++++++---- > migration/ram.c | 21 +++++++++++++++++++++ > migration/ram.h | 1 + > 3 files changed, 49 insertions(+), 4 deletions(-) > > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c > index 2e9697bdd2..38cdfc09c3 100644 > --- a/migration/postcopy-ram.c > +++ b/migration/postcopy-ram.c > @@ -671,6 +671,29 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd, > return ret; > } > > +static int postcopy_request_page(MigrationIncomingState *mis, RAMBlock *rb, > + ram_addr_t start, uint64_t haddr) > +{ > + void *aligned = (void *)(uintptr_t)(haddr & -qemu_ram_pagesize(rb)); > + > + /* > + * Discarded pages (via RamDiscardManager) are never migrated. On unlikely > + * access, place a zeropage, which will also set the relevant bits in the > + * recv_bitmap accordingly, so we won't try placing a zeropage twice. > + * > + * Checking a single bit is sufficient to handle pagesize > TPS as either > + * all relevant bits are set or not. > + */ > + assert(QEMU_IS_ALIGNED(start, qemu_ram_pagesize(rb))); Is this check for ramblock_page_is_discarded()? If so, shall we move this into it, e.g. after memory_region_has_ram_discard_manager() returned true? Other than that it looks good to me, thanks. > + if (ramblock_page_is_discarded(rb, start)) { > + bool received = ramblock_recv_bitmap_test_byte_offset(rb, start); > + > + return received ? 0 : postcopy_place_page_zero(mis, aligned, rb); > + } > + > + return migrate_send_rp_req_pages(mis, rb, start, haddr); > +} > + > /* > * Callback from shared fault handlers to ask for a page, > * the page must be specified by a RAMBlock and an offset in that rb > @@ -690,7 +713,7 @@ int postcopy_request_shared_page(struct PostCopyFD *pcfd, RAMBlock *rb, > qemu_ram_get_idstr(rb), rb_offset); > return postcopy_wake_shared(pcfd, client_addr, rb); > } > - migrate_send_rp_req_pages(mis, rb, aligned_rbo, client_addr); > + postcopy_request_page(mis, rb, aligned_rbo, client_addr); > return 0; > } > > @@ -984,8 +1007,8 @@ retry: > * Send the request to the source - we want to request one > * of our host page sizes (which is >= TPS) > */ > - ret = migrate_send_rp_req_pages(mis, rb, rb_offset, > - msg.arg.pagefault.address); > + ret = postcopy_request_page(mis, rb, rb_offset, > + msg.arg.pagefault.address); > if (ret) { > /* May be network failure, try to wait for recovery */ > if (ret == -EIO && postcopy_pause_fault_thread(mis)) { > @@ -993,7 +1016,7 @@ retry: > goto retry; > } else { > /* This is a unavoidable fault */ > - error_report("%s: migrate_send_rp_req_pages() get %d", > + error_report("%s: postcopy_request_page() get %d", > __func__, ret); > break; > } > diff --git a/migration/ram.c b/migration/ram.c > index 9776919faa..01cea01774 100644 > --- a/migration/ram.c > +++ b/migration/ram.c > @@ -912,6 +912,27 @@ static uint64_t ramblock_dirty_bitmap_clear_discarded_pages(RAMBlock *rb) > return cleared_bits; > } > > +/* > + * Check if a host-page aligned page falls into a discarded range as managed by > + * a RamDiscardManager responsible for the mapped memory region of the RAMBlock. > + * > + * Note: The result is only stable while migration (precopy/postcopy). > + */ > +bool ramblock_page_is_discarded(RAMBlock *rb, ram_addr_t start) > +{ > + if (rb->mr && memory_region_has_ram_discard_manager(rb->mr)) { > + RamDiscardManager *rdm = memory_region_get_ram_discard_manager(rb->mr); > + MemoryRegionSection section = { > + .mr = rb->mr, > + .offset_within_region = start, > + .size = int128_get64(qemu_ram_pagesize(rb)), > + }; > + > + return !ram_discard_manager_is_populated(rdm, §ion); > + } > + return false; > +} > + > /* Called with RCU critical section */ > static void ramblock_sync_dirty_bitmap(RAMState *rs, RAMBlock *rb) > { > diff --git a/migration/ram.h b/migration/ram.h > index 4833e9fd5b..dda1988f3d 100644 > --- a/migration/ram.h > +++ b/migration/ram.h > @@ -72,6 +72,7 @@ void ramblock_recv_bitmap_set_range(RAMBlock *rb, void *host_addr, size_t nr); > int64_t ramblock_recv_bitmap_send(QEMUFile *file, > const char *block_name); > int ram_dirty_bitmap_reload(MigrationState *s, RAMBlock *rb); > +bool ramblock_page_is_discarded(RAMBlock *rb, ram_addr_t start); > > /* ram cache */ > int colo_init_ram_cache(void); > -- > 2.31.1 > -- Peter Xu