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

[Christoph Hellwig <hch@infradead.org>]

On Tue, Oct 07, 2025 at 08:15:05PM +0800, Ming Lei wrote:
> NOWAIT is obviously interface provided by FS, here loop just wants to try
> NOWAIT first in block layer dispatch context for avoiding the extra wq
> schedule latency.

Yes.

> But for write on sparse file, trying NOWAIT first may bring extra retry
> cost, that is why the hint is added. It is very coarse, but potential
> regression can be avoided.

And that is absolutely not a property of loop, and loop should not have
to know about.  So this logic needs to be in common code, preferably
triggered by a fs flag.  Note that this isn't about holes - it is about
allocating blocks.  For most file systems filling holes or extending
past i_size is what requires allocating blocks.  But for a out of place
write file systems like btrfs, or zoned xfs we always need to allocate
blocks for now.  But I have work that I need to finish off that allows
for non-blocking block allocation in zoned XFS, at which point you
don't need this.  I think some of this might be true for network file
systems already.

> 
> > rather have a flag similar FOP_DIO_PARALLEL_WRITE that makes this
> > limitation clear rather then opencoding it in the loop driver while
> 
> What is the limitation?

See above.

> > leabing the primary user of RWF_NOWAIT out in the cold.
> 
> FOP_DIO_PARALLEL_WRITE is one static FS feature,

It actually isn't :( I need to move it to be a bit more dynamic on a
per-file basis.

> but here it is FS
> runtime behavior, such as if the write can be blocked because of space
> allocation, so it can't be done by one static flag.

Yes, that's why you want a flag to indicate that a file, or maybe file
operations instance can do non-blocking fill of blocks.  But that's
for the future, for now I just want your logic lifted to common code
and shared with io_uring so that we don't have weird hardcoded
assumptions about file system behavior inside the loop driver.

> io-uring shares nothing with loop in this area, it is just one policy wrt.
> use NOWAIT or not. I don't understand why you insist on covering both
> from FS internal...

It's really about all IOCB_NOWAIT users, io_uring being the prime one,
and the one that we can actually easily write tests for.