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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe