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bh=UPxtBuHlqNyWCKxhUQYfGvkpauJtaSJEq7HxeTZHi/M=; b=PZBSzRfHWfDQv2FtagnWaXvyQe7UdezPFfX6YBQaYKabye4ANh1wDPsAFFRds2S1KK7yCd VD9sx7jdiAPFOzppz0zR17o2RMDUw/oj2/gCW0eEzgPDKPSA16B1X593FCg+//6uC7uyAs RzlC7b/aLR2CTZh9Js8250nNKoA5g7Y= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-333-vvQ8a8XTNbiCJeqgcDsVFw-1; Thu, 15 Oct 2020 11:41:29 -0400 X-MC-Unique: vvQ8a8XTNbiCJeqgcDsVFw-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B6AE980364C; Thu, 15 Oct 2020 15:41:28 +0000 (UTC) Received: from merkur.fritz.box (ovpn-114-163.ams2.redhat.com [10.36.114.163]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8962455764; Thu, 15 Oct 2020 15:41:24 +0000 (UTC) Date: Thu, 15 Oct 2020 17:41:23 +0200 From: Kevin Wolf To: Max Reitz Subject: Re: [PATCH v2 02/20] fuse: Allow exporting BDSs via FUSE Message-ID: <20201015154123.GK4610@merkur.fritz.box> References: <20200922104932.46384-1-mreitz@redhat.com> <20200922104932.46384-3-mreitz@redhat.com> <20201015085703.GB4610@merkur.fritz.box> <2db9dbfe-137a-4cdc-08ab-aaa1aed26423@redhat.com> MIME-Version: 1.0 In-Reply-To: <2db9dbfe-137a-4cdc-08ab-aaa1aed26423@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="5mCyUwZo2JvN/JJP" Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/10/15 02:38:26 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] 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_H4=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Stefan Hajnoczi , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --5mCyUwZo2JvN/JJP Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Am 15.10.2020 um 16:46 hat Max Reitz geschrieben: > On 15.10.20 10:57, Kevin Wolf wrote: > > Am 22.09.2020 um 12:49 hat Max Reitz geschrieben: > >> block-export-add type=3Dfuse allows mounting block graph nodes via FUS= E on > >> some existing regular file. That file should then appears like a raw > >> disk image, and accesses to it result in accesses to the exported BDS. > >> > >> Right now, we only implement the necessary block export functions to s= et > >> it up and shut it down. We do not implement any access functions, so > >> accessing the mount point only results in errors. This will be > >> addressed by a followup patch. > >> > >> We keep a hash table of exported mount points, because we want to be > >> able to detect when users try to use a mount point twice. This is > >> because we invoke stat() to check whether the given mount point is a > >> regular file, but if that file is served by ourselves (because it is > >> already used as a mount point), then this stat() would have to be serv= ed > >> by ourselves, too, which is impossible to do while we (as the caller) > >> are waiting for it to settle. Therefore, keep track of mount point > >> paths to at least catch the most obvious instances of that problem. > >> > >> Signed-off-by: Max Reitz > >> +/** > >> + * Callback to be invoked when the FUSE session FD can be read from. > >> + * (This is basically the FUSE event loop.) > >> + */ > >> +static void read_from_fuse_export(void *opaque) > >> +{ > >> + FuseExport *exp =3D opaque; > >> + int ret; > >> + > >> + blk_exp_ref(&exp->common); > >> + > >> + ret =3D fuse_session_receive_buf(exp->fuse_session, &exp->fuse_bu= f); > >=20 > > The fuse_session_loop() implementation seems to imply that we should > > retry on EINTR. >=20 > OK. I see you=E2=80=99re digging into libfuse already. :) Well, I have to review against something, and the documentation tends to be terse... > >> + if (ret < 0) { > >> + goto out; > >> + } > >> + > >> + fuse_session_process_buf(exp->fuse_session, &exp->fuse_buf); > >> + > >> +out: > >> + blk_exp_unref(&exp->common); > >> +} > >> + > >> +static void fuse_export_shutdown(BlockExport *blk_exp) > >> +{ > >> + FuseExport *exp =3D container_of(blk_exp, FuseExport, common); > >> + > >> + if (exp->fuse_session) { > >> + fuse_session_exit(exp->fuse_session); > >> + > >> + if (exp->mounted) { > >> + fuse_session_unmount(exp->fuse_session); > >> + exp->mounted =3D false; > >> + } > >> + > >> + if (exp->fd_handler_set_up) { > >> + aio_set_fd_handler(exp->common.ctx, > >> + fuse_session_fd(exp->fuse_session), tr= ue, > >> + NULL, NULL, NULL, NULL); > >> + exp->fd_handler_set_up =3D false; > >> + } > >> + > >> + fuse_session_destroy(exp->fuse_session); > >> + exp->fuse_session =3D NULL; > >=20 > > What happens if a request is still in flight? > >=20 > > Oh, can't happen because the driver is fully synchronous after this > > series. Fair enough, making it asynchronous can come on top of it. >=20 > (I had multiple approaches of handling parallel requests, but none made > a substantial performance difference, which is why I left the driver in > the most simple form for this first proposal.) I think the more relevant part is that we'd block the guest or anything else running in the main loop while doing I/O. Not a problem if you spawn a new qemu-storage-daemon just for this FUSE export, but if you want to have multiple exports, or export from the system emulator, you probably don't want to have synchronous operations. 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