Re: [Qemu-devel] [PATCH 1/8] block: prepare bdrv_co_do_write_zeroes to deal with large bl.max_write_zeroes

["Denis V. Lunev" <den@openvz.org>]

On 05/01/15 10:34, Peter Lieven wrote:
> On 30.12.2014 10:20, Denis V. Lunev wrote:
>> bdrv_co_do_write_zeroes split writes using bl.max_write_zeroes or
>> 16 MiB as a chunk size. This is implemented in this way to tolerate
>> buggy block backends which do not accept too big requests.
>>
>> Though if the bdrv_co_write_zeroes callback is not good enough, we
>> fallback to write data explicitely using bdrv_co_writev and we
>> create buffer to accomodate zeroes inside. The size of this buffer
>> is the size of the chunk. Thus if the underlying layer will have
>> bl.max_write_zeroes high enough, f.e. 4 GiB, the allocation can fail.
>>
>> Actually, there is no need to allocate such a big amount of memory.
>> We could simply allocate 1 MiB buffer and create iovec, which will
>> point to the same memory.
>>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Kevin Wolf <kwolf@redhat.com>
>> CC: Stefan Hajnoczi <stefanha@redhat.com>
>> CC: Peter Lieven <pl@kamp.de>
>> ---
>>   block.c | 35 ++++++++++++++++++++++++-----------
>>   1 file changed, 24 insertions(+), 11 deletions(-)
>>
>> diff --git a/block.c b/block.c
>> index 4165d42..d69c121 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -3173,14 +3173,18 @@ int coroutine_fn 
>> bdrv_co_copy_on_readv(BlockDriverState *bs,
>>    * of 32768 512-byte sectors (16 MiB) per request.
>>    */
>>   #define MAX_WRITE_ZEROES_DEFAULT 32768
>> +/* allocate iovec with zeroes using 1 MiB chunks to avoid to big 
>> allocations */
>> +#define MAX_ZEROES_CHUNK (1024 * 1024)
>>     static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState 
>> *bs,
>>       int64_t sector_num, int nb_sectors, BdrvRequestFlags flags)
>>   {
>>       BlockDriver *drv = bs->drv;
>>       QEMUIOVector qiov;
>> -    struct iovec iov = {0};
>>       int ret = 0;
>> +    void *chunk = NULL;
>> +
>> +    qemu_iovec_init(&qiov, 0);
>>         int max_write_zeroes = bs->bl.max_write_zeroes ?
>>                              bs->bl.max_write_zeroes : 
>> MAX_WRITE_ZEROES_DEFAULT;
>> @@ -3217,27 +3221,35 @@ static int coroutine_fn 
>> bdrv_co_do_write_zeroes(BlockDriverState *bs,
>>           }
>>             if (ret == -ENOTSUP) {
>> +            int64_t num_bytes = (int64_t)num << BDRV_SECTOR_BITS;
>> +            int chunk_size = MIN(MAX_ZEROES_CHUNK, num_bytes);
>> +
>>               /* Fall back to bounce buffer if write zeroes is 
>> unsupported */
>> -            iov.iov_len = num * BDRV_SECTOR_SIZE;
>> -            if (iov.iov_base == NULL) {
>> -                iov.iov_base = qemu_try_blockalign(bs, num * 
>> BDRV_SECTOR_SIZE);
>> -                if (iov.iov_base == NULL) {
>> +            if (chunk == NULL) {
>> +                chunk = qemu_try_blockalign(bs, chunk_size);
>> +                if (chunk == NULL) {
>>                       ret = -ENOMEM;
>>                       goto fail;
>>                   }
>> -                memset(iov.iov_base, 0, num * BDRV_SECTOR_SIZE);
>> +                memset(chunk, 0, chunk_size);
>> +            }
>> +
>> +            while (num_bytes > 0) {
>> +                int to_add = MIN(chunk_size, num_bytes);
>> +                qemu_iovec_add(&qiov, chunk, to_add);
>
> This can and likely will fail for big num_bytes if you exceed IOV_MAX 
> vectors.
>
> I would stick to the old method and limit the num to a reasonable 
> value e.g. MAX_WRITE_ZEROES_DEFAULT.
> This becomes necessary as you set INT_MAX for max_write_zeroes. That 
> hasn't been considered before in
> the original patch.
>
> Peter
>

hmm. You are right, but I think that it would be better to limit iovec size
to 32 and this will solve the problem. Allocation of 32 Mb could be a 
real problem
on loaded system could be a problem.

What do you think on this? May be we could consider 16 as a limit...