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 based on arm64.

Without the patch:
|--99.57%-- oom_reaper
|    |--0.28%-- [hit in function]
|    |--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
|    |    |--1.66%-- folio_mark_accessed
|    |    |--0.74%-- free_swap_and_cache_nr
|    |    |--0.69%-- __tlb_remove_folio_pages
|    |--19.94%-- tlb_finish_mmu
|    |--3.21%-- folio_remove_rmap_ptes
|    |--1.16%-- __tlb_remove_folio_pages
|    |--1.16%-- folio_mark_accessed
|    |--0.36%-- __pte_offset_map_lock

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
|    |    |--2.65%-- __tlb_remove_folio_pages
|    |    |--1.37%-- _raw_spin_lock
|    |    |--0.68%-- __mod_lruvec_page_state
|    |    |--0.51%-- __pte_offset_map_lock
|    |--32.21%-- tlb_finish_mmu
|    |--6.93%-- folio_remove_rmap_ptes
|    |--1.90%-- __tlb_remove_folio_pages
|    |--1.55%-- folio_mark_accessed
|    |--0.69%-- __pte_offset_map_lock

Link: https://lkml.kernel.org/r/20250909090659.26400-4-zhongjinji@honor.com
Signed-off-by: zhongjinji <zhongjinji@honor.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.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>

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 58bd4cf71d527c8900e39aab229a441e2bad6e65..5656dcc11bbcbf5c97c87428a630657c1df5628b 100644 (file)
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -516,7 +516,7 @@ static bool __oom_reap_task_mm(struct mm_struct *mm)
 {
        struct vm_area_struct *vma;
        bool ret = true;
-       VMA_ITERATOR(vmi, mm, 0);
+       MA_STATE(mas, &mm->mm_mt, ULONG_MAX, ULONG_MAX);
 
        /*
         * Tell all users of get_user/copy_from_user etc... that the content
@@ -526,7 +526,13 @@ static bool __oom_reap_task_mm(struct mm_struct *mm)
         */
        mm_flags_set(MMF_UNSTABLE, mm);
 
-       for_each_vma(vmi, vma) {
+       /*
+        * It 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.
+        */
+       mas_for_each_rev(&mas, vma, 0) {
                if (vma->vm_flags & (VM_HUGETLB|VM_PFNMAP))
                        continue;