block: Reject misaligned write requests with BDRV_REQ_NO_FALLBACK

The BDRV_REQ_NO_FALLBACK flag means that an operation should only be
performed if it can be offloaded or otherwise performed efficiently.

However a misaligned write request requires a RMW so we should return
an error and let the caller decide how to proceed.

This hits an assertion since commit c8bb23cbdb if the required
alignment is larger than the cluster size:

qemu-img create -f qcow2 -o cluster_size=2k img.qcow2 4G
qemu-io -c "open -o driver=qcow2,file.align=4k blkdebug::img.qcow2" \
        -c 'write 0 512'
qemu-io: block/io.c:1127: bdrv_driver_pwritev: Assertion `!(flags & BDRV_REQ_NO_FALLBACK)' failed.
Aborted

The reason is that when writing to an unallocated cluster we try to
skip the copy-on-write part and zeroize it using BDRV_REQ_NO_FALLBACK
instead, resulting in a write request that is too small (2KB cluster
size vs 4KB required alignment).

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/io.c

@@ -2090,6 +2090,13 @@ int coroutine_fn bdrv_co_pwritev_part(BdrvChild *child,
         return ret;
     }
 
+    /* If the request is misaligned then we can't make it efficient */
+    if ((flags & BDRV_REQ_NO_FALLBACK) &&
+        !QEMU_IS_ALIGNED(offset | bytes, align))
+    {
+        return -ENOTSUP;
+    }
+
     bdrv_inc_in_flight(bs);
     /*
      * Align write if necessary by performing a read-modify-write cycle.

tests/qemu-iotests/268

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+#
+# Test write request with required alignment larger than the cluster size
+#
+# Copyright (C) 2019 Igalia, S.L.
+# Author: Alberto Garcia <berto@igalia.com>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+# creator
+owner=berto@igalia.com
+
+seq=`basename $0`
+echo "QA output created by $seq"
+
+status=1	# failure is the default!
+
+_cleanup()
+{
+    _cleanup_test_img
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common.rc
+. ./common.filter
+
+_supported_fmt qcow2
+_supported_proto file
+
+echo
+echo "== Required alignment larger than cluster size =="
+
+CLUSTER_SIZE=2k _make_test_img 1M
+# Since commit c8bb23cbdb writing to an unallocated cluster fills the
+# empty COW areas with bdrv_write_zeroes(flags=BDRV_REQ_NO_FALLBACK)
+$QEMU_IO -c "open -o driver=$IMGFMT,file.align=4k blkdebug::$TEST_IMG" \
+         -c "write 0 512" | _filter_qemu_io
+
+# success, all done
+echo "*** done"
+rm -f $seq.full
+status=0

tests/qemu-iotests/268.out

@@ -0,0 +1,7 @@
+QA output created by 268
+
+== Required alignment larger than cluster size ==
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576
+wrote 512/512 bytes at offset 0
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+*** done

tests/qemu-iotests/group

@@ -278,3 +278,4 @@
 265 rw auto quick
 266 rw quick
 267 rw auto quick snapshot
+268 rw auto quick