linux-kernel - Re: [PATCH 1/2] dax: pass bdev argument to dax_clear

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Date:	Mon, 8 Feb 2016 08:34:43 -0700
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Jan Kara <jack@...e.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-nvdimm@...ts.01.org, xfs@....sgi.com
Subject: Re: [PATCH 1/2] dax: pass bdev argument to dax_clear_blocks()

On Mon, Feb 08, 2016 at 04:17:25PM +1100, Dave Chinner wrote:
> On Sun, Feb 07, 2016 at 06:44:09PM -0700, Ross Zwisler wrote:
> > On Mon, Feb 08, 2016 at 09:03:29AM +1100, Dave Chinner wrote:
> > > On Sun, Feb 07, 2016 at 12:19:12AM -0700, Ross Zwisler wrote:
> > > > dax_clear_blocks() needs a valid struct block_device and previously it was
> > > > using inode->i_sb->s_bdev in all cases.  This is correct for normal inodes
> > > > on mounted ext2, ext4 and XFS filesystems, but is incorrect for DAX raw
> > > > block devices and for XFS real-time devices.
> > > > 
> > > > Instead, have the caller pass in a struct block_device pointer which it
> > > > knows to be correct.
> > > ....
> > > > diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> > > > index 07ef29b..f722ba2 100644
> > > > --- a/fs/xfs/xfs_bmap_util.c
> > > > +++ b/fs/xfs/xfs_bmap_util.c
> > > > @@ -73,9 +73,11 @@ xfs_zero_extent(
> > > >  	xfs_daddr_t	sector = xfs_fsb_to_db(ip, start_fsb);
> > > >  	sector_t	block = XFS_BB_TO_FSBT(mp, sector);
> > > >  	ssize_t		size = XFS_FSB_TO_B(mp, count_fsb);
> > > > +	struct inode	*inode = VFS_I(ip);
> > > >  
> > > >  	if (IS_DAX(VFS_I(ip)))
> > > > -		return dax_clear_blocks(VFS_I(ip), block, size);
> > > > +		return dax_clear_blocks(inode, xfs_find_bdev_for_inode(inode),
> > > > +				block, size);
> > > 
> > > Get rid of the local inode variable and use VFS_I(ip) like the code
> > > originally did. Do not change code that is unrelated to the
> > > modifcation being made, especially when it results in making
> > > the code an inconsistent mess of mixed pointer constructs....
> > 
> > The local 'inode' variable was added to avoid multiple calls for VFS_I() for
> > the same 'ip'.
> 
> My point is you didn't achieve that. The end result of your patch
> is:
> 
> 	struct inode	*inode = VFS_I(ip);
> 
> 	if (IS_DAX(VFS_I(ip)))
> 		return dax_clear_blocks(inode, xfs_find_bdev_for_inode(inode),
> 					block, size);
> 
> So now we have a local variable, but we still have 2 calls to
> VFS_I(ip). i.e. this makes the code harder to read and understand
> than before for no benefit.

*facepalm*  Yep, thanks for the correction.