Re: [PATCH 1/2] xfs: cache dax_device lookup result

[Darrick J. Wong]

On Thu, Aug 24, 2017 at 05:35:48PM -0700, Dan Williams wrote:
> The ->iomap_begin() operation is a hot path, so cache the
> fs_dax_get_by_host() result to avoid the incurring the hash lookup
> overhead.

Just out of curiosity (and sorry if this has already been discussed to
death and I'm merely ignorant) but I was wondering why the daxdev isn't
simply attached to the block_device?  Is it not a 1:1 mapping?

--D

> 
> Cc: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
> Reported-by: Christoph Hellwig <hch@xxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  fs/xfs/xfs_aops.c  |   24 ++++++++++++++++++++++++
>  fs/xfs/xfs_aops.h  |    1 +
>  fs/xfs/xfs_buf.h   |    1 +
>  fs/xfs/xfs_iomap.c |    9 +--------
>  4 files changed, 27 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 6bf120bb1a17..d16e013e1b8c 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -80,6 +80,30 @@ xfs_find_bdev_for_inode(
>  		return mp->m_ddev_targp->bt_bdev;
>  }
>  
> +struct dax_device *
> +xfs_find_daxdev_for_inode(
> +	struct inode		*inode)
> +{
> +	struct xfs_inode	*ip = XFS_I(inode);
> +	struct xfs_mount	*mp = ip->i_mount;
> +	struct block_device	*bdev;
> +	xfs_buftarg_t		*bt;
> +
> +	if (XFS_IS_REALTIME_INODE(ip))
> +		bt = mp->m_rtdev_targp;
> +	else
> +		bt = mp->m_ddev_targp;
> +
> +	bdev = bt->bt_bdev;
> +	if (!blk_queue_dax(bdev->bd_queue))
> +		return NULL;
> +
> +	if (!bt->bt_daxdev)
> +		bt->bt_daxdev = fs_dax_get_by_host(bdev->bd_disk->disk_name);
> +
> +	return bt->bt_daxdev;
> +}
> +
>  /*
>   * We're now finished for good with this page.  Update the page state via the
>   * associated buffer_heads, paying attention to the start and end offsets that
> diff --git a/fs/xfs/xfs_aops.h b/fs/xfs/xfs_aops.h
> index cc174ec6c2fd..88c85ea63da0 100644
> --- a/fs/xfs/xfs_aops.h
> +++ b/fs/xfs/xfs_aops.h
> @@ -59,5 +59,6 @@ int	xfs_setfilesize(struct xfs_inode *ip, xfs_off_t offset, size_t size);
>  
>  extern void xfs_count_page_state(struct page *, int *, int *);
>  extern struct block_device *xfs_find_bdev_for_inode(struct inode *);
> +extern struct dax_device *xfs_find_daxdev_for_inode(struct inode *);
>  
>  #endif /* __XFS_AOPS_H__ */
> diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
> index 20721261dae5..fc95a02f272a 100644
> --- a/fs/xfs/xfs_buf.h
> +++ b/fs/xfs/xfs_buf.h
> @@ -108,6 +108,7 @@ typedef unsigned int xfs_buf_flags_t;
>  typedef struct xfs_buftarg {
>  	dev_t			bt_dev;
>  	struct block_device	*bt_bdev;
> +	struct dax_device	*bt_daxdev;
>  	struct xfs_mount	*bt_mount;
>  	unsigned int		bt_meta_sectorsize;
>  	size_t			bt_meta_sectormask;
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 813394c62849..3e8b9ec9e802 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -69,6 +69,7 @@ xfs_bmbt_to_iomap(
>  	iomap->offset = XFS_FSB_TO_B(mp, imap->br_startoff);
>  	iomap->length = XFS_FSB_TO_B(mp, imap->br_blockcount);
>  	iomap->bdev = xfs_find_bdev_for_inode(VFS_I(ip));
> +	iomap->dax_dev = xfs_find_daxdev_for_inode(VFS_I(ip));
>  }
>  
>  xfs_extlen_t
> @@ -976,7 +977,6 @@ xfs_file_iomap_begin(
>  	int			nimaps = 1, error = 0;
>  	bool			shared = false, trimmed = false;
>  	unsigned		lockmode;
> -	struct block_device	*bdev;
>  
>  	if (XFS_FORCED_SHUTDOWN(mp))
>  		return -EIO;
> @@ -1087,13 +1087,6 @@ xfs_file_iomap_begin(
>  
>  	xfs_bmbt_to_iomap(ip, iomap, &imap);
>  
> -	/* optionally associate a dax device with the iomap bdev */
> -	bdev = iomap->bdev;
> -	if (blk_queue_dax(bdev->bd_queue))
> -		iomap->dax_dev = fs_dax_get_by_host(bdev->bd_disk->disk_name);
> -	else
> -		iomap->dax_dev = NULL;
> -
>  	if (shared)
>  		iomap->flags |= IOMAP_F_SHARED;
>  	return 0;
>