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

[Jens Axboe <axboe@kernel.dk>]

On 8/31/20 8:12 AM, Christoph Hellwig wrote:
> On Mon, Aug 31, 2020 at 08:02:43AM -0600, Jens Axboe wrote:
>>>> The use case is async submissions, going through ->read_iter() again.
>>>> Or ->write_iter().
>>>
>>> But how does a bio flag help there?  If we go through the file ops
>>> again the next submission will be a new bio structure.
>>
>> Yeah the patch is garbage, can't work. The previous suggestion is here:
>>
>> https://lore.kernel.org/linux-block/395b4c19-cc80-eebb-f6ab-04687110c84a@kernel.dk/T/
>>
>> which isn't super pretty either, but at least it works. Not sure there's
>> a better solution, outside of marking the iocb as retry and then
>> carrying that flag forward for the bio as well. And that seems a bit
>> much for this case.
> 
> We'll still need a flag with the above to skip the submit_bio_noacct
> bios.  But I think it is the right way to go.  Eventually we'll also
> need to push the accounting down into the individual bio based drivers.

For the iocb propagation, we'd really need the caller to mark the iocb
as IOCB_ACCOUNTED (or whatever) if BIO_ACCOUNTED is set, since we can't
do that further down the stack as we really don't know if we hit -EAGAIN
before or after the bio was accounted... Which kind of sucks, as it'll
be hard to contain in a generic fashion.

-- 
Jens Axboe