Re: [PATCH] xfs: coalesce contiguous unwritten suffix into iomap for NOWAIT writes

[Pankaj Raghav (Samsung)]

On Fri, May 29, 2026 at 03:11:00PM -0400, Brandon Allard wrote:
> In append-only direct-IO workloads with fallocate(with KEEP_SIZE +
> ZERO_RANGE) writes can span the written/unwritten boundary of the
> preallocated extent. This is from callers needing to overwrite the
> previously written block if the last write didn't completely fill it.
> 
> Prior to commit 883a790a8440 ("xfs: don't allow NOWAIT DIO across extent
> boundaries") these writes succeeded by issuing two bios: one against the

Also add this info in as Fixes tag.

> NORM head, one against the UNWRITTEN tail. That commit closed off the
> two-bio path to prevent NOWAIT callers from observing short writes when
> one bio succeeded and another returned -EAGAIN. The side effect is that
> the workload above now returns -EAGAIN on the spanning write.
> 
>  static bool
>  xfs_bmap_hw_atomic_write_possible(
>  	struct xfs_inode	*ip,
> @@ -961,7 +996,14 @@ xfs_direct_write_iomap_begin(
>  	 */
>  	if (flags & (IOMAP_NOWAIT | IOMAP_OVERWRITE_ONLY)) {

I am wondering if adding this imap_extend_into_unwritten() inside
(IOMAP_NOWAIT | IOMAP_OVERWRITE_ONLY) will have some side effect in the 
next check for IOMAP_OVERWRITE_ONLY [1]:
	if (flags & IOMAP_OVERWRITE_ONLY) {
		error = -EAGAIN;
		if (imap.br_state != XFS_EXT_NORM &&
	            ((offset | length) & mp->m_blockmask))
			goto out_unlock;
	}

We are changing the br_state to NORM->UNWRITTEN. Should you guard the
imap_extend_into_unwritten() only for NOWAIT case? Did you also test
this scenario?

>  		error = -EAGAIN;
> -		if (!imap_spans_range(&imap, offset_fsb, end_fsb))
> +		/*
> +		 * Sequential append workloads with adaptive preallocation can
> +		 * span the NORM->UNWRITTEN boundary that unwritten conversion
> +		 * leaves behind. Try to coalesce the next contiguous unwritten record
> +		 * into imap before returning -EAGAIN.
> +		 */
> +		if (!imap_spans_range(&imap, offset_fsb, end_fsb) &&
> +		    !imap_extend_into_unwritten(ip, &imap, end_fsb))
>  			goto out_unlock;
>  	}
>  
> -- 
> 2.48.1
> 

[1] https://elixir.bootlin.com/linux/v7.0.11/source/fs/xfs/xfs_iomap.c#L974

-- 
Pankaj