Re: [PATCH 5/5] block/io: auto-no-fallback for write-zeroes

[Eric Blake <eblake@redhat.com>]

On 3/2/20 4:05 AM, Vladimir Sementsov-Ogievskiy wrote:
> NBD driver may has max_pwrite_zeroes but doesn't has
> max_pwrite_zeroes_no_fallback limit. This means, that (when
> BDRV_REQ_NO_FALLBACK is supported) it is beneficial to try send request
> with BDRV_REQ_NO_FALLBACK instead of splitting the request accordingly
> to max_pwrite_zeroes.
> 
> If failed, fallback to old behavior.

Grammar:

When BDRV_REQ_NO_FALLBACK is supported, the NBD driver supports a larger 
request size.  Add code to try large zero requests with a NO_FALLBACK 
request prior to having to split a request into chunks according to 
max_pwrite_zeroes.

> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   block/io.c | 30 ++++++++++++++++++++++++++----
>   1 file changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/block/io.c b/block/io.c
> index c64566b4cf..48d71b0883 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1752,17 +1752,28 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
>       int head = 0;
>       int tail = 0;
>   
> -    int max_write_zeroes = MIN_NON_ZERO((flags & BDRV_REQ_NO_FALLBACK) ?
> -                                        bs->bl.max_pwrite_zeroes_no_fallback :
> -                                        bs->bl.max_pwrite_zeroes, INT_MAX);
> +    int max_write_zeroes;
>       int alignment = MAX(bs->bl.pwrite_zeroes_alignment,
>                           bs->bl.request_alignment);
>       int max_transfer = MIN_NON_ZERO(bs->bl.max_transfer, MAX_BOUNCE_BUFFER);
> +    bool auto_no_fallback;
>   
>       if (!drv) {
>           return -ENOMEDIUM;
>       }
>   
> +    if (!(flags & BDRV_REQ_NO_FALLBACK) &&
> +        (bs->supported_zero_flags & BDRV_REQ_NO_FALLBACK) &&
> +        bs->bl.max_pwrite_zeroes &&
> +        bs->bl.max_pwrite_zeroes < bytes &&
> +        (bs->bl.max_pwrite_zeroes < bs->bl.max_pwrite_zeroes_no_fallback ||
> +         bs->bl.max_pwrite_zeroes_no_fallback == 0))

Why are we letting max_pwrite_zeroes_no_fallback ever be 0?  It might be 
more convenient if it is always guaranteed to be >= max_pwrite_zeroes by 
the block layer.

> +    {
> +        assert(drv->bdrv_co_pwrite_zeroes);
> +        flags |= BDRV_REQ_NO_FALLBACK;
> +        auto_no_fallback = true;
> +    }
> +
>       if ((flags & ~bs->supported_zero_flags) & BDRV_REQ_NO_FALLBACK) {
>           return -ENOTSUP;
>       }
> @@ -1770,7 +1781,11 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
>       assert(alignment % bs->bl.request_alignment == 0);
>       head = offset % alignment;
>       tail = (offset + bytes) % alignment;
> -    max_write_zeroes = QEMU_ALIGN_DOWN(max_write_zeroes, alignment);
> +    max_write_zeroes =
> +        QEMU_ALIGN_DOWN(MIN_NON_ZERO((flags & BDRV_REQ_NO_FALLBACK) ?
> +                                     bs->bl.max_pwrite_zeroes_no_fallback :
> +                                     bs->bl.max_pwrite_zeroes, INT_MAX),
> +                        alignment);
>       assert(max_write_zeroes >= bs->bl.request_alignment);
>   
>       while (bytes > 0 && !ret) {
> @@ -1801,6 +1816,13 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
>           if (drv->bdrv_co_pwrite_zeroes) {
>               ret = drv->bdrv_co_pwrite_zeroes(bs, offset, num,
>                                                flags & bs->supported_zero_flags);
> +            if (ret == -ENOTSUP && auto_no_fallback) {
> +                flags &= ~BDRV_REQ_NO_FALLBACK;
> +                max_write_zeroes =
> +                    QEMU_ALIGN_DOWN(MIN_NON_ZERO(bs->bl.max_pwrite_zeroes,
> +                                                 INT_MAX), alignment);
> +                continue;
> +            }
>               if (ret != -ENOTSUP && (flags & BDRV_REQ_FUA) &&
>                   !(bs->supported_zero_flags & BDRV_REQ_FUA)) {
>                   need_flush = true;
> 

Otherwise makes sense.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org