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

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH 04/11] xfs: remove the if_ext_max field in struct xfs_ifork
From: Ben Myers <bpm@xxxxxxx>
Date: Tue, 17 Jan 2012 09:16:39 -0600
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20120116224527.GD16581@xxxxxxx>
References: <20111218200003.557507716@xxxxxxxxxxxxxxxxxxxxxx> <20111218200131.321997628@xxxxxxxxxxxxxxxxxxxxxx> <20120106165818.GD6390@xxxxxxx> <20120116224527.GD16581@xxxxxxx>
User-agent: Mutt/1.5.18 (2008-05-17)
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@xxxxxx>
> > > Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
> > 
> > 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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