Commit ebb718a5 authored by Eric Blake's avatar Eric Blake Committed by Kevin Wolf
Browse files

qcow2: Catch more unaligned write_zero into zero cluster 



is_zero_cluster() and is_zero_cluster_top_locked() are used only
by qcow2_co_write_zeroes().  The former is too broad (we don't
care if the sectors we are about to overwrite are non-zero, only
that all other sectors in the cluster are zero), so it needs to
be called up to twice but with smaller limits - rename it along
with adding the neeeded parameter.  The latter can be inlined for
more compact code.

The testsuite change shows that we now have a sparser top file
when an unaligned write_zeroes overwrites the only portion of
the backing file with data.

Based on a patch proposal by Denis V. Lunev.

CC: Denis V. Lunev <den@openvz.org>
Signed-off-by: default avatarEric Blake <eblake@redhat.com>
Reviewed-by: default avatarDenis V. Lunev <den@openvz.org>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
parent 31ad4fdf

block/qcow2.c

+23 −24
Original line number Diff line number Diff line
@@ -2406,26 +2406,19 @@ finish: 
}





static bool is_zero_cluster(BlockDriverState *bs, int64_t start)

static bool is_zero_sectors(BlockDriverState *bs, int64_t start,

                            uint32_t count)

{

    BDRVQcow2State *s = bs->opaque;

    int nr;

    BlockDriverState *file;

    int64_t res = bdrv_get_block_status_above(bs, NULL, start,

                                              s->cluster_sectors, &nr, &file);

    return res >= 0 && (res & BDRV_BLOCK_ZERO) && nr == s->cluster_sectors;

}



static bool is_zero_cluster_top_locked(BlockDriverState *bs, int64_t start)

{

    BDRVQcow2State *s = bs->opaque;

    int nr = s->cluster_sectors;

    uint64_t off;

    int ret;

    int64_t res;



    ret = qcow2_get_cluster_offset(bs, start << BDRV_SECTOR_BITS, &nr, &off);

    assert(nr == s->cluster_sectors);

    return ret == QCOW2_CLUSTER_UNALLOCATED || ret == QCOW2_CLUSTER_ZERO;

    if (!count) {

        return true;

    }

    res = bdrv_get_block_status_above(bs, NULL, start, count,

                                      &nr, &file);

    return res >= 0 && (res & BDRV_BLOCK_ZERO) && nr == count;

}



static coroutine_fn int qcow2_co_write_zeroes(BlockDriverState *bs,
@@ -2434,27 +2427,33 @@ static coroutine_fn int qcow2_co_write_zeroes(BlockDriverState *bs, 
    int ret;

    BDRVQcow2State *s = bs->opaque;



    int head = sector_num % s->cluster_sectors;

    int tail = (sector_num + nb_sectors) % s->cluster_sectors;

    uint32_t head = sector_num % s->cluster_sectors;

    uint32_t tail = (sector_num + nb_sectors) % s->cluster_sectors;



    trace_qcow2_write_zeroes_start_req(qemu_coroutine_self(), sector_num,

                                       nb_sectors);



    if (head != 0 || tail != 0) {

    if (head || tail) {

        int64_t cl_start = sector_num - head;

        uint64_t off;

        int nr;



        assert(cl_start + s->cluster_sectors >= sector_num + nb_sectors);



        sector_num = cl_start;

        nb_sectors = s->cluster_sectors;



        if (!is_zero_cluster(bs, sector_num)) {

        /* check whether remainder of cluster already reads as zero */

        if (!(is_zero_sectors(bs, cl_start, head) &&

              is_zero_sectors(bs, sector_num + nb_sectors,

                              -tail & (s->cluster_sectors - 1)))) {

            return -ENOTSUP;

        }



        qemu_co_mutex_lock(&s->lock);

        /* We can have new write after previous check */

        if (!is_zero_cluster_top_locked(bs, sector_num)) {

        sector_num = cl_start;

        nb_sectors = nr = s->cluster_sectors;

        ret = qcow2_get_cluster_offset(bs, cl_start << BDRV_SECTOR_BITS,

                                       &nr, &off);

        if (ret != QCOW2_CLUSTER_UNALLOCATED && ret != QCOW2_CLUSTER_ZERO) {

            qemu_co_mutex_unlock(&s->lock);

            return -ENOTSUP;

        }

tests/qemu-iotests/154.out

+4 −4
Original line number Diff line number Diff line
@@ -102,13 +102,13 @@ wrote 2048/2048 bytes at offset 29696 
read 4096/4096 bytes at offset 28672

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

[{ "start": 0, "length": 4096, "depth": 1, "zero": true, "data": false},

{ "start": 4096, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": 20480},

{ "start": 4096, "length": 4096, "depth": 0, "zero": true, "data": false},

{ "start": 8192, "length": 4096, "depth": 1, "zero": true, "data": false},

{ "start": 12288, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": 24576},

{ "start": 12288, "length": 4096, "depth": 0, "zero": true, "data": false},

{ "start": 16384, "length": 4096, "depth": 1, "zero": true, "data": false},

{ "start": 20480, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": 28672},

{ "start": 20480, "length": 4096, "depth": 0, "zero": true, "data": false},

{ "start": 24576, "length": 4096, "depth": 1, "zero": true, "data": false},

{ "start": 28672, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": 32768},

{ "start": 28672, "length": 4096, "depth": 0, "zero": true, "data": false},

{ "start": 32768, "length": 134184960, "depth": 1, "zero": true, "data": false}]



== spanning two clusters, non-zero before request ==