Commit a5675f39 authored by Alberto Garcia's avatar Alberto Garcia Committed by Max Reitz
Browse files

qcow2: Fix preallocation on images with unaligned sizes 



When resizing an image with qcow2_co_truncate() using the falloc or
full preallocation modes the code assumes that both the old and new
sizes are cluster-aligned.

There are two problems with this:

  1) The calculation of how many clusters are involved does not always
     get the right result.

     Example: creating a 60KB image and resizing it (with
     preallocation=full) to 80KB won't allocate the second cluster.

  2) No copy-on-write is performed, so in the previous example if
     there is a backing file then the first 60KB of the first cluster
     won't be filled with data from the backing file.

This patch fixes both issues.

Signed-off-by: default avatarAlberto Garcia <berto@igalia.com>
Message-Id: <20200617140036.20311-1-berto@igalia.com>
Reviewed-by: default avatarEric Blake <eblake@redhat.com>
Signed-off-by: default avatarMax Reitz <mreitz@redhat.com>
parent e8de7ba9

block/qcow2.c

+14 −3
Original line number Diff line number Diff line
@@ -4239,8 +4239,8 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset, 
            old_file_size = ROUND_UP(old_file_size, s->cluster_size);

        }



        nb_new_data_clusters = DIV_ROUND_UP(offset - old_length,

                                            s->cluster_size);

        nb_new_data_clusters = (ROUND_UP(offset, s->cluster_size) -

            start_of_cluster(s, old_length)) >> s->cluster_bits;



        /* This is an overestimation; we will not actually allocate space for

         * these in the file but just make sure the new refcount structures are
@@ -4317,10 +4317,21 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset, 
            int64_t nb_clusters = MIN(

                nb_new_data_clusters,

                s->l2_slice_size - offset_to_l2_slice_index(s, guest_offset));

            QCowL2Meta allocation = {

            unsigned cow_start_length = offset_into_cluster(s, guest_offset);

            QCowL2Meta allocation;

            guest_offset = start_of_cluster(s, guest_offset);

            allocation = (QCowL2Meta) {

                .offset       = guest_offset,

                .alloc_offset = host_offset,

                .nb_clusters  = nb_clusters,

                .cow_start    = {

                    .offset       = 0,

                    .nb_bytes     = cow_start_length,

                },

                .cow_end      = {

                    .offset       = nb_clusters << s->cluster_bits,

                    .nb_bytes     = 0,

                },

            };

            qemu_co_queue_init(&allocation.dependent_requests);

tests/qemu-iotests/125

+24 −0
Original line number Diff line number Diff line
@@ -164,6 +164,30 @@ for GROWTH_SIZE in 16 48 80; do 
done

done



# Test image resizing using preallocation and unaligned offsets

$QEMU_IMG create -f raw "$TEST_IMG.base" 128k | _filter_img_create

$QEMU_IO -c 'write -q -P 1 0 128k' -f raw "$TEST_IMG.base"

for orig_size in 31k 33k; do

    echo "--- Resizing image from $orig_size to 96k ---"

    _make_test_img -F raw -b "$TEST_IMG.base" -o cluster_size=64k "$orig_size"

    $QEMU_IMG resize -f "$IMGFMT" --preallocation=full "$TEST_IMG" 96k

    # The first part of the image should contain data from the backing file

    $QEMU_IO -c "read -q -P 1 0 ${orig_size}" "$TEST_IMG"

    # The resized part of the image should contain zeroes

    $QEMU_IO -c "read -q -P 0 ${orig_size} 63k" "$TEST_IMG"

    # If the image does not have an external data file we can also verify its

    # actual size. The resized image should have 7 clusters:

    # header, L1 table, L2 table, refcount table, refcount block, 2 data clusters

    if ! _get_data_file "$TEST_IMG" > /dev/null; then

        expected_file_length=$((65536 * 7))

        file_length=$(stat -c '%s' "$TEST_IMG_FILE")

        if [ "$file_length" != "$expected_file_length" ]; then

            echo "ERROR: file length $file_length (expected $expected_file_length)"

        fi

    fi

    echo

done



# success, all done

echo '*** done'

rm -f $seq.full

tests/qemu-iotests/125.out

+9 −0
Original line number Diff line number Diff line
@@ -767,4 +767,13 @@ wrote 2048000/2048000 bytes at offset 0 
wrote 81920/81920 bytes at offset 2048000

80 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)



Formatting 'TEST_DIR/t.IMGFMT.base', fmt=raw size=131072

--- Resizing image from 31k to 96k ---

Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=31744 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=raw

Image resized.



--- Resizing image from 33k to 96k ---

Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33792 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=raw

Image resized.



*** done