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Tue, 17 Sep 2024 19:29:48 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id mu8fIazY6WYoFgAAD6G6ig (envelope-from ); Tue, 17 Sep 2024 19:29:48 +0000 From: Fabiano Rosas To: Peter Xu Cc: qemu-devel@nongnu.org, Peter Maydell , qemu-stable@nongnu.org Subject: Re: [PATCH 2/2] migration/multifd: Fix rb->receivedmap cleanup race In-Reply-To: References: <20240917185802.15619-1-farosas@suse.de> <20240917185802.15619-3-farosas@suse.de> Date: Tue, 17 Sep 2024 16:29:46 -0300 Message-ID: <877cbayt79.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Queue-Id: 474C322464 X-Spamd-Result: default: False [-6.51 / 50.00]; BAYES_HAM(-3.00)[99.99%]; DWL_DNSWL_MED(-2.00)[suse.de:dkim]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MISSING_XM_UA(0.00)[]; MIME_TRACE(0.00)[0:+]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; TO_DN_SOME(0.00)[]; RCVD_TLS_ALL(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCPT_COUNT_THREE(0.00)[4]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; RCVD_COUNT_TWO(0.00)[2]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; DKIM_TRACE(0.00)[suse.de:+] X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Action: no action Received-SPF: pass client-ip=2a07:de40:b251:101:10:150:64:1; envelope-from=farosas@suse.de; helo=smtp-out1.suse.de 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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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 Peter Xu writes: > 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). > */ Yeah, that's ok. > >> + 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 >>