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

[Ben Myers <bpm@sgi.com>]

On Mon, Jan 16, 2012 at 04:45:27PM -0600, Ben Myers wrote:
> 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?

Here is what I propose to use here:

@@ -5322,7 +5319,8 @@ xfs_bunmapi(
                 */
                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 &&
+                   XFS_IFORK_NEXTENTS(ip, whichfork) >= /* Note the >= */
+                       XFS_IFORK_MAXEXT(ip, whichfork) &&
                    del.br_startoff > got.br_startoff &&
                    del.br_startoff + del.br_blockcount <
                    got.br_startoff + got.br_blockcount) {

-Ben

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