[PATCH] maple_tree: Fix mas_spanning_rebalance() on insufficient data

[Liam Howlett]

* Andrew Morton <akpm at linux-foundation.org> [221216 14:01]:
> On Fri, 16 Dec 2022 18:53:15 +0000 Liam Howlett <liam.howlett at oracle.com> wrote:
> 
> > Mike Rapoport contacted me off-list with a regression in running criu.
> > Periodic tests fail with an RCU stall during execution.  Although rare,
> > it is possible to hit this with other uses so this patch should be
> > backported to fix the regression.
> > 
> > An insufficient node was causing an out-of-bounds access on the node in
> > mas_leaf_max_gap().  The cause was the faulty detection of the new node
> > being a root node when overwriting many entries at the end of the tree.
> > 
> > Fix the detection of a new root and ensure there is sufficient data
> > prior to entering the spanning rebalance loop.
> > 
> > Add a testcase to the maple tree test suite for this issue.
> 
> Shall do.
> 
> > Cc: Andrei Vagin <avagin at gmail.com>
> > Cc: usama.anjum at collabora.com
> > Reported-by: Mike Rapoport <rppt at kernel.org>
> > Fixes: 54a611b60590 ("Maple Tree: add new data structure")
> > Signed-off-by: Liam R. Howlett <Liam.Howlett at oracle.com>
> 
> I guess we want a cc:stable there?

Yes, please.  I'll be sure to do that in the future for backports.

> 
> >  lib/test_maple_tree.c | 23 +++++++++++++++++++++++
> 
> Belated review: all this code runs at __init time, so every dang
> function in there really should be marked __init, data marked
> __initdata, etc.  Like lib/test_bitmap.c.
> 
> I wonder if there's some trick we can do external to the .c file to
> have the same effect.

This can be built as a module and also runs in userspace via
tools/testing/radix-tree.  I'm not sure about using __init in this
context but I'll look into it.

> 
> Also, maple_tree_seed():set[] could be static ;).  So we don't have to
> initialize it at runtime.  Better would be static const.  Nitpick.

I'll have a look.  I'm sure there are more areas within my test code
that could be optimised like this.