Re: [PATCH v7 15/15] xfs: Add async buffered write support

[Jens Axboe <axboe@kernel.dk>]

On 7/1/22 8:30 AM, Jens Axboe wrote:
> On 7/1/22 8:19 AM, Jens Axboe wrote:
>> On 6/30/22 10:39 PM, Al Viro wrote:
>>> On Wed, Jun 01, 2022 at 02:01:41PM -0700, Stefan Roesch wrote:
>>>> This adds the async buffered write support to XFS. For async buffered
>>>> write requests, the request will return -EAGAIN if the ilock cannot be
>>>> obtained immediately.
>>>
>>> breaks generic/471...
>>
>> That test case is odd, because it makes some weird assumptions about
>> what RWF_NOWAIT means. Most notably that it makes it mean if we should
>> instantiate blocks or not. Where did those assumed semantics come from?
>> On the read side, we have clearly documented that it should "not wait
>> for data which is not immediately available".
>>
>> Now it is possible that we're returning a spurious -EAGAIN here when we
>> should not be. And that would be a bug imho. I'll dig in and see what's
>> going on.
> 
> This is the timestamp update that needs doing which will now return
> -EAGAIN if IOCB_NOWAIT is set as it may block.
> 
> I do wonder if we should just allow inode time updates with IOCB_NOWAIT,
> even on the io_uring side. Either that, or passed in RWF_NOWAIT
> semantics don't map completely to internal IOCB_NOWAIT semantics. At
> least in terms of what generic/471 is doing, but I'm not sure who came
> up with that and if it's established semantics or just some made up ones
> from whomever wrote that test. I don't think they make any sense, to be
> honest.

Further support that generic/471 is just randomly made up semantics,
it needs to special case btrfs with nocow or you'd get -EAGAIN anyway
for that test.

And it's relying on some random timing to see if this works. I really
think that test case is just hot garbage, and doesn't test anything
meaningful.

-- 
Jens Axboe