Re: [PATCH 3/6] xfs: simplify xfs_dax_fault

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Jun 19, 2024 at 01:53:53PM +0200, Christoph Hellwig wrote:
> Replace the separate stub with an IS_ENABLED check, and take the call to
> dax_finish_sync_fault into xfs_dax_fault instead of leaving it in the
> caller.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_file.c | 34 +++++++++++++---------------------
>  1 file changed, 13 insertions(+), 21 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 74c2c8d253e69b..8aab2f66fe016f 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1251,31 +1251,27 @@ xfs_file_llseek(
>  	return vfs_setpos(file, offset, inode->i_sb->s_maxbytes);
>  }
>  
> -#ifdef CONFIG_FS_DAX
>  static inline vm_fault_t
>  xfs_dax_fault(
>  	struct vm_fault		*vmf,
>  	unsigned int		order,
> -	bool			write_fault,
> -	pfn_t			*pfn)
> +	bool			write_fault)
>  {
> -	return dax_iomap_fault(vmf, order, pfn, NULL,
> +	vm_fault_t		ret;
> +	pfn_t			pfn;
> +
> +	if (!IS_ENABLED(CONFIG_FS_DAX)) {
> +		ASSERT(0);
> +		return VM_FAULT_SIGBUS;

Does this actually work if FS_DAX=n?  AFAICT there's no !DAX stub for
dax_iomap_fault, so won't that cause a linker error?

> +	}
> +	ret = dax_iomap_fault(vmf, order, &pfn, NULL,
>  			(write_fault && !vmf->cow_page) ?
>  				&xfs_dax_write_iomap_ops :
>  				&xfs_read_iomap_ops);
> +	if (ret & VM_FAULT_NEEDDSYNC)
> +		ret = dax_finish_sync_fault(vmf, order, pfn);
> +	return ret;

I /almost/ wondered if these ought to be separate helpers for read and
write faults, but then I realized the (write && cow_page) case is a
"read" and ... lol.  So the only question I have is about linker errors.

--D

>  }
> -#else
> -static inline vm_fault_t
> -xfs_dax_fault(
> -	struct vm_fault		*vmf,
> -	unsigned int		order,
> -	bool			write_fault,
> -	pfn_t			*pfn)
> -{
> -	ASSERT(0);
> -	return VM_FAULT_SIGBUS;
> -}
> -#endif
>  
>  /*
>   * Locking for serialisation of IO during page faults. This results in a lock
> @@ -1309,11 +1305,7 @@ __xfs_filemap_fault(
>  		lock_mode = xfs_ilock_for_write_fault(XFS_I(inode));
>  
>  	if (IS_DAX(inode)) {
> -		pfn_t pfn;
> -
> -		ret = xfs_dax_fault(vmf, order, write_fault, &pfn);
> -		if (ret & VM_FAULT_NEEDDSYNC)
> -			ret = dax_finish_sync_fault(vmf, order, pfn);
> +		ret = xfs_dax_fault(vmf, order, write_fault);
>  	} else if (write_fault) {
>  		ret = iomap_page_mkwrite(vmf, &xfs_page_mkwrite_iomap_ops);
>  	} else {
> -- 
> 2.43.0
> 
>