linux-kernel - Re: [PATCH v4 7/7] fs/xfs: Add dedupe support for fsdax

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Message-ID: <20210408230451.GD3957620@magnolia>
Date:   Thu, 8 Apr 2021 16:04:51 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-fsdevel@...r.kernel.org,
        darrick.wong@...cle.com, dan.j.williams@...el.com,
        willy@...radead.org, jack@...e.cz, viro@...iv.linux.org.uk,
        linux-btrfs@...r.kernel.org, david@...morbit.com, hch@....de,
        rgoldwyn@...e.de
Subject: Re: [PATCH v4 7/7] fs/xfs: Add dedupe support for fsdax

On Thu, Apr 08, 2021 at 08:04:32PM +0800, Shiyang Ruan wrote:
> Add xfs_break_two_dax_layouts() to break layout for tow dax files.  Then
> call compare range function only when files are both DAX or not.
> 
> Signed-off-by: Shiyang Ruan <ruansy.fnst@...itsu.com>
> ---
>  fs/xfs/xfs_file.c    | 20 ++++++++++++++++++++
>  fs/xfs/xfs_inode.c   |  8 +++++++-
>  fs/xfs/xfs_inode.h   |  1 +
>  fs/xfs/xfs_reflink.c |  5 +++--
>  4 files changed, 31 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 5795d5d6f869..1fd457167c12 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -842,6 +842,26 @@ xfs_break_dax_layouts(
>  			0, 0, xfs_wait_dax_page(inode));
>  }
>  
> +int
> +xfs_break_two_dax_layouts(
> +	struct inode		*src,
> +	struct inode		*dest)
> +{
> +	int			error;
> +	bool			retry = false;
> +
> +retry:
> +	error = xfs_break_dax_layouts(src, &retry);
> +	if (error || retry)
> +		goto retry;
> +
> +	error = xfs_break_dax_layouts(dest, &retry);
> +	if (error || retry)
> +		goto retry;
> +
> +	return error;
> +}
> +
>  int
>  xfs_break_layouts(
>  	struct inode		*inode,
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index f93370bd7b1e..c01786917eef 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -3713,8 +3713,10 @@ xfs_ilock2_io_mmap(
>  	struct xfs_inode	*ip2)
>  {
>  	int			ret;
> +	struct inode		*inode1 = VFS_I(ip1);
> +	struct inode		*inode2 = VFS_I(ip2);
>  
> -	ret = xfs_iolock_two_inodes_and_break_layout(VFS_I(ip1), VFS_I(ip2));
> +	ret = xfs_iolock_two_inodes_and_break_layout(inode1, inode2);
>  	if (ret)
>  		return ret;
>  	if (ip1 == ip2)
> @@ -3722,6 +3724,10 @@ xfs_ilock2_io_mmap(
>  	else
>  		xfs_lock_two_inodes(ip1, XFS_MMAPLOCK_EXCL,
>  				    ip2, XFS_MMAPLOCK_EXCL);
> +
> +	if (IS_DAX(inode1) && IS_DAX(inode2))
> +		ret = xfs_break_two_dax_layouts(inode1, inode2);

This is wrong on many levels.

The first problem is that xfs_break_two_dax_layouts calls
xfs_break_dax_layouts twice even if inode1 == inode2, which is
unnecessary.

The second problem is that xfs_break_dax_layouts can cycle the MMAPLOCK
on the inode that it's processing.  Since there are two inodes in play
here, you must be /very/ careful about maintaining correct locking order,
which for the MMAPLOCK is increasing order of xfs_inode.i_ino.  If you
drop the MMAPLOCK for the lower-numbered inode for any reason, you have
to drop both MMAPLOCKs and try again.

In other words, you have to replace all that nice MMAPLOCK code with a
new xfs_mmaplock_two_inodes_and_break_dax_layouts function that is
structured similarly to what xfs_iolock_two_inodes_and_break_layout
does for the IOLOCK and PNFS layouts.

> +
>  	return 0;
>  }
>  
> diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> index 88ee4c3930ae..5ef21924dddc 100644
> --- a/fs/xfs/xfs_inode.h
> +++ b/fs/xfs/xfs_inode.h
> @@ -435,6 +435,7 @@ enum xfs_prealloc_flags {
>  
>  int	xfs_update_prealloc_flags(struct xfs_inode *ip,
>  				  enum xfs_prealloc_flags flags);
> +int	xfs_break_two_dax_layouts(struct inode *inode1, struct inode *inode2);
>  int	xfs_break_layouts(struct inode *inode, uint *iolock,
>  		enum layout_break_reason reason);
>  
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index a4cd6e8a7aa0..4426bcc8a985 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -29,6 +29,7 @@
>  #include "xfs_iomap.h"
>  #include "xfs_sb.h"
>  #include "xfs_ag_resv.h"
> +#include <linux/dax.h>

Why is this necessary?

--D

>  
>  /*
>   * Copy on Write of Shared Blocks
> @@ -1324,8 +1325,8 @@ xfs_reflink_remap_prep(
>  	if (XFS_IS_REALTIME_INODE(src) || XFS_IS_REALTIME_INODE(dest))
>  		goto out_unlock;
>  
> -	/* Don't share DAX file data for now. */
> -	if (IS_DAX(inode_in) || IS_DAX(inode_out))
> +	/* Don't share DAX file data with non-DAX file. */
> +	if (IS_DAX(inode_in) != IS_DAX(inode_out))
>  		goto out_unlock;
>  
>  	if (!IS_DAX(inode_in))
> -- 
> 2.31.0
> 
> 
>