Re: [PATCH V4 6/6] loop: add hint for handling aio via IOCB_NOWAIT

[Christoph Hellwig <hch@infradead.org>]

On Mon, Oct 06, 2025 at 10:18:12PM +0800, Ming Lei wrote:
> On Fri, Oct 03, 2025 at 12:06:44AM -0700, Christoph Hellwig wrote:
> > On Sun, Sep 28, 2025 at 09:29:25PM +0800, Ming Lei wrote:
> > > - there isn't any queued blocking async WRITEs, because NOWAIT won't cause
> > > contention with blocking WRITE, which often implies exclusive lock
> > 
> > Isn't this a generic thing we should be doing in core code so that
> > it applies to io_uring I/O as well?
> 
> No.
> 
> It is just policy of using NOWAIT or not, so far:
> 
> - RWF_NOWAIT can be set from preadv/pwritev
> 
> - used for handling io_uring FS read/write
> 
> Even though loop's situation is similar with io-uring, however, both two are
> different subsystem, and there is nothing `core code` for both, more importantly
> it is just one policy: use it or not use it, each subsystem can make its
> own decision based on subsystem internal.

I fail to parse what you say here.  You are encoding special magic
about what underlying file systems do in an upper layer.  I'd much
rather have a flag similar FOP_DIO_PARALLEL_WRITE that makes this
limitation clear rather then opencoding it in the loop driver while
leabing the primary user of RWF_NOWAIT out in the cold.