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

Darrick J. Wong
2021-10-12T18:49:26Z

On Tue, Sep 28, 2021 at 08:01:36AM +1000, Dave Chinner wrote:
> 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.

Agree. I'll add an assert to the functions that compute the per-mount
maxlevels.

--D

> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com