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dkim=none (message not signed) header.d=none;nongnu.org; dmarc=none action=none header.from=virtuozzo.com; Received: from AM7PR08MB5494.eurprd08.prod.outlook.com (2603:10a6:20b:dc::15) by AM6PR08MB3221.eurprd08.prod.outlook.com (2603:10a6:209:49::33) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4195.24; Mon, 7 Jun 2021 05:39:36 +0000 Received: from AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::f928:f4f2:77c0:74b4]) by AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::f928:f4f2:77c0:74b4%9]) with mapi id 15.20.4195.030; Mon, 7 Jun 2021 05:39:36 +0000 Subject: Re: [PATCH v3 1/7] file-posix: fix max_iov for /dev/sg devices To: Paolo Bonzini , qemu-devel@nongnu.org Cc: kwolf@redhat.com, qemu-block@nongnu.org References: <20210603133722.218465-1-pbonzini@redhat.com> <20210603133722.218465-2-pbonzini@redhat.com> From: Vladimir Sementsov-Ogievskiy Message-ID: <6426cb5d-9b3e-ca6b-35d4-c4cd5488c448@virtuozzo.com> Date: Mon, 7 Jun 2021 08:39:34 +0300 User-Agent: Mozilla/5.0 (X11; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR05-VI1-obe.outbound.protection.outlook.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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-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: 2.1.23 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" 03.06.2021 16:37, Paolo Bonzini wrote: > Even though it was only called for devices that have bs->sg set (which > must be character devices), > sg_get_max_segments looked at /sys/dev/block which only works for > block devices. > > On Linux the sg driver has its own way to provide the maximum number of > iovecs in a scatter/gather list. > > Signed-off-by: Paolo Bonzini > --- > block/file-posix.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) > > diff --git a/block/file-posix.c b/block/file-posix.c > index f37dfc10b3..58db526cc2 100644 > --- a/block/file-posix.c > +++ b/block/file-posix.c > @@ -1180,6 +1180,17 @@ static int sg_get_max_segments(int fd) > goto out; > } > > + if (S_ISCHR(st->st_mode)) { > + if (ioctl(fd, SG_GET_SG_TABLESIZE, &ret) == 0) { > + return ret; So, this is a new most-probable path for char-dev, yes? Is it better do it at start of the function to avoid extra fstat() ? > + } > + return -EIO; > + } > + > + if (!S_ISBLK(st->st_mode)) { > + return -ENOTSUP; > + } > + > sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments", > major(st.st_rdev), minor(st.st_rdev)); > sysfd = open(sysfspath, O_RDONLY); > -- Best regards, Vladimir