Re: [PATCH v2 1/1] iomap: propagate nowait to block layer

[Christoph Hellwig <hch@infradead.org>]

On Tue, Mar 04, 2025 at 12:18:07PM +0000, Pavel Begunkov wrote:
>  	    ((dio->flags & IOMAP_DIO_NEED_SYNC) && !use_fua) ||
> -	    ((dio->flags & IOMAP_DIO_WRITE) && pos >= i_size_read(inode)))
> +	    ((dio->flags & IOMAP_DIO_WRITE) && pos >= i_size_read(inode))) {
>  		dio->flags &= ~IOMAP_DIO_CALLER_COMP;
>  
> +		if (!is_sync_kiocb(dio->iocb) &&
> +		    (dio->iocb->ki_flags & IOCB_NOWAIT))
> +			return -EAGAIN;

Black magic without comments explaining it.

> +	if (!is_sync_kiocb(dio->iocb) && (dio->iocb->ki_flags & IOCB_NOWAIT)) {
> +		/*
> +		 * This is nonblocking IO, and we might need to allocate
> +		 * multiple bios. In this case, as we cannot guarantee that
> +		 * one of the sub bios will not fail getting issued FOR NOWAIT
> +		 * and as error results are coalesced across all of them, ask
> +		 * for a retry of this from blocking context.
> +		 */
> +		if (bio_iov_vecs_to_alloc(dio->submit.iter, BIO_MAX_VECS + 1) >
> +					  BIO_MAX_VECS)

This is not very accurate in times of multi-page bvecs and large order
folios all over.

I think you really need to byte the bullet and support for early returns
from the non-blocking bio submission path.