Re: [Qemu-devel] [PATCH v6 1/2] mirror: Rewrite mirror_iteration

[Max Reitz <mreitz@redhat.com>]

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On 20.11.2015 11:12, Fam Zheng wrote:
> The "pnum < nb_sectors" condition in deciding whether to actually copy
> data is unnecessarily strict, and the qiov initialization is
> unnecessarily for bdrv_aio_write_zeroes and bdrv_aio_discard.
> 
> Rewrite mirror_iteration to fix both flaws.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> 
> ---
> v5: Address Max's review comments:
>     - Fix parameter name of mirror_do_read().
>     - Simplify the buffer waiting loop in mirror_do_read.
>     - Don't skip next dirty chunk when collecting consective dirty
>       chunks.
>     - Check sector range when collecting consective dirty chunks.
>     - Don't misuse a negative return value of
>       bdrv_get_block_status_above.
> ---
>  block/mirror.c | 307 +++++++++++++++++++++++++++++++++++----------------------
>  1 file changed, 187 insertions(+), 120 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index 52c9abf..ff8149d 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -45,7 +45,6 @@ typedef struct MirrorBlockJob {
>      BlockdevOnError on_source_error, on_target_error;
>      bool synced;
>      bool should_complete;
> -    int64_t sector_num;
>      int64_t granularity;
>      size_t buf_size;
>      int64_t bdev_length;
> @@ -157,113 +156,76 @@ static void mirror_read_complete(void *opaque, int ret)
>                      mirror_write_complete, op);
>  }
>  
> -static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
> +/* Round sector_num and/or nb_sectors to target cluster if COW is needed, and
> + * return the offset of the adjusted ending sector against
> + * sector_num + nb_sectors. */
> +static int mirror_cow_align(MirrorBlockJob *s,
> +                            int64_t *sector_num,
> +                            int *nb_sectors)
> +{
> +    bool head_need_cow, tail_need_cow;
> +    int diff = 0;
> +    int sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS;
> +
> +    head_need_cow = !test_bit(*sector_num / sectors_per_chunk, s->cow_bitmap);
> +    tail_need_cow = !test_bit((*sector_num + *nb_sectors) / sectors_per_chunk,

Should this be (*sector_num + *nb_sectors - 1) so that we actually check
the last chunk of the sector range (instead of the next chunk if
*sector_num + *nb_sectors is divisible by sectors_per_chunk).

> +                             s->cow_bitmap);
> +    if (head_need_cow || tail_need_cow) {
> +        int64_t rounded_sector_num;
> +        int rounded_nb_sectors;
> +        bdrv_round_to_clusters(s->target, *sector_num, *nb_sectors,
> +                               &rounded_sector_num, &rounded_nb_sectors);
> +        assert(*sector_num >= rounded_sector_num);
> +        assert(rounded_nb_sectors >= *nb_sectors);

You could move these assertions into the following conditional blocks,
that would make more sense to me:

> +        if (tail_need_cow) {
> +            int diff = rounded_sector_num + rounded_nb_sectors
> +                        - (*sector_num + *nb_sectors);

assert(diff >= 0);

Also, I don't like shadowing of variables very much.

> +            *nb_sectors += diff;
> +        }
> +        if (head_need_cow) {
> +            int diff = *sector_num - rounded_sector_num;

assert(diff >= 0);

> +            *sector_num = rounded_sector_num;
> +            *nb_sectors += diff;
> +        }
> +    }
> +    return diff;

diff is always 0.

(This is why I don't like shadowing of variables very much.)

