From b7ab0fea37c15ca9e249c42c46f5c48fd1a0943c Mon Sep 17 00:00:00 2001 From: Kevin Wolf Date: Tue, 22 May 2012 17:01:20 +0200 Subject: [PATCH] qcow2: Fix avail_sectors in cluster allocation code avail_sectors should really be the number of sectors from the start of the allocation, not from the start of the write request. We're lucky enough that this mistake didn't cause any real bug. avail_sectors is only used in the intialiser of QCowL2Meta: .nb_available = MIN(requested_sectors, avail_sectors), m->nb_available in turn is only used for COW at the end of the allocation. A COW occurs only if the request wasn't cluster aligned, which in turn would imply that requested_sectors was less than avail_sectors (both in the original and in the fixed version). In this case avail_sectors is ignored and therefore the mistake doesn't cause any misbehaviour. Signed-off-by: Kevin Wolf --- block/qcow2-cluster.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) 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) { -- 2.7.4