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[99.254.121.117]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7ab3eb65f9bsm390275185a.120.2024.09.17.12.20.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Sep 2024 12:20:12 -0700 (PDT) Date: Tue, 17 Sep 2024 15:20:10 -0400 From: Peter Xu To: Fabiano Rosas Cc: qemu-devel@nongnu.org, Peter Maydell , qemu-stable@nongnu.org Subject: Re: [PATCH 2/2] migration/multifd: Fix rb->receivedmap cleanup race Message-ID: References: <20240917185802.15619-1-farosas@suse.de> <20240917185802.15619-3-farosas@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20240917185802.15619-3-farosas@suse.de> 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable 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, Sep 17, 2024 at 03:58:02PM -0300, Fabiano Rosas wrote: > Fix a segmentation fault in multifd when rb->receivedmap is cleared > too early. > > After commit 5ef7e26bdb ("migration/multifd: solve zero page causing > multiple page faults"), multifd started using the rb->receivedmap > bitmap, which belongs to ram.c and is initialized and *freed* from the > ram SaveVMHandlers. > > Multifd threads are live until migration_incoming_state_destroy(), > which is called after qemu_loadvm_state_cleanup(), leading to a crash > when accessing rb->receivedmap. > > process_incoming_migration_co() ... > qemu_loadvm_state() multifd_nocomp_recv() > qemu_loadvm_state_cleanup() ramblock_recv_bitmap_set_offset() > rb->receivedmap = NULL set_bit_atomic(..., rb->receivedmap) > ... > migration_incoming_state_destroy() > multifd_recv_cleanup() > multifd_recv_terminate_threads(NULL) > > Move the loadvm cleanup into migration_incoming_state_destroy(), after > multifd_recv_cleanup() to ensure multifd threads have already exited > when rb->receivedmap is cleared. > > Adjust the postcopy listen thread comment to indicate that we still > want to skip the cpu synchronization. > > CC: qemu-stable@nongnu.org > Fixes: 5ef7e26bdb ("migration/multifd: solve zero page causing multiple page faults") > Signed-off-by: Fabiano Rosas Reviewed-by: Peter Xu One trivial question below.. > --- > migration/migration.c | 1 + > migration/savevm.c | 6 ++++-- > 2 files changed, 5 insertions(+), 2 deletions(-) > > diff --git a/migration/migration.c b/migration/migration.c > index 3dea06d577..b190a574b1 100644 > --- a/migration/migration.c > +++ b/migration/migration.c > @@ -378,6 +378,7 @@ void migration_incoming_state_destroy(void) > struct MigrationIncomingState *mis = migration_incoming_get_current(); > > multifd_recv_cleanup(); Would you mind I add a comment squashed here when queue? /* * RAM state cleanup needs to happen after multifd cleanup, because * multifd threads can use some of its states (receivedmap). */ > + qemu_loadvm_state_cleanup(); > > if (mis->to_src_file) { > /* Tell source that we are done */ > diff --git a/migration/savevm.c b/migration/savevm.c > index d0759694fd..7e1e27182a 100644 > --- a/migration/savevm.c > +++ b/migration/savevm.c > @@ -2979,7 +2979,10 @@ int qemu_loadvm_state(QEMUFile *f) > trace_qemu_loadvm_state_post_main(ret); > > if (mis->have_listen_thread) { > - /* Listen thread still going, can't clean up yet */ > + /* > + * Postcopy listen thread still going, don't synchronize the > + * cpus yet. > + */ > return ret; > } > > @@ -3022,7 +3025,6 @@ int qemu_loadvm_state(QEMUFile *f) > } > } > > - qemu_loadvm_state_cleanup(); > cpu_synchronize_all_post_init(); > > return ret; > -- > 2.35.3 > -- Peter Xu