linux-ext4 - Re: [PATCH 1/2] dax: pass bdev argument to dax_clear

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Message-ID: <20160208051725.GM31407@dastard>
Date:	Mon, 8 Feb 2016 16:17:25 +1100
From:	Dave Chinner <david@...morbit.com>
To:	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 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.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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