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Date: Mon, 5 Jan 2015 14:51:14 +0300
From: "Denis V. Lunev" <den-lists@parallels.com>
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Subject: Re: [Qemu-devel] [PATCH] block: limited request size in write
 zeroes unsupported path
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To: Peter Lieven <pl@kamp.de>, qemu-devel@nongnu.org
Cc: kwolf@redhat.com, den@openvz.org, famz@redhat.com, stefanha@redhat.com, mreitz@redhat.com

On 05/01/15 14:29, Peter Lieven wrote:
> If bs->bl.max_write_zeroes is large and we end up in the unsupported
> path we might allocate a lot of memory for the iovector and/or even
> generate an oversized requests.
>
> Fix this by limiting the request by the minimum of the reported
> maximum transfer size or 16MB (32768 sectors).
>
> Reported-by: Denis V. Lunev <den@openvz.org>
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>   block.c |    5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/block.c b/block.c
> index a612594..8009478 100644
> --- a/block.c
> +++ b/block.c
> @@ -3203,6 +3203,9 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
>
>           if (ret == -ENOTSUP) {
>               /* Fall back to bounce buffer if write zeroes is unsupported */
> +            int max_xfer_len = MIN_NON_ZERO(bs->bl.max_transfer_length,
> +                                            MAX_WRITE_ZEROES_DEFAULT);
> +            num = MIN(num, max_xfer_len);
>               iov.iov_len = num * BDRV_SECTOR_SIZE;
>               if (iov.iov_base == NULL) {
>                   iov.iov_base = qemu_try_blockalign(bs, num * BDRV_SECTOR_SIZE);
> @@ -3219,7 +3222,7 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
>               /* Keep bounce buffer around if it is big enough for all
>                * all future requests.
>                */
> -            if (num < max_write_zeroes) {
> +            if (num < max_xfer_len) {
>                   qemu_vfree(iov.iov_base);
>                   iov.iov_base = NULL;
>               }
>
this is not going to work IMHO. num is the number in sectors.
max_xfer_len is in bytes.

I will send my updated version using your approach in a
couple of minutes. Would like to test it a bit.