Re: [RFC PATCH v1 06/18] xfs: add iomap async buffered write support

[Dave Chinner <david@fromorbit.com>]

On Tue, Apr 26, 2022 at 10:43:23AM -0700, Stefan Roesch wrote:
> This adds the async buffered write support to the iomap layer of XFS. If
> a lock cannot be acquired or additional reads need to be performed, the
> request will return -EAGAIN in case this is an async buffered write request.
> 
> Signed-off-by: Stefan Roesch <shr@fb.com>
> ---
>  fs/xfs/xfs_iomap.c | 33 +++++++++++++++++++++++++++++++--
>  1 file changed, 31 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index e552ce541ec2..80b6c48e88af 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -881,18 +881,28 @@ xfs_buffered_write_iomap_begin(
>  	bool			eof = false, cow_eof = false, shared = false;
>  	int			allocfork = XFS_DATA_FORK;
>  	int			error = 0;
> +	bool			no_wait = (flags & IOMAP_NOWAIT);
>  
>  	if (xfs_is_shutdown(mp))
>  		return -EIO;
>  
>  	/* we can't use delayed allocations when using extent size hints */
> -	if (xfs_get_extsz_hint(ip))
> +	if (xfs_get_extsz_hint(ip)) {
> +		if (no_wait)
> +			return -EAGAIN;
> +
>  		return xfs_direct_write_iomap_begin(inode, offset, count,
>  				flags, iomap, srcmap);

Why can't we do IOMAP_NOWAIT allocation here?
xfs_direct_write_iomap_begin() supports IOMAP_NOWAIT semantics just
fine - that's what the direct IO path uses...

> +	}
>  
>  	ASSERT(!XFS_IS_REALTIME_INODE(ip));
>  
> -	xfs_ilock(ip, XFS_ILOCK_EXCL);
> +	if (no_wait) {
> +		if (!xfs_ilock_nowait(ip, XFS_ILOCK_EXCL))
> +			return -EAGAIN;
> +	} else {
> +		xfs_ilock(ip, XFS_ILOCK_EXCL);
> +	}

handled by xfs_ilock_for_iomap().
>  
>  	if (XFS_IS_CORRUPT(mp, !xfs_ifork_has_extents(&ip->i_df)) ||
>  	    XFS_TEST_ERROR(false, mp, XFS_ERRTAG_BMAPIFORMAT)) {
> @@ -902,6 +912,11 @@ xfs_buffered_write_iomap_begin(
>  
>  	XFS_STATS_INC(mp, xs_blk_mapw);
>  
> +	if (no_wait && xfs_need_iread_extents(XFS_IFORK_PTR(ip, XFS_DATA_FORK))) {
> +		error = -EAGAIN;
> +		goto out_unlock;
> +	}
> +

Also handled by xfs_ilock_for_iomap().

>  	error = xfs_iread_extents(NULL, ip, XFS_DATA_FORK);
>  	if (error)
>  		goto out_unlock;
> @@ -933,6 +948,10 @@ xfs_buffered_write_iomap_begin(
>  	if (xfs_is_cow_inode(ip)) {
>  		if (!ip->i_cowfp) {
>  			ASSERT(!xfs_is_reflink_inode(ip));
> +			if (no_wait) {
> +				error = -EAGAIN;
> +				goto out_unlock;
> +			}
>  			xfs_ifork_init_cow(ip);
>  		}

Also handled by xfs_ilock_for_iomap().

>  		cow_eof = !xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb,
> @@ -956,6 +975,11 @@ xfs_buffered_write_iomap_begin(
>  			goto found_imap;
>  		}
>  
> +		if (no_wait) {
> +			error = -EAGAIN;
> +			goto out_unlock;
> +		}
> +

Won't get here because this is COW path, and xfs_ilock_for_iomap()
handles returning -EAGAIN for that case.

>  		xfs_trim_extent(&imap, offset_fsb, end_fsb - offset_fsb);
>  
>  		/* Trim the mapping to the nearest shared extent boundary. */
> @@ -993,6 +1017,11 @@ xfs_buffered_write_iomap_begin(
>  			allocfork = XFS_COW_FORK;
>  	}
>  
> +	if (no_wait) {
> +		error = -EAGAIN;
> +		goto out_unlock;
> +	}

Why? Delayed allocation doesn't block...

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com