Re: __io_file_supports_nowait for regular files

[Christoph Hellwig <hch@lst.de>]

On Thu, Jul 21, 2022 at 12:46:16PM -0600, Jens Axboe wrote:
> Looking a bit deeper at this, FMODE_NOWAIT is about the file.

Yes.

> The nowait
> check for the bdev is about whether the driver honors NOWAIT
> submissions. Any blk-mq driver will be fine, bio based ones probably
> not. You could very well end up blocking off the submit path in that
> case.

But do these submissions even matter for the high level interface?
We'd get -EAGAIN way more often without them (or all the time
for direct I/O), but does that strictly matter for the interface?

Note that for mny file systems (at least btrfs, f2fs and xfs) just
checking s_bdev is not enough any way as they can use multiple block
devices.

I'm also a little confused now that I'm looking more into this,
as iomap only uses REQ_NOWAIT for polled direct I/O to start with.
The legacy direct I/O code uses it for all writes as long as
IOCB_NOWAIT is set, so it seems like only the block device code
really makes extensive and most likely correct use of the
REQ_NOWAIT flag anyway.