[merged mm-stable] mm-kmemleak-move-the-struct-page-scan-into-a-helper.patch removed from -mm tree

[merged mm-stable] mm-kmemleak-move-the-struct-page-scan-into-a-helper.patch removed from -mm tree

[Andrew Morton]

The quilt patch titled
     Subject: mm: kmemleak: move the struct page scan into a helper
has been removed from the -mm tree.  Its filename was
     mm-kmemleak-move-the-struct-page-scan-into-a-helper.patch

This patch was dropped because it was merged into the mm-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Breno Leitao <leitao@debian.org>
Subject: mm: kmemleak: move the struct page scan into a helper
Date: Mon, 21 Sep 2026 05:30:26 -0700

Patch series "mm: kmemleak: batch the struct page scan".

kmemleak walks the struct page array as a scan root and hands it to
scan_block() one page at a time.

scan_block() takes kmemleak_lock with interrupts disabled for the duration
of the call, so the scanner acquires the lock once per online PFN in order
to look at a single 64-byte struct page.

Batching adjacent pages up to that size keeps exactly the same pages under
the same maximum lock hold time, with MAX_SCAN_SIZE / pagesize fewer
acquisitions.

Patch 1 lifts the loop out of __kmemleak_scan() into scan_zone_pages()
with no functional change.  Patch 2 does the batching, which is then
contained in that one function.

This improves the performance due to less atomic operations, which is not
a big deal on a regular machine, but, given kmemleak usually comes with
extra debug options, such as PROVE_LOCKING, DEBUG_SPINLOCK, etc.

For instance, measuring Meta's "debug kernel flavor" on an arm64 hosts,
this improve the scan time by 20%.

It is safe to get more work into scan_block(), given it has the
protections, added by commit eb11f56eeca560 ("mm/kmemleak: stop the task
stack scan early when interrupted")

MAX_SCAN_SIZE is also not a new maximum for a single scan_block() call. 
kmemleak_scan_task_stacks() already hands it a whole task stack in one go.
On arm64 and x86_64 THREAD_SIZE is never below 16 KiB, four times
MAX_SCAN_SIZE, and it is 64 KiB on arm64 with 64K pages.


This patch (of 2):

The struct page scanning loop sits inline in __kmemleak_scan(), nested
three levels deep and sharing the caller's "stop" variable with the
zone walk around it. Move it into scan_zone_pages(), which scans one
zone and returns 1 if scanning should stop.

No functional change; this only makes room for changing how the pages
are handed to scan_block().

Link: https://lore.kernel.org/20260921-b4-kmemleak-page-scan-v1-1-fb97d4801b3a@debian.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
---

 mm/kmemleak.c |   57 ++++++++++++++++++++++++++++--------------------
 1 file changed, 34 insertions(+), 23 deletions(-)

--- a/mm/kmemleak.c~mm-kmemleak-move-the-struct-page-scan-into-a-helper
+++ a/mm/kmemleak.c
@@ -1856,6 +1856,39 @@ static void dedup_flush(struct xarray *d
 }
 
 /*
+ * Scan the struct pages of a zone, skipping memory holes, pages that belong to
+ * another zone and pages that are not in use. Returns 1 if the scan should be
+ * stopped.
+ */
+static int scan_zone_pages(struct zone *zone)
+{
+	unsigned long start_pfn = zone->zone_start_pfn;
+	unsigned long end_pfn = zone_end_pfn(zone);
+	unsigned long pfn;
+
+	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
+		struct page *page = pfn_to_online_page(pfