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Date: Mon, 5 Jan 2015 15:28:41 +0300
From: "Denis V. Lunev" <den@openvz.org>
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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>, "Denis V. Lunev" <den-lists@parallels.com>, qemu-devel@nongnu.org
Cc: kwolf@redhat.com, famz@redhat.com, stefanha@redhat.com, mreitz@redhat.com

On 05/01/15 15:14, Peter Lieven wrote:
> On 05.01.2015 12:51, Denis V. Lunev wrote:
>> 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.
>
> bs->bl.max_transfer_length is in sectors.
>
> Peter
>

oops. you are right...