Re: [Qemu-devel] [PATCH 1/2] block: sync bdrv_co_get_block_status_above() with bdrv_is_allocated_above()

From: Roman Kagan <rkagan@virtuozzo.com>
To: "Denis V. Lunev" <den@openvz.org>
Cc: qemu-block@nongnu.org, Kevin Wolf <kwolf@redhat.com>,
	Fam Zheng <famz@redhat.com>, Jeff Cody <jcody@redhat.com>,
	qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>
Subject: Re: [Qemu-devel] [PATCH 1/2] block: sync bdrv_co_get_block_status_above() with bdrv_is_allocated_above()
Date: Mon, 12 Sep 2016 15:41:39 +0300	[thread overview]
Message-ID: <20160912124139.oqgj2fie7oa4cmqb@rkaganb.sw.ru> (raw)
In-Reply-To: <1473424308-19812-2-git-send-email-den@openvz.org>

On Fri, Sep 09, 2016 at 03:31:47PM +0300, Denis V. Lunev wrote:
> They should work very similar, covering same areas if backing store is
> shorter than the image. This change is necessary for the followup patch
> switching to bdrv_get_block_status_above() in mirror to avoid assert
> in check_block.

I wonder why bdrv_is_allocated_above has to be a separate function
rather than a trivial wrapper around bdrv_get_block_status_above() (like
bdrv_is_allocated() is over bdrv_get_block_status())?

> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> CC: Fam Zheng <famz@redhat.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Max Reitz <mreitz@redhat.com>
> CC: Jeff Cody <jcody@redhat.com>
> ---
>  block/io.c | 26 ++++++++++++++++++++------
>  1 file changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/block/io.c b/block/io.c
> index 420944d..0422123 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1745,14 +1745,28 @@ static int64_t coroutine_fn bdrv_co_get_block_status_above(BlockDriverState *bs,
>  
>      assert(bs != base);
>      for (p = bs; p != base; p = backing_bs(p)) {
> -        ret = bdrv_co_get_block_status(p, sector_num, nb_sectors, pnum, file);
> -        if (ret < 0 || ret & BDRV_BLOCK_ALLOCATED) {
> -            break;
> +        int sc;
> +        ret = bdrv_co_get_block_status(p, sector_num, nb_sectors, &sc, file);
> +        if (ret < 0) {
> +            return ret;
> +        } else if (ret & BDRV_BLOCK_ALLOCATED) {
> +            *pnum = sc;
> +            return ret;
> +        }
> +
> +        /*
> +         * [sector_num, nb_sectors] is unallocated on top but intermediate
> +         * might have
> +         *
> +         * [sector_num+x, nr_sectors] allocated.
> +         */
> +        if (nb_sectors > sc &&
> +            (p == bs || sector_num + sc < p->total_sectors)) {
> +            nb_sectors = sc;
>          }
> -        /* [sector_num, pnum] unallocated on this layer, which could be only
> -         * the first part of [sector_num, nb_sectors].  */
> -        nb_sectors = MIN(nb_sectors, *pnum);
>      }
> +
> +    *pnum = nb_sectors;
>      return ret;

IIUC in the chain image->backing_1->backing_2, where size(image) >
size(backing_1) and size(backing_1) < size(backing_2), if the status of
blocks beyond size(backing_1) is requested we'll start falling through
to backing_2.  I'm not certain this is desirable.  (And yes, this is
already the case in bdrv_is_allocated_above).

Roman.