Re: Possible bug for ring-mapped provided buffer

[Hao Xu <hao.xu@linux.dev>]

On 6/9/22 18:06, Jens Axboe wrote:
> On 6/9/22 1:53 AM, Hao Xu wrote:
>> Hi all,
>> I haven't done tests to demonstrate it. It is for partial io case, we
>> don't consume/release the buffer before arm_poll in ring-mapped mode.
>> But seems we should? Otherwise ring head isn't moved and other requests
>> may take that buffer. What do I miss?
> 
> On vacation this week, so can't take a look at the code. But the
> principle is precisely not to consume the buffer if we arm poll, because
> then the next one can grab it instead. We don't want to consume a buffer
> over poll, as that defeats the purpose of a provided buffer. It should
> be grabbed and consumed only if we can use it right now.
> 
> Hence the way it should work is that we DON'T consume the buffer in this
> case, and that someone else can just use it. At the same time, we should
> ensure that we grab a NEW buffer for this case, whenever the poll

If we grab a new buffer for it, then we have to copy the data since we
have done partial io...this also defeats the purpose of this feature.
What the legacy provided buffer mode do in this case is just
keep/consume that buffer. So I'd think we should keep the consistency.
But yes, there may be a better way.

> triggers and we can retry the IO. As mentioned I can't check the code
> right now, but perhaps you can take a look.
>