> +}
> +
> +/* Submit async read while handling COW.
> + * Returns: nb_sectors if no alignment is necessary, or
> + *          (new_end - sector_num) if tail is rounded up or down due to
> + *          alignment or buffer limit.
> + */
> +static int mirror_do_read(MirrorBlockJob *s, int64_t sector_num,
> +                          int nb_sectors)
>  {
>      BlockDriverState *source = s->common.bs;
> -    int nb_sectors, sectors_per_chunk, nb_chunks;
> -    int64_t end, sector_num, next_chunk, next_sector, hbitmap_next_sector;
> -    uint64_t delay_ns = 0;
> +    int sectors_per_chunk, nb_chunks;
> +    int ret = nb_sectors;
>      MirrorOp *op;
> -    int pnum;
> -    int64_t ret;
>  
> -    s->sector_num = hbitmap_iter_next(&s->hbi);
> -    if (s->sector_num < 0) {
> -        bdrv_dirty_iter_init(s->dirty_bitmap, &s->hbi);
> -        s->sector_num = hbitmap_iter_next(&s->hbi);
> -        trace_mirror_restart_iter(s, bdrv_get_dirty_count(s->dirty_bitmap));
> -        assert(s->sector_num >= 0);
> -    }
> -
> -    hbitmap_next_sector = s->sector_num;
> -    sector_num = s->sector_num;
>      sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS;
> -    end = s->bdev_length / BDRV_SECTOR_SIZE;
>  
> -    /* Extend the QEMUIOVector to include all adjacent blocks that will
> -     * be copied in this operation.
> -     *
> -     * We have to do this if we have no backing file yet in the destination,
> -     * and the cluster size is very large.  Then we need to do COW ourselves.
> -     * The first time a cluster is copied, copy it entirely.  Note that,
> -     * because both the granularity and the cluster size are powers of two,
> -     * the number of sectors to copy cannot exceed one cluster.
> -     *
> -     * We also want to extend the QEMUIOVector to include more adjacent
> -     * dirty blocks if possible, to limit the number of I/O operations and
> -     * run efficiently even with a small granularity.
> -     */
> -    nb_chunks = 0;
> -    nb_sectors = 0;
> -    next_sector = sector_num;
> -    next_chunk = sector_num / sectors_per_chunk;
> +    if (s->cow_bitmap) {
> +        ret += mirror_cow_align(s, &sector_num, &nb_sectors);

mirror_cow_align() always returns 0, but I assume it is supposed to
return the difference of nb_sectors before and after the call (in which
case this line is correct).

> +    }
> +    /* We can only handle as much as buf_size at a time. */
> +    nb_sectors = MIN(s->buf_size >> BDRV_SECTOR_BITS, nb_sectors);
> +    assert(nb_sectors);

Maybe we should move these three lines before the mirror_cow_align()
call. I don't think it will make a difference in practice, but it seems
cleaner to me that way.

> +    /* The sector range must meet granularity because:
> +     * 1) Caller passes in aligned values;
> +     * 2) mirror_cow_align is used only when target cluster is larger. */
> +    assert(!(nb_sectors % sectors_per_chunk));
> +    assert(!(sector_num % sectors_per_chunk));
> +    nb_chunks = nb_sectors / sectors_per_chunk;

[...]

> +static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
> +{

[...]

> +    while (nb_chunks > 0 && sector_num < end) {
> +        int ret;
> +        int io_sectors;
> +        enum MirrorMethod {
> +            MIRROR_METHOD_COPY,
> +            MIRROR_METHOD_ZERO,
> +            MIRROR_METHOD_DISCARD
> +        } mirror_method = MIRROR_METHOD_COPY;
> +
> +        assert(!(sector_num % sectors_per_chunk));
> +        ret = bdrv_get_block_status_above(source, NULL, sector_num,
> +                                          nb_chunks * sectors_per_chunk,
> +                                          &io_sectors);
> +        if (ret < 0) {
> +            io_sectors = nb_chunks * sectors_per_chunk;
> +        }
> +
> +        io_sectors -= io_sectors % sectors_per_chunk;
> +        if (io_sectors < sectors_per_chunk) {
> +            io_sectors = sectors_per_chunk;
> +        } else if (ret > 0 && !(ret & BDRV_BLOCK_DATA)) {

Why > 0 and not >= 0?

Max

> +            int64_t target_sector_num;
> +            int target_nb_sectors;
> +            bdrv_round_to_clusters(s->target, sector_num, io_sectors,
> +                                   &target_sector_num, &target_nb_sectors);
> +            if (target_sector_num == sector_num &&
> +                target_nb_sectors == io_sectors) {
> +                mirror_method = ret & BDRV_BLOCK_ZERO ?
> +                                    MIRROR_METHOD_ZERO :
> +                                    MIRROR_METHOD_DISCARD;
> +            }
> +        }
> +
> +        switch (mirror_method) {
> +        case MIRROR_METHOD_COPY:
> +            io_sectors = mirror_do_read(s, sector_num, io_sectors);
> +            break;
> +        case MIRROR_METHOD_ZERO:
> +            mirror_do_zero_or_discard(s, sector_num, io_sectors, false);
> +            break;
> +        case MIRROR_METHOD_DISCARD:
> +            mirror_do_zero_or_discard(s, sector_num, io_sectors, true);
> +            break;
> +        default:
> +            abort();
> +        }
> +        assert(io_sectors);
> +        sector_num += io_sectors;
> +        nb_chunks -= io_sectors / sectors_per_chunk;
> +        delay_ns += ratelimit_calculate_delay(&s->limit, io_sectors);
>      }
>      return delay_ns;
>  }
> 



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