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

[Peter Lieven]

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 <address@hidden>
> CC: Kevin Wolf <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> CC: Peter Lieven <address@hidden>
> ---
>   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

> +                num_bytes -= to_add;
>               }
> -            qemu_iovec_init_external(&qiov, &iov, 1);
>   
>               ret = drv->bdrv_co_writev(bs, sector_num, num, &qiov);
>   
>               /* Keep bounce buffer around if it is big enough for all
>                * all future requests.
>                */
> -            if (num < max_write_zeroes) {
> -                qemu_vfree(iov.iov_base);
> -                iov.iov_base = NULL;
> +            if (chunk_size != MAX_ZEROES_CHUNK) {
> +                qemu_vfree(chunk);
> +                chunk = NULL;
>               }
> +            qemu_iovec_reset(&qiov);
>           }
>   
>           sector_num += num;
> @@ -3245,7 +3257,8 @@ static int coroutine_fn 
> bdrv_co_do_write_zeroes(BlockDriverState *bs,
>       }
>   
>   fail:
> -    qemu_vfree(iov.iov_base);
> +    qemu_iovec_destroy(&qiov);
> +    qemu_vfree(chunk);
>       return ret;
>   }
>   


--

Mit freundlichen Grüßen

Peter Lieven

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