From patchwork Wed Dec 12 13:25:36 2012 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: [BUG] qemu-1.1.2 [FIXED-BY] qcow2: Fix avail_sectors in cluster allocation code From: Philipp Hahn X-Patchwork-Id: 205511 Message-Id: <201212121425.41850.hahn@univention.de> To: qemu-devel@nongnu.org, Michael Tokarev Cc: Kevin Wolf Date: Wed, 12 Dec 2012 14:25:36 +0100 Hello Kevin, hello Michael, hello *, we noticed a data corruption bug in qemu-1.1.2, which will be shipped by Debian and our own Debian based distibution. The corruption mostly manifests while installing large Debian package files and seems to be reladed to memory preasure: As long as the file is still in the page cache, everything looks fine, but when the file is re-read from the virtual hard disk using a qcow2 file backed by another qcow2 file, the file is corrupted: dpkg complains that the .tar.gz file inside the Debian archive file is corrupted and the md5sum no longer matches. I tracked this down using "git bisect" to your patch attached below, which fixed this bug, so everything is fine with qemu-kvm-1.2.0. >From my reading this seems to explain our problems, since during my own testing during development I never used backing chains and the problem only showed up when my collegues started using qemu-kvm-1.1.2 with their VMs using backing chains. @Kevin: Do you thinks that's a valid explanation and your patch should fix that problem? I'd like to get your expertise before filing a bug with Debian and asking Michael to include that patch with his next stable update for 1.1. Thanks in advance. Sincerely Philipp diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c index 98fba71..d7e0e19 100644 --- a/block/qcow2-cluster.c +++ b/block/qcow2-cluster.c @@ -947,8 +947,16 @@ again: /* save info needed for meta data update */ if (nb_clusters > 0) { + /* + * requested_sectors: Number of sectors from the start of the first + * newly allocated cluster to the end of the (possibly shortened + * before) write request. + * + * avail_sectors: Number of sectors from the start of the first + * newly allocated to the end of the last newly allocated cluster. + */ int requested_sectors = n_end - keep_clusters * s->cluster_sectors; - int avail_sectors = (keep_clusters + nb_clusters) + int avail_sectors = nb_clusters << (s->cluster_bits - BDRV_SECTOR_BITS); *m = (QCowL2Meta) {