mm: mmap_lock: replace get_memcg_path_buf() with on-stack buffer

mm: mmap_lock: replace get_memcg_path_buf() with on-stack buffer

Commit 2b5067a8143e ("mm: mmap_lock: add tracepoints around lock
acquisition") introduced TRACE_MMAP_LOCK_EVENT() macro using
preempt_disable() in order to let get_mm_memcg_path() return a percpu
buffer exclusively used by normal, softirq, irq and NMI contexts
respectively.

Commit 832b50725373 ("mm: mmap_lock: use local locks instead of disabling
preemption") replaced preempt_disable() with local_lock(&memcg_paths.lock)
based on an argument that preempt_disable() has to be avoided because
get_mm_memcg_path() might sleep if PREEMPT_RT=y.

But syzbot started reporting

  inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.

and

  inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.

messages, for local_lock() does not disable IRQ.

We could replace local_lock() with local_lock_irqsave() in order to
suppress these messages.  But this patch instead replaces percpu buffers
with on-stack buffer, for the size of each buffer returned by
get_memcg_path_buf() is only 256 bytes which is tolerable for allocating
from current thread's kernel stack memory.

Link: https://lkml.kernel.org/r/ef22d289-eadb-4ed9-863b-fbc922b33d8d@I-love.SAKURA.ne.jp
Reported-by: syzbot <syzbot+40905bca570ae6784745@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=40905bca570ae6784745
Fixes: 832b50725373 ("mm: mmap_lock: use local locks instead of disabling preemption")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>
Cc: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>