Re: [PATCH 04/11] xfs: remove the if_ext_max field in struct xfs_ifork

From: Ben Myers <bpm@sgi.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: xfs@oss.sgi.com
Subject: Re: [PATCH 04/11] xfs: remove the if_ext_max field in struct xfs_ifork
Date: Mon, 16 Jan 2012 16:45:27 -0600	[thread overview]
Message-ID: <20120116224527.GD16581@sgi.com> (raw)
In-Reply-To: <20120106165818.GD6390@sgi.com>

Hey Christoph,

On Fri, Jan 06, 2012 at 10:58:18AM -0600, Ben Myers wrote:
> On Sun, Dec 18, 2011 at 03:00:07PM -0500, Christoph Hellwig wrote:
> > We spent a lot of effort to maintain this field, but it always equalts to the
> 								 equals the
> > fork size divided by the constant size of an extent.  The prime use of it is
> > to assert that the two stay in sync.  Just divide the fork size by the extent
> > size in the few places that we actually use it and remove the overhead
> > of maintaining it.  Also introduce a few helpers to consolidate the places
> > where we actually care about the value.
> > 
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > Reviewed-by: Dave Chinner <dchinner@redhat.com>
> 
> After reviewing this patch it's not crystal clear to me why we were
> putting all that effort into keeping this counter uptodate on the inode
> instead of using helpers like you've implemented.  Maybe a question of
> integer division as Dave suggested.  This is a nice improvement.
> 
> > Index: xfs/fs/xfs/xfs_bmap.c
> > ===================================================================
> > --- xfs.orig/fs/xfs/xfs_bmap.c	2011-12-12 10:33:55.748696870 -0800
> > +++ xfs/fs/xfs/xfs_bmap.c	2011-12-14 05:15:20.612373687 -0800
> > @@ -249,7 +249,27 @@ xfs_bmbt_lookup_ge(
> >  }
> >  
> >  /*
> > -* Update the record referred to by cur to the value given
> > + * Check if the inode needs to be converted to btree format.
> > + */
> > +static inline bool xfs_bmap_needs_btree(struct xfs_inode *ip, int whichfork)
> > +{
> > +	return XFS_IFORK_FORMAT(ip, whichfork) == XFS_DINODE_FMT_EXTENTS &&
> > +		XFS_IFORK_NEXTENTS(ip, whichfork) >
> > +			XFS_IFORK_MAXEXT(ip, whichfork);
> > +}
> > +
> > +/*
> > + * Check if the inode should be converted to extent format.
> > + */
> > +static inline bool xfs_bmap_wants_extents(struct xfs_inode *ip, int whichfork)
> > +{
> > +	return XFS_IFORK_FORMAT(ip, whichfork) == XFS_DINODE_FMT_BTREE &&
> > +		XFS_IFORK_NEXTENTS(ip, whichfork) <=
> > +			XFS_IFORK_MAXEXT(ip, whichfork);
> > +}
> 
> The logic in these two appears to be equivalent to the code you've
> replaced in all but one case...
> 
> ...
> 
> > @@ -5321,8 +5318,7 @@ xfs_bunmapi(
> >  		 * will be dirty.
> >  		 */
> >  		if (!wasdel && xfs_trans_get_block_res(tp) == 0 &&
> > -		    XFS_IFORK_FORMAT(ip, whichfork) == XFS_DINODE_FMT_EXTENTS &&
> > -		    XFS_IFORK_NEXTENTS(ip, whichfork) >= ifp->if_ext_max &&
> 						      ^^ 
> All other tests for this were:
> XFS_IFORK_NEXTENTS(ip, whichfork) > ifp->if_ext_max
> 
> Did you just fix a lurking off-by-one or insert one?
> 
> xfs_bmap_needs_btree needs ip->i_d.di_nextents to have been incremented 
> already in order to detect that we need to convert to btree format.  In
> this case we haven't done that yet and are checking to see if doing so
> would require conversion to btree format... 
> 
> Looks to me like we can't use xfs_bmap_needs_btree here and should use
> the old logic.  Right?

HCH, I have a question for you here that I feel needs to be resolved.
Can you take a look?

Thanks,
	Ben

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