maple_tree: be more cautious about dead nodes

Patch series "Fix VMA tree modification under mmap read lock".

Syzbot reported a BUG_ON in mm/mmap.c which was found to be caused by an
inconsistency between threads walking the VMA maple tree.  The
inconsistency is caused by the page fault handler modifying the maple tree
while holding the mmap_lock for read.

This only happens for stack VMAs.  We had thought this was safe as it only
modifies a single pivot in the tree.  Unfortunately, syzbot constructed a
test case where the stack had no guard page and grew the stack to abut the
next VMA.  This causes us to delete the NULL entry between the two VMAs
and rewrite the node.

We considered several options for fixing this, including dropping the
mmap_lock, then reacquiring it for write; and relaxing the definition of
the tree to permit a zero-length NULL entry in the node.  We decided the
best option was to backport some of the RCU patches from -next, which
solve the problem by allocating a new node and RCU-freeing the old node. 
Since the problem exists in 6.1, we preferred a solution which is similar
to the one we intended to merge next merge window.

These patches have been in -next since next-20230301, and have received
intensive testing in Android as part of the RCU page fault patchset.  They
were also sent as part of the "Per-VMA locks" v4 patch series.  Patches 1
to 7 are bug fixes for RCU mode of the tree and patch 8 enables RCU mode
for the tree.

Performance v6.3-rc3 vs patched v6.3-rc3: Running these changes through
mmtests showed there was a 15-20% performance decrease in
will-it-scale/brk1-processes.  This tests creating and inserting a single
VMA repeatedly through the brk interface and isn't representative of any
real world applications.


This patch (of 8):

ma_pivots() and ma_data_end() may be called with a dead node.  Ensure to
that the node isn't dead before using the returned values.

This is necessary for RCU mode of the maple tree.

Link: https://lkml.kernel.org/r/20230327185532.2354250-1-Liam.Howlett@oracle.com
Link: https://lkml.kernel.org/r/20230227173632.3292573-1-surenb@google.com
Link: https://lkml.kernel.org/r/20230227173632.3292573-2-surenb@google.com


Fixes: 54a611b6 ("Maple Tree: add new data structure")
Signed-off-by: Liam Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arjun Roy <arjunroy@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Chris Li <chriscli@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: David Rientjes <rientjes@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: freak07 <michalechner92@googlemail.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Laurent Dufour <ldufour@linux.ibm.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Minchan Kim <minchan@google.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Oskolkov <posk@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Punit Agrawal <punit.agrawal@bytedance.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>