From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34013) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VF1iZ-0002Fn-Bt for qemu-devel@nongnu.org; Thu, 29 Aug 2013 08:53:01 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1VF1iT-0001gi-AX for qemu-devel@nongnu.org; Thu, 29 Aug 2013 08:52:55 -0400 Received: from mx1.redhat.com ([209.132.183.28]:30066) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VF1iT-0001ge-1o for qemu-devel@nongnu.org; Thu, 29 Aug 2013 08:52:49 -0400 Received: from int-mx09.intmail.prod.int.phx2.redhat.com (int-mx09.intmail.prod.int.phx2.redhat.com [10.5.11.22]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id r7TCqmTN005780 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Thu, 29 Aug 2013 08:52:48 -0400 Date: Thu, 29 Aug 2013 14:53:01 +0200 From: Kevin Wolf Message-ID: <20130829125301.GJ2961@dhcp-200-207.str.redhat.com> References: <1377771363-5198-1-git-send-email-mreitz@redhat.com> <20130829123343.GH2961@dhcp-200-207.str.redhat.com> <521F40D3.8000302@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <521F40D3.8000302@redhat.com> Subject: Re: [Qemu-devel] [PATCH] qcow2-refcount: Snapshot update for zero clusters List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Max Reitz Cc: qemu-devel@nongnu.org, Stefan Hajnoczi Am 29.08.2013 um 14:38 hat Max Reitz geschrieben: > Am 29.08.2013 14:33, schrieb Kevin Wolf: > >Am 29.08.2013 um 12:16 hat Max Reitz geschrieben: > >>Do not try to update the refcount for zero clusters in > >>qcow2_update_snapshot_refcount. > >> > >>Signed-off-by: Max Reitz > >>--- > >> block/qcow2-refcount.c | 16 +++++++++++++--- > >> 1 file changed, 13 insertions(+), 3 deletions(-) > >Please don't forget to add a test case for v2. > Okay. > > >>diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c > >>index 1244693..7555242 100644 > >>--- a/block/qcow2-refcount.c > >>+++ b/block/qcow2-refcount.c > >>@@ -863,9 +863,13 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs, > >> for(j = 0; j < s->l2_size; j++) { > >> offset = be64_to_cpu(l2_table[j]); > >> if (offset != 0) { > >>+ uint64_t cluster_index; > >>+ > >> old_offset = offset; > >> offset &= ~QCOW_OFLAG_COPIED; > >>- if (offset & QCOW_OFLAG_COMPRESSED) { > >>+ > >>+ switch (qcow2_get_cluster_type(offset)) { > >>+ case QCOW2_CLUSTER_COMPRESSED: > >> nb_csectors = ((offset >> s->csize_shift) & > >> s->csize_mask) + 1; > >> if (addend != 0) { > >>@@ -880,8 +884,10 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs, > >> } > >> /* compressed clusters are never modified */ > >> refcount = 2; > >>- } else { > >>- uint64_t cluster_index = (offset & L2E_OFFSET_MASK) >> s->cluster_bits; > >>+ break; > >>+ > >>+ case QCOW2_CLUSTER_NORMAL: > >>+ cluster_index = (offset & L2E_OFFSET_MASK) >> s->cluster_bits; > >> if (addend != 0) { > >> refcount = update_cluster_refcount(bs, cluster_index, addend, > >> QCOW2_DISCARD_SNAPSHOT); > >>@@ -893,6 +899,10 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs, > >> ret = refcount; > >> goto fail; > >> } > >>+ break; > >I think this part isn't quite right. Even zero clusters can have an > >allocated offset, so I think what's really needed is that the same code > >runs from QCOW2_CLUSTER_ZERO. However, add an if (cluster_index == 0) > >case for the unallocated case. > So then it would be enough to change: > > - if (offset != 0) { > + if ((offset & L2E_OFFSET_MASK) != 0) { > > and just execute the same code for QCOW2_CLUSTER_NORMAL and > QCOW2_CLUSTER_ZERO? Doing only this change in the original code might happen to work, but it's not really clean as the L2 entry format is different for compressed clusters (L2E_OFFSET_MASK isn't valid for it). So having the switch for the cluster type is a good thing, but the implementation for normal and zero clusters should be shared indeed. The offset != 0 check ends up redundant when you introduce the switch, so in theory it could as well be removed. Kevin