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dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:48964 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1khFol-0000OD-5o for qemu-devel@archiver.kernel.org; Mon, 23 Nov 2020 12:39:59 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:45564) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1khFnu-0008AP-Ot for qemu-devel@nongnu.org; Mon, 23 Nov 2020 12:39:06 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:41017) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1khFnt-0002Ar-3I for qemu-devel@nongnu.org; Mon, 23 Nov 2020 12:39:06 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1606153144; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; 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auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable 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: , Cc: vsementsov@virtuozzo.com, Alberto Garcia , qemu-block@nongnu.org, zhang_youjia@126.com, qemu-devel@nongnu.org, Max Reitz , andrey.shinkevich@virtuozzo.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 23.11.2020 um 16:49 hat Maxim Levitsky geschrieben: > Commit 205fa50750 ("qcow2: Add subcluster support to zero_in_l2_slice()") > introduced a subtle change to code in zero_in_l2_slice: > > It swapped the order of > > 1. qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); > 2. set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO); > 3. qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST); > > To > > 1. qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); > 2. qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST); > 3. set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO); > > It seems harmless, however the call to qcow2_free_any_clusters > can trigger a cache flush which can mark the L2 table as clean, > and assuming that this was the last write to it, > a stale version of it will remain on the disk. Do you have more details on this last paragraph? I'm trying to come up with a reproducer, but I don't see how qcow2_free_any_clusters() could flush the L2 table cache. (It's easy to get it to flush the refcount block cache, but that's useless for a reproducer.) The only way I see to flush any cache with it is in update_refcount() the qcow2_cache_set_dependency() call. This will always flush the cache that the L2 cache depends on - which will never be the L2 cache itself, but always either the refcount cache or nothing. There are more options in alloc_refcount_block() if we're allocating a new refcount block, but in the context of freeing clusters we'll never need to do that. Whatever I tried, at the end of zero_in_l2_slice(), I have a dirty L2 table and a dirty refcount block in the cache, with a dependency that makes sure that the L2 table will be written out first. If you don't have the information yet, can you try to debug your manual reproducer a bit more to find out how this happens? Kevin > Now we have a valid L2 entry pointing to a freed cluster. Oops. > > Fixes: 205fa50750 ("qcow2: Add subcluster support to zero_in_l2_slice()") > Signed-off-by: Maxim Levitsky > --- > block/qcow2-cluster.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c > index 485b4cb92e..267b46a4ca 100644 > --- a/block/qcow2-cluster.c > +++ b/block/qcow2-cluster.c > @@ -2010,11 +2010,11 @@ static int zero_in_l2_slice(BlockDriverState *bs, uint64_t offset, > continue; > } > > - qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); > if (unmap) { > qcow2_free_any_cluster(bs, old_l2_entry, QCOW2_DISCARD_REQUEST); > } > set_l2_entry(s, l2_slice, l2_index + i, new_l2_entry); > + qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); > if (has_subclusters(s)) { > set_l2_bitmap(s, l2_slice, l2_index + i, new_l2_bitmap); > } > -- > 2.26.2 >