mm/oom_kill: the OOM reaper traverses the VMA maple tree in reverse order

Although the oom_reaper is delayed and it gives the oom victim chance to
clean up its address space this might take a while especially for
processes with a large address space footprint.  In those cases oom_reaper
might start racing with the dying task and compete for shared resources -
e.g.  page table lock contention has been observed.

Reduce those races by reaping the oom victim from the other end of the
address space.

It is also a significant improvement for process_mrelease().  When a
process is killed, process_mrelease is used to reap the killed process and
often runs concurrently with the dying task.  The test data shows that
after applying the patch, lock contention is greatly reduced during the
procedure of reaping the killed process.

The test is conducted on arm64.  The following basic perf numbers show
that applying this patch significantly reduces pte spin lock contention.

Without the patch:
|--99.57%-- oom_reaper
|    |--73.58%-- unmap_page_range
|    |    |--8.67%-- [hit in function]
|    |    |--41.59%-- __pte_offset_map_lock
|    |    |--29.47%-- folio_remove_rmap_ptes
|    |    |--16.11%-- tlb_flush_mmu
|    |--19.94%-- tlb_finish_mmu
|    |--3.21%-- folio_remove_rmap_ptes

With the patch:
|--99.53%-- oom_reaper
|    |--55.77%-- unmap_page_range
|    |    |--20.49%-- [hit in function]
|    |    |--58.30%-- folio_remove_rmap_ptes
|    |    |--11.48%-- tlb_flush_mmu
|    |    |--3.33%-- folio_mark_accessed
|    |--32.21%-- tlb_finish_mmu
|    |--6.93%-- folio_remove_rmap_ptes
|    |--0.69%-- __pte_offset_map_lock

Detailed breakdowns for both scenarios are provided below.  The cumulative
time for oom_reaper plus exit_mmap(victim) in both cases is also
summarized, making the performance improvements clear.

+----------------------------------------------------------------+
| Category                      | Applying patch | Without patch |
+-------------------------------+----------------+---------------+
| Total running time            |    132.6       |    167.1      |
|   (exit_mmap + reaper work)   |  72.4 + 60.2   |  90.7 + 76.4  |
+-------------------------------+----------------+---------------+
| Time waiting for pte spinlock |     1.0        |    33.1       |
|   (exit_mmap + reaper work)   |   0.4 + 0.6    |  10.0 + 23.1  |
+-------------------------------+----------------+---------------+
| folio_remove_rmap_ptes time   |    42.0        |    41.3       |
|   (exit_mmap + reaper work)   |  18.4 + 23.6   |  22.4 + 18.9  |
+----------------------------------------------------------------+

From this report, we can see that:

1. The reduction in total time comes mainly from the decrease in time
   spent on pte spinlock and other locks.

2. oom_reaper performs more work in some areas, but at the same time,
   exit_mmap also handles certain tasks more efficiently, such as
   folio_remove_rmap_ptes.

Here is a more detailed perf report. [1]

Link: https://lkml.kernel.org/r/20250915162946.5515-3-zhongjinji@honor.com
Link: https://lore.kernel.org/all/20250915162619.5133-1-zhongjinji@honor.com/ [1]
Signed-off-by: zhongjinji <zhongjinji@honor.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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