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

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Date:   Thu, 2 Sep 2021 08:33:09 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Shiyang Ruan <ruansy.fnst@...itsu.com>, 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 Thu, Sep 02, 2021 at 09:43:08AM +0200, Christoph Hellwig wrote:
> 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);

Seeing as written either contains iter->processed if it's positive, or
zero if nothing got written or there were errors, I wonder why this
isn't just:

	if (!xfs_is_cow_inode(ip));
		return 0;

	if (!written) {
		xfs_reflink_cancel_cow_range(ip, pos, length, true);
		return 0;
	}

	return xfs_reflink_end_cow(ip, pos, written);

? (He says while cleaning up trying to leave for vacation, pardon me
if this comment is totally boneheaded...)

--D

> > +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.