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Lunev" Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, Peter Xu , Fabiano Rosas , Vladimir Sementsov-Ogievskiy , qemu-stable@nongnu.org Subject: Re: [PATCH 1/2] migration/block-dirty-bitmap: fix postcopy abort with bitmaps on several nodes Message-ID: References: <20260924115611.4098417-1-den@openvz.org> <20260924115611.4098417-2-den@openvz.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260924115611.4098417-2-den@openvz.org> User-Agent: NeoMutt/20260616 X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 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: 10 X-Spam_score: 1.0 X-Spam_bar: + X-Spam_report: (1.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.24, 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, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no 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-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 Thu, Sep 24, 2026 at 01:56:10PM +0200, Denis V. Lunev wrote: > From: Denis V. Lunev > > With dirty bitmaps on more than one node, the source switches nodes > during the postcopy bulk phase and sends the node name again. The > destination reads those chunks in the postcopy listen thread, which does > not hold the BQL, and resolves the name through bdrv_lookup_bs(): > > qemu-system-x86_64: ../block.c:6400: bdrv_lookup_bs: Assertion > `qemu_in_main_thread()' failed. > > The bdrv_find_dirty_bitmap() call right after it needs the BQL or the > dirty bitmap lock as well. Taking the BQL there would deadlock: the > listen thread holds s->lock, and dirty_bitmap_mig_before_vm_start() > takes s->lock with the BQL held. > > Every node and bitmap that a non-START chunk refers to was set up by a > START chunk, and those are loaded under the BQL during setup. Record the > node alias there, and resolve all other chunks against s->bitmaps > instead of the block graph. > > Bitmaps on a single node never resend the node name after setup, which > is why migrate-bitmaps-postcopy-test does not catch this. > > Fixes: f791bf7f93f2 ("assertions for block global state API") > Signed-off-by: Denis V. Lunev > Cc: Peter Xu > Cc: Fabiano Rosas > Cc: Eric Blake > Cc: Vladimir Sementsov-Ogievskiy > Cc: qemu-stable@nongnu.org > --- > migration/block-dirty-bitmap.c | 59 +++++++++++++++++++++++++++++++--- > 1 file changed, 55 insertions(+), 4 deletions(-) > > diff --git a/migration/block-dirty-bitmap.c b/migration/block-dirty-bitmap.c > index 1b8f39c12b..61691ee4cc 100644 > --- a/migration/block-dirty-bitmap.c > +++ b/migration/block-dirty-bitmap.c > @@ -136,6 +136,7 @@ typedef struct DBMSaveState { > } DBMSaveState; > > typedef struct LoadBitmapState { > + char node_alias[256]; Is making this fixed-length rather than malloc'd char* correct? /me checks migration/block-dirty-bitmap.c: char node_alias[256]; Yep. I'm a bit surprised we did it that way earlier, but this is correct. That said, if enough structs reference the same size, we may want a #define or enum named value in a header, rather than a magic number at each use site. > +/* > + * Only START chunks are guaranteed to be loaded under the BQL; the rest may > + * come from the postcopy listen thread and must not touch the block graph. > + */ > +static BlockDriverState *dirty_bitmap_load_find_bs(DBMLoadState *s, > + const char *device, > + const char *node_name, > + Error **errp) > +{ > + GSList *item; > + > + if (s->flags & DIRTY_BITMAP_MIG_FLAG_START) { > + return bdrv_lookup_bs(device, node_name, errp); > + } > + > + for (item = s->bitmaps; item; item = g_slist_next(item)) { > + LoadBitmapState *b = item->data; > + > + if (!strcmp(b->node_alias, s->node_alias)) { > + return b->bs; > + } > + } > + > + error_setg(errp, "Error: no migrated bitmaps on node alias '%s'", > + s->node_alias); Drop the "Error: " prefix; it will be redundant if the caller reports rather than ignores the error. Otherwise, this makes sense. -- Eric Blake, Principal Software Engineer Red Hat, Inc. Virtualization: qemu.org | libguestfs.org