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Lunev" Cc: qemu-devel@nongnu.org, Vladimir Sementsov-Ogievskiy , John Snow , Peter Xu , Fabiano Rosas , qemu-block@nongnu.org Subject: Re: [PATCH] migration/block-dirty-bitmap: don't hold the load lock across stream reads Message-ID: References: <20261110155113.2721134-1-den@openvz.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261110155113.2721134-1-den@openvz.org> User-Agent: NeoMutt/20260616 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 Received-SPF: pass client-ip=170.10.129.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 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_LOW=-0.7, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-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: qemu development List-Archive: List-Post: List-Help: List-Subscribe: , Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org X-Spam-Status: No, score=-17.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Tue, Nov 10, 2026 at 07:24:08PM +0100, Denis V. Lunev wrote: > From: Denis V. Lunev > > dirty_bitmap_load() takes dbm_state.load.lock for each chunk and holds > it while it reads the chunk from the migration stream. > > In precopy the load runs in the incoming migration coroutine, which > yields in qemu_fill_buffer() while it waits for data, still holding > the lock. A quit on the destination at that point never completes: > migration_shutdown() calls dirty_bitmap_mig_cancel_incoming(), which > takes the same lock in the same main thread. > > In postcopy the load runs in the listen thread, and the main thread > takes the lock in dirty_bitmap_mig_before_vm_start(), called from > loadvm_postcopy_handle_run_bh(), and in migration_shutdown(). The main > loop then waits until the source sends the next chunk. > > Read each chunk into a DBMLoadChunk first and take the lock only to > resolve the names and apply the chunk. The lock protects the bitmaps > list and the resolved node and bitmap, which the main thread uses. The > flags and alias strings read from the stream are only touched by the > code doing the load. I'm guessing this should be considered for 11.2, and necessary as a followup for the recent commit 4d11088a "migration/block-dirty-bitmap: fix postcopy abort...")? > > Signed-off-by: Denis V. Lunev > Cc: Vladimir Sementsov-Ogievskiy > Cc: Eric Blake > Cc: John Snow > Cc: Peter Xu > Cc: Fabiano Rosas > Cc: qemu-block@nongnu.org > --- > migration/block-dirty-bitmap.c | 222 +++++++++++++++++++-------------- > 1 file changed, 130 insertions(+), 92 deletions(-) > > diff --git a/migration/block-dirty-bitmap.c b/migration/block-dirty-bitmap.c > index 35623379eb..14e5a984e4 100644 > --- a/migration/block-dirty-bitmap.c > +++ b/migration/block-dirty-bitmap.c > -static int dirty_bitmap_load_bits(QEMUFile *f, DBMLoadState *s) > +static int dirty_bitmap_load_bits(DBMLoadState *s, DBMLoadChunk *c) > { ... > - needed_size = bdrv_dirty_bitmap_serialization_size(s->bitmap, > - first_byte, > - nr_bytes); > - > - if (needed_size > buf_size || > - buf_size > QEMU_ALIGN_UP(needed_size, 4 * sizeof(long)) > - /* Here used same alignment as in send_bitmap_bits */ > - ) { It looks odd to see a comment appear mid-argument, which results in unusual formatting... > - error_report("Migrated bitmap granularity doesn't " > - "match the destination bitmap '%s' granularity", > - bdrv_dirty_bitmap_name(s->bitmap)); > - cancel_incoming_locked(s); > - return 0; > - } > + if (s->cancelled) { > + return 0; > + } > > - bdrv_dirty_bitmap_deserialize_part(s->bitmap, buf, first_byte, nr_bytes, > - false); > + needed_size = bdrv_dirty_bitmap_serialization_size(s->bitmap, > + c->first_byte, > + c->nr_bytes); > + > + if (needed_size > c->buf_size || > + c->buf_size > QEMU_ALIGN_UP(needed_size, 4 * sizeof(long)) > + /* Here used same alignment as in send_bitmap_bits */ > + ) { ...which then got preserved with an unindent. Not worth cleaning up, but it did make me do a double-take. Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. Virtualization: qemu.org | libguestfs.org