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Date: Mon, 05 Jan 2015 13:14:48 +0100
From: Peter Lieven <pl@kamp.de>
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Subject: Re: [Qemu-devel] [PATCH] block: limited request size in write
 zeroes unsupported path
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To: "Denis V. Lunev" <den-lists@parallels.com>, qemu-devel@nongnu.org
Cc: kwolf@redhat.com, den@openvz.org, famz@redhat.com, stefanha@redhat.com, mreitz@redhat.com

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

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


-- 

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Peter Lieven

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