linux-kernel - [PATCH v7 2/2] mm/oom_kill: The OOM reaper traverses the VMA maple tree in reverse order

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Message-ID: <20250903092729.10611-3-zhongjinji@honor.com>
Date: Wed, 3 Sep 2025 17:27:29 +0800
From: zhongjinji <zhongjinji@...or.com>
To: <mhocko@...e.com>
CC: <rientjes@...gle.com>, <shakeel.butt@...ux.dev>,
	<akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <tglx@...utronix.de>,
	<liam.howlett@...cle.com>, <lorenzo.stoakes@...cle.com>, <surenb@...gle.com>,
	<liulu.liu@...or.com>, <feng.han@...or.com>, <zhongjinji@...or.com>
Subject: [PATCH v7 2/2] 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.

Without the patch:
|--99.74%-- oom_reaper
|  |--76.67%-- unmap_page_range
|  |  |--33.70%-- __pte_offset_map_lock
|  |  |  |--98.46%-- _raw_spin_lock
|  |  |--27.61%-- free_swap_and_cache_nr
|  |  |--16.40%-- folio_remove_rmap_ptes
|  |  |--12.25%-- tlb_flush_mmu
|  |--12.61%-- tlb_finish_mmu

With the patch:
|--98.84%-- oom_reaper
|  |--53.45%-- unmap_page_range
|  |  |--24.29%-- [hit in function]
|  |  |--48.06%-- folio_remove_rmap_ptes
|  |  |--17.99%-- tlb_flush_mmu
|  |  |--1.72%-- __pte_offset_map_lock
|  |--30.43%-- tlb_finish_mmu

Signed-off-by: zhongjinji <zhongjinji@...or.com>
---
 mm/oom_kill.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 3caaafc896d4..540b1e5e0e46 100644
--- 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)
 	 */
 	set_bit(MMF_UNSTABLE, &mm->flags);
 
-	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;
 
-- 
2.17.1