linux-kernel - Re: [PATCH v8 6/7] xfs: support CoW in fsdax mode

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Date:   Thu, 2 Sep 2021 09:43:08 +0200
From:   Christoph Hellwig <hch@....de>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     djwong@...nel.org, hch@....de, linux-xfs@...r.kernel.org,
        dan.j.williams@...el.com, david@...morbit.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        nvdimm@...ts.linux.dev, rgoldwyn@...e.de, viro@...iv.linux.org.uk,
        willy@...radead.org
Subject: Re: [PATCH v8 6/7] xfs: support CoW in fsdax mode

On Sun, Aug 29, 2021 at 08:25:16PM +0800, Shiyang Ruan wrote:
> In fsdax mode, WRITE and ZERO on a shared extent need CoW performed.
> After that, new allocated extents needs to be remapped to the file.  Add
> an implementation of ->iomap_end() for dax write ops to do the remapping
> work.

Please split the new dax infrastructure from the XFS changes.

>  static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> -			       int *iomap_errp, const struct iomap_ops *ops)
> +		int *iomap_errp, const struct iomap_ops *ops)
>  {
>  	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
>  	XA_STATE(xas, &mapping->i_pages, vmf->pgoff);
> @@ -1631,7 +1664,7 @@ static bool dax_fault_check_fallback(struct vm_fault *vmf, struct xa_state *xas,
>  }
>  
>  static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
> -			       const struct iomap_ops *ops)
> +		const struct iomap_ops *ops)

These looks like unrelated whitespace changes.

> -static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
> +loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
>  {
>  	const struct iomap *iomap = &iter->iomap;
>  	const struct iomap *srcmap = iomap_iter_srcmap(iter);
> @@ -918,6 +918,7 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
>  
>  	return written;
>  }
> +EXPORT_SYMBOL_GPL(iomap_zero_iter);

I don't see why this would have to be exported.

> +	unsigned 		flags,
> +	struct iomap 		*iomap)
> +{
> +	int			error = 0;
> +	struct xfs_inode	*ip = XFS_I(inode);
> +	bool			cow = xfs_is_cow_inode(ip);

The cow variable is only used once, so I think we can drop it.

> +	const struct iomap_iter *iter =
> +				container_of(iomap, typeof(*iter), iomap);

Please comment this as it is a little unusual.

> +
> +	if (cow) {
> +		if (iter->processed <= 0)
> +			xfs_reflink_cancel_cow_range(ip, pos, length, true);
> +		else
> +			error = xfs_reflink_end_cow(ip, pos, iter->processed);
> +	}
> +	return error ?: iter->processed;

The ->iomap_end convention is to return 0 or a negative error code.
Also i'd much prefer to just spell this out in a normal sequential way:

	if (!xfs_is_cow_inode(ip))
		return 0;

	if (iter->processed <= 0) {
		xfs_reflink_cancel_cow_range(ip, pos, length, true);
		return 0;
	}

	return xfs_reflink_end_cow(ip, pos, iter->processed);

> +static inline int
> +xfs_iomap_zero_range(
> +	struct xfs_inode	*ip,
> +	loff_t			pos,
> +	loff_t			len,
> +	bool			*did_zero)
> +{
> +	struct inode		*inode = VFS_I(ip);
> +
> +	return IS_DAX(inode)
> +			? dax_iomap_zero_range(inode, pos, len, did_zero,
> +					       &xfs_dax_write_iomap_ops)
> +			: iomap_zero_range(inode, pos, len, did_zero,
> +					       &xfs_buffered_write_iomap_ops);
> +}

	if (IS_DAX(inode))
		return dax_iomap_zero_range(inode, pos, len, did_zero,
					    &xfs_dax_write_iomap_ops);
	return iomap_zero_range(inode, pos, len, did_zero,
				&xfs_buffered_write_iomap_ops);

> +static inline int
> +xfs_iomap_truncate_page(
> +	struct xfs_inode	*ip,
> +	loff_t			pos,
> +	bool			*did_zero)
> +{
> +	struct inode		*inode = VFS_I(ip);
> +
> +	return IS_DAX(inode)
> +			? dax_iomap_truncate_page(inode, pos, did_zero,
> +					       &xfs_dax_write_iomap_ops)
> +			: iomap_truncate_page(inode, pos, did_zero,
> +					       &xfs_buffered_write_iomap_ops);
> +}

Same here.