Re: [PATCH v11 09/13] copy-on-read: skip non-guest reads if no copy needed

[Max Reitz]

On 14.10.20 17:22, Vladimir Sementsov-Ogievskiy wrote:
> 14.10.2020 15:51, Max Reitz wrote:
>> On 12.10.20 19:43, Andrey Shinkevich wrote:
>>> If the flag BDRV_REQ_PREFETCH was set, pass it further to the
>>> COR-driver to skip unneeded reading. It can be taken into account for
>>> the COR-algorithms optimization. That check is being made during the
>>> block stream job by the moment.
>>>
>>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
>>> ---
>>>   block/copy-on-read.c | 13 +++++++++----
>>>   block/io.c           |  3 ++-
>>>   2 files changed, 11 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/block/copy-on-read.c b/block/copy-on-read.c
>>> index b136895..278a11a 100644
>>> --- a/block/copy-on-read.c
>>> +++ b/block/copy-on-read.c
>>> @@ -148,10 +148,15 @@ static int coroutine_fn
>>> cor_co_preadv_part(BlockDriverState *bs,
>>>               }
>>>           }
>>>   -        ret = bdrv_co_preadv_part(bs->file, offset, n, qiov,
>>> qiov_offset,
>>> -                                  local_flags);
>>> -        if (ret < 0) {
>>> -            return ret;
>>> +        if (!!(flags & BDRV_REQ_PREFETCH) &
>>
>> How about dropping the double negation and using a logical && instead of
>> the binary &?
>>
>>> +            !(local_flags & BDRV_REQ_COPY_ON_READ)) {
>>> +            /* Skip non-guest reads if no copy needed */
>>> +        } else {
>>
>> Hm.  I would have just written the negated form
>>
>> (!(flags & BDRV_REQ_PREFETCH) || (local_flags & BDRV_REQ_COPY_ON_READ))
>>
>> and put the “skip” comment above that condition.
>>
>> (Since local_flags is initialized to flags, it can be written as a
>> single comparison, but that’s a matter of taste and I’m not going to
>> recommend either over the other:
>>
>> ((local_flags & (BDRV_REQ_PREFETCH | BDRV_REQ_COPY_ON_READ)) !=
>> BDRV_REQ_PREFETCH)
>>
>> )
>>
>>> +            ret = bdrv_co_preadv_part(bs->file, offset, n, qiov,
>>> qiov_offset,
>>> +                                      local_flags);
>>> +            if (ret < 0) {
>>> +                return ret;
>>> +            }
>>>           }
>>>             offset += n;
>>> diff --git a/block/io.c b/block/io.c
>>> index 11df188..bff1808 100644
>>> --- a/block/io.c
>>> +++ b/block/io.c
>>> @@ -1512,7 +1512,8 @@ static int coroutine_fn
>>> bdrv_aligned_preadv(BdrvChild *child,
>>>         max_bytes = ROUND_UP(MAX(0, total_bytes - offset), align);
>>>       if (bytes <= max_bytes && bytes <= max_transfer) {
>>> -        ret = bdrv_driver_preadv(bs, offset, bytes, qiov,
>>> qiov_offset, 0);
>>> +        ret = bdrv_driver_preadv(bs, offset, bytes, qiov, qiov_offset,
>>> +                                 flags & bs->supported_read_flags);
> 
> 
> When BDRV_REQ_PREFETCH is passed, qiov may be (and generally should be)
> NULL. This means, that we can't just drop the flag when call the driver
> that doesn't support it.

True. :/

> Actually, if driver doesn't support the PREFETCH flag we should do nothing.

Hm.  But at least in the case of COR, PREFETCH is not something that can
be optimized to be a no-op (unless the COR is a no-op); it still denotes
a command that must be executed.

So if we can’t pass it to the driver, I don’t think we should do
nothing, but to return an error.  Or maybe we could even assert that it
isn’t set for drivers that don’t support it, because at least right now
such a case would just be a bug.

>> Ah, OK.  I see.  I expected this to be a separate patch.  I still wonder
>> why it isn’t.
>>
> 
> 
> Could it be part of patch 07? I mean introduce new field
> supported_read_flags and handle it in generic code in one patch, prior
> to implementing support for it in COR driver.

Sure.

Max

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