Re: [PATCH 4/4] xfs: use separate btree cursor slab for each btree type

[Dave Chinner <david@fromorbit.com>]

On Mon, Sep 27, 2021 at 11:21:22AM -0700, Darrick J. Wong wrote:
> On Sun, Sep 26, 2021 at 10:47:21AM +1000, Dave Chinner wrote:
> > On Thu, Sep 23, 2021 at 06:27:59PM -0700, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <djwong@kernel.org>
> > > 
> > > Now that we have the infrastructure to track the max possible height of
> > > each btree type, we can create a separate slab zone for cursors of each
> > > type of btree.  For smaller indices like the free space btrees, this
> > > means that we can pack more cursors into a slab page, improving slab
> > > utilization.
> > > 
> > > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > > ---
> > >  fs/xfs/libxfs/xfs_btree.c |   12 ++++++------
> > >  fs/xfs/libxfs/xfs_btree.h |    9 +--------
> > >  fs/xfs/xfs_super.c        |   33 ++++++++++++++++++++++++---------
> > >  3 files changed, 31 insertions(+), 23 deletions(-)
> > > 
> > > 
> > > diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> > > index 120280c998f8..3131de9ae631 100644
> > > --- a/fs/xfs/libxfs/xfs_btree.c
> > > +++ b/fs/xfs/libxfs/xfs_btree.c
> > > @@ -26,7 +26,6 @@
> > >  /*
> > >   * Cursor allocation zone.
> > >   */
> > > -kmem_zone_t	*xfs_btree_cur_zone;
> > >  struct xfs_btree_cur_zone xfs_btree_cur_zones[XFS_BTNUM_MAX] = {
> > >  	[XFS_BTNUM_BNO]		= { .name = "xfs_alloc_btree_cur" },
> > >  	[XFS_BTNUM_INO]		= { .name = "xfs_ialloc_btree_cur" },
> > > @@ -364,6 +363,7 @@ xfs_btree_del_cursor(
> > >  	struct xfs_btree_cur	*cur,		/* btree cursor */
> > >  	int			error)		/* del because of error */
> > >  {
> > > +	struct xfs_btree_cur_zone *bczone = &xfs_btree_cur_zones[cur->bc_btnum];
> > >  	int			i;		/* btree level */
> > >  
> > >  	/*
> > > @@ -386,10 +386,10 @@ xfs_btree_del_cursor(
> > >  		kmem_free(cur->bc_ops);
> > >  	if (!(cur->bc_flags & XFS_BTREE_LONG_PTRS) && cur->bc_ag.pag)
> > >  		xfs_perag_put(cur->bc_ag.pag);
> > > -	if (cur->bc_maxlevels > XFS_BTREE_CUR_ZONE_MAXLEVELS)
> > > +	if (cur->bc_maxlevels > bczone->maxlevels)
> > >  		kmem_free(cur);
> > >  	else
> > > -		kmem_cache_free(xfs_btree_cur_zone, cur);
> > > +		kmem_cache_free(bczone->zone, cur);
> > >  }
> > >  
> > >  /*
> > > @@ -5021,12 +5021,12 @@ xfs_btree_alloc_cursor(
> > >  {
> > >  	struct xfs_btree_cur	*cur;
> > >  	unsigned int		maxlevels = xfs_btree_maxlevels(mp, btnum);
> > > +	struct xfs_btree_cur_zone *bczone = &xfs_btree_cur_zones[btnum];
> > >  
> > > -	if (maxlevels > XFS_BTREE_CUR_ZONE_MAXLEVELS)
> > > +	if (maxlevels > bczone->maxlevels)
> > >  		cur = kmem_zalloc(xfs_btree_cur_sizeof(maxlevels), KM_NOFS);
> > >  	else
> > > -		cur = kmem_cache_zalloc(xfs_btree_cur_zone,
> > > -				GFP_NOFS | __GFP_NOFAIL);
> > > +		cur = kmem_cache_zalloc(bczone->zone, GFP_NOFS | __GFP_NOFAIL);
> > 
> > When will maxlevels ever be greater than bczone->maxlevels? Isn't
> > the bczone->maxlevels case always supposed to be the tallest
> > possible height for that btree?
> 
> It should never happen, provided that the maxlevels computation and
> verification are all correct.  I thought it was important to leave the
> heap allocation in here as a fallback, since the consequence for getting
> the size calculations wrong is corrupt kernel memory.

I think that this is the wrong approach. Static debug-only testing
of btree size calculations at init time is needed here, not runtime
fallbacks that hide the fact that we got fundamental calculations
wrong. A mistake here should be loud and obvious, not hidden away in
a fallback path that might never, ever be hit in the real world.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com