file-posix: Fix alignment after reopen changing O_DIRECT

At the end of a reopen, we already call bdrv_refresh_limits(), which
should update bs->request_alignment according to the new file
descriptor. However, raw_probe_alignment() relies on s->needs_alignment
and just uses 1 if it isn't set. We neglected to update this field, so
starting with cache=writeback and then reopening with cache=none means
that we get an incorrect bs->request_alignment == 1 and unaligned
requests fail instead of being automatically aligned.

Fix this by recalculating s->needs_alignment in raw_refresh_limits()
before calling raw_probe_alignment().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211104113109.56336-1-kwolf@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211115145409.176785-13-kwolf@redhat.com>
[hreitz: Fix iotest 142 for block sizes greater than 512 by operating on
         a file with a size of 1 MB]
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211116101431.105252-1-hreitz@redhat.com>

block/file-posix.c

@@ -167,6 +167,7 @@ typedef struct BDRVRawState {
     int page_cache_inconsistent; /* errno from fdatasync failure */
     bool has_fallocate;
     bool needs_alignment;
+    bool force_alignment;
     bool drop_cache;
     bool check_cache_dropped;
     struct {
@@ -351,6 +352,17 @@ static bool dio_byte_aligned(int fd)
     return false;
 }
 
+static bool raw_needs_alignment(BlockDriverState *bs)
+{
+    BDRVRawState *s = bs->opaque;
+
+    if ((bs->open_flags & BDRV_O_NOCACHE) != 0 && !dio_byte_aligned(s->fd)) {
+        return true;
+    }
+
+    return s->force_alignment;
+}
+
 /* Check if read is allowed with given memory buffer and length.
  *
  * This function is used to check O_DIRECT memory buffer and request alignment.
@@ -728,9 +740,6 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
 
     s->has_discard = true;
     s->has_write_zeroes = true;
-    if ((bs->open_flags & BDRV_O_NOCACHE) != 0 && !dio_byte_aligned(s->fd)) {
-        s->needs_alignment = true;
-    }
 
     if (fstat(s->fd, &st) < 0) {
         ret = -errno;
@@ -784,9 +793,10 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
          * so QEMU makes sure all IO operations on the device are aligned
          * to sector size, or else FreeBSD will reject them with EINVAL.
          */
-        s->needs_alignment = true;
+        s->force_alignment = true;
     }
 #endif
+    s->needs_alignment = raw_needs_alignment(bs);
 
 #ifdef CONFIG_XFS
     if (platform_test_xfs_fd(s->fd)) {
@@ -1251,7 +1261,9 @@ static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
     BDRVRawState *s = bs->opaque;
     struct stat st;
 
+    s->needs_alignment = raw_needs_alignment(bs);
     raw_probe_alignment(bs, s->fd, errp);
+
     bs->bl.min_mem_alignment = s->buf_align;
     bs->bl.opt_mem_alignment = MAX(s->buf_align, qemu_real_host_page_size);
 

tests/qemu-iotests/142

@@ -350,6 +350,35 @@ info block backing-file"
 
 echo "$hmp_cmds" | run_qemu -drive "$files","$ids" | grep "Cache"
 
+echo
+echo "--- Alignment after changing O_DIRECT ---"
+echo
+
+# Directly test the protocol level: Can unaligned requests succeed even if
+# O_DIRECT was only enabled through a reopen and vice versa?
+
+# Ensure image size is a multiple of the sector size (required for O_DIRECT)
+$QEMU_IMG create -f file "$TEST_IMG" 1M | _filter_img_create
+
+# And write some data (not strictly necessary, but it feels better to actually
+# have something to be read)
+$QEMU_IO -f file -c 'write 0 4096' "$TEST_IMG" | _filter_qemu_io
+
+$QEMU_IO --cache=writeback -f file $TEST_IMG <<EOF | _filter_qemu_io
+read 42 42
+reopen -o cache.direct=on
+read 42 42
+reopen -o cache.direct=off
+read 42 42
+EOF
+$QEMU_IO --cache=none -f file $TEST_IMG <<EOF | _filter_qemu_io
+read 42 42
+reopen -o cache.direct=off
+read 42 42
+reopen -o cache.direct=on
+read 42 42
+EOF
+
 # success, all done
 echo "*** done"
 rm -f $seq.full

tests/qemu-iotests/142.out

@@ -747,4 +747,22 @@ cache.no-flush=on on backing-file
     Cache mode:       writeback
     Cache mode:       writeback, direct
     Cache mode:       writeback, ignore flushes
+
+--- Alignment after changing O_DIRECT ---
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=file size=1048576
+wrote 4096/4096 bytes at offset 0
+4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 42/42 bytes at offset 42
+42 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 42/42 bytes at offset 42
+42 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 42/42 bytes at offset 42
+42 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 42/42 bytes at offset 42
+42 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 42/42 bytes at offset 42
+42 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 42/42 bytes at offset 42
+42 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 *** done