[PATCH v5] mm: shrink skip folio mapped by an exiting process

[Zhiguo Jiang]

The releasing process of the non-shared anonymous folio mapped solely by
an exiting process may go through two flows: 1) the anonymous folio is
firstly is swaped-out into swapspace and transformed into a swp_entry
in shrink_folio_list; 2) then the swp_entry is released in the process
exiting flow. This will increase the cpu load of releasing a non-shared
anonymous folio mapped solely by an exiting process, because the folio
go through swap-out and the releasing the swapspace and swp_entry.

When system is low memory, it is more likely to occur, because more
backend applidatuions will be killed.

The modification is that shrink skips the non-shared anonymous folio
solely mapped by an exting process and the folio is only released
directly in the process exiting flow, which will save swap-out time
and alleviate the load of the process exiting. 

Signed-off-by: Zhiguo Jiang <justinjiang@xxxxxxxx>
---

Change log:
v4->v5:
1.Modify to skip non-shared anonymous folio only.
2.Update comments for pra->referenced = -1.
v3->v4:
1.Modify that the unshared folios mapped only in exiting task are skip.
v2->v3:
Nothing.
v1->v2:
1.The VM_EXITING added in v1 patch is removed, because it will fail
to compile in 32-bit system.

 mm/rmap.c   | 13 +++++++++++++
 mm/vmscan.c |  7 ++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 26806b49a86f..5b5281d71dbb
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -843,6 +843,19 @@ static bool folio_referenced_one(struct folio *folio,
 	int referenced = 0;
 	unsigned long start = address, ptes = 0;
 
+	/*
+	 * Skip the non-shared anonymous folio mapped solely by
+	 * the single exiting process, and release it directly
+	 * in the process exiting.
+	 */
+	if ((!atomic_read(&vma->vm_mm->mm_users) ||
+		test_bit(MMF_OOM_SKIP, &vma->vm_mm->flags)) &&
+		folio_test_anon(folio) && folio_test_swapbacked(folio) &&
+		!folio_likely_mapped_shared(folio)) {
+		pra->referenced = -1;
+		return false;
+	}
+
 	while (page_vma_mapped_walk(&pvmw)) {
 		address = pvmw.address;
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0761f91b407f..bae7a8bf6b3d
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -863,7 +863,12 @@ static enum folio_references folio_check_references(struct folio *folio,
 	if (vm_flags & VM_LOCKED)
 		return FOLIOREF_ACTIVATE;
 
-	/* rmap lock contention: rotate */
+	/*
+	 * There are two cases to consider.
+	 * 1) Rmap lock contention: rotate.
+	 * 2) Skip the non-shared anonymous folio mapped solely by
+	 *    the single exiting process.
+	 */
 	if (referenced_ptes == -1)
 		return FOLIOREF_KEEP;
 
-- 
2.39.0