Re: [PATCH] block: fix -EAGAIN IOPOLL task/vm accounting

[Jens Axboe <axboe@kernel.dk>]

On 8/30/20 12:26 AM, Christoph Hellwig wrote:
> On Sat, Aug 29, 2020 at 10:51:11AM -0600, Jens Axboe wrote:
>> We currently increment the task/vm counts when we first attempt to queue a
>> bio. But this isn't necessarily correct - if the request allocation fails
>> with -EAGAIN, for example, and the caller retries, then we'll over-account
>> by as many retries as are done.
>>
>> This can happen for polled IO, where we cannot wait for requests. Hence
>> retries can get aggressive, if we're running out of requests. If this
>> happens, then watching the IO rates in vmstat are incorrect as they count
>> every issue attempt as successful and hence the stats are inflated by
>> quite a lot potentially.
>>
>> Add a bio flag to know if we've done accounting or not. This prevents
>> the same bio from being accounted potentially many times, when retried.
> 
> Can't the resubmitter just use submit_bio_noacct?  What is the call
> stack here?

The resubmitter is way higher than that. You could potentially have that
done in the block layer, but not higher up.

The use case is async submissions, going through ->read_iter() again.
Or ->write_iter().

-- 
Jens Axboe