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

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

14.03.2020 0:56, Eric Blake wrote:
> 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.


All other limits may be 0 too, which means some default.. So, if we want to set the default for max_pwrite_zeroes_no_fallback explicitly, I think we should do it for all other limits too. And it should be separate thing..

> 
>> +    {
>> +        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.
> 


-- 
Best regards,
Vladimir