Re: [RFC PATCH v1 3/3] xfs: example of devirtualizing buffered write iomap callbacks

[Christoph Hellwig <hch@lst.de>]

On Wed, Jun 24, 2026 at 07:47:23PM -0700, Joanne Koong wrote:
> This is an example of what removing the indirect call would look like.
> With the inlined iomap_buffered_write_inline() that gets passed
> xfs_buffered_write_iomap_begin() and xfs_buffered_write_iomap_end() as
> constant callbacks, the compiler is able to call the callbacks directly
> and drop the indirect call through struct iomap_ops:

Nice.  Btw, a more critical path would be direct I/O.

Fengnan, maybe you can play around with this series and your direct read
fast path?

> +	/*
> +	 * Call inlined iomap buffered write and pass constant begin/end
> +	 * function pointers so the compiler can devirtualize them and avoid the
> +	 * indirect call through struct iomap_ops.
> +	 */
> +	iter = (struct iomap_iter){
> +		.inode		= inode,
> +		.pos		= iocb->ki_pos,
> +		.len		= iov_iter_count(from),
> +		.flags		= IOMAP_WRITE,
> +	};
> +	if (iocb->ki_flags & IOCB_NOWAIT)
> +		iter.flags |= IOMAP_NOWAIT;
> +	if (iocb->ki_flags & IOCB_DONTCACHE)
> +		iter.flags |= IOMAP_DONTCACHE;

We'll probably want a helper for this initializion boilerplate.
Or make it part of iomap_buffered_write_inline somehow?

> +	ret = iomap_buffered_write_inline(&iter, from,
> +			xfs_buffered_write_iomap_begin,
> +			xfs_buffered_write_iomap_end,

So we're not even doing the _next scheme here and already get
results, nice!

> +	if (ret > 0)
> +		iocb->ki_pos = iter.pos;

Similarly here, it would be great if we could somehow avoid
too much boilerplate.