From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34747) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dBnzj-0002pB-Ah for qemu-devel@nongnu.org; Fri, 19 May 2017 15:55:28 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dBnzi-0002J2-2I for qemu-devel@nongnu.org; Fri, 19 May 2017 15:55:27 -0400 Received: from indium.canonical.com ([91.189.90.7]:58941) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dBnzh-0002In-SY for qemu-devel@nongnu.org; Fri, 19 May 2017 15:55:26 -0400 Received: from loganberry.canonical.com ([91.189.90.37]) by indium.canonical.com with esmtp (Exim 4.76 #1 (Debian)) id 1dBnzg-0006zs-TM for ; Fri, 19 May 2017 19:55:24 +0000 Received: from loganberry.canonical.com (localhost [127.0.0.1]) by loganberry.canonical.com (Postfix) with ESMTP id DC0342E800E for ; Fri, 19 May 2017 19:55:24 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Date: Fri, 19 May 2017 19:36:52 -0000 From: Thomas Huth <721825@bugs.launchpad.net> Reply-To: Bug 721825 <721825@bugs.launchpad.net> Sender: bounces@canonical.com References: <20110219161957.9055.80104.malonedeb@gandwana.canonical.com> Message-Id: <149522261208.12451.14164199324746532419.malone@gac.canonical.com> Errors-To: bounces@canonical.com Subject: [Qemu-devel] [Bug 721825] Re: VDI block driver bugs List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Is this still an issue with the latest version of QEMU, or could we close this bug nowadays? ** Changed in: qemu Status: Confirmed =3D> Incomplete -- = You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/721825 Title: VDI block driver bugs Status in QEMU: Incomplete Bug description: Chunqiang Tang reports the following issues with the VDI block driver, these are present in QEMU 0.14: "Bug 1. The most serious bug is caused by race condition in updating a ne= w = bmap entry in memory and on disk. Considering the following operation = sequence. = O1: VM issues a write to sector X O2: VDI allocates a new bmap entry and updates in-memory s->bmap O3: VDI writes data to disk O4: The disk I/O for writing sector X fails O5: VDI reports error to VM and returns. Note that the bmap entry is updated in memory, but not persisted on disk. = Now consider another write that immediately follows: P1: VM issues a write to sector X+1, which locates in the same block as = the previously used sector X. P2: s->bmap already has one entry for the block, and hence VDI writes = data directly without persisting the new s->bmap entry on disk. P3: The write disk I/O succeeds P4: VDI report success to VM, but the bitmap entry is still not = persisted on disk. Now suppose the VM powers off gracefully (i.e., the QEMU process quits) = and reboots. The second write to sector X+1, which is reported as finishe= d = successfully, is simply lost, because the corresponding in-memory s->bmap = entry is never persisted on disk. This is exactly what FVD's testing tool = discovers. After the block device is closed and then re-opened, disk = content verification fails. This is just one example of the problem. Race condition plus host crash = also causes problems. Consider another example below. Q1: VM issues a write to sector X Q2: VDI allocates a new bmap entry and updates in-memory s->bmap Q3: VDI writes sector X to disk and waits for the callback Q4: VM issues a write to another sector X+1, which is in the same block = as sector X. Q5: VDI sees the bitmap entry in s->bmap is already allocated, and = writes sector X+1 to disk. Q6: Write to sector X+1 finishes, and VDI's callback is invoked. Q7: VDI acknowledges to the VM the completion of writing sector X+1 Q8: After observing the completion of writing sector X+1, VM issues a = flush to ensure that sector X+1 is persisted on disk. Q9: VDI finishes the flush and acknowledge the completion of the = operation. Q10: ... (some other arbitrary operations, but the disk I/O for writing = sector X is still not finished....) Q11: The host crashes Now the new bitmap entry is not persisted on disk, while both writing to = sector X+1 and the flush has been acknowledged as finished. Sector X+1 is = lost, which is a corruption. This problem exists even if it uses O_DSYNC. = The root cause of the problem is that, if a request updates in-memory = s->bmap, another request that sees this update assumes that the update is = already persisted on disk, which is not. Bug 2: Similar to the bugs the FVD testing tool found for QCOW2, there ar= e = several cases of the code below on failure handling path without setting = error return code, which mistakenly reports failure as success. This = mistake is caught by FVD when doing image content validation. if (acb->hd_aiocb =3D=3D NULL) { /* missing ret =3D -EIO; */ goto done; = } = Bug 3: Similar to the bugs the FVD testing tool found for QCOW2, = vdi_aio_cancel does not perform a complete clean up and there are several = related bugs. First, memory buffer is not freed, acb->orig_buf and = acb->block_buffer. Second, acb->bh is not cancelled. Third, = vdi_aio_setup() does not initialize acb->bh to NULL so that when a reques= t = acb is cancelled and then later reused for another request, its acb->bh != =3D = NULL and the new request fails in vdi_schedule_bh(). This is caught by = FVD's testing tool, when it observes that no I/O failure is injected but = VDI reports a failed I/O request, which indicates a bug in the driver." http://permalink.gmane.org/gmane.comp.emulators.qemu/94340 To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/721825/+subscriptions