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.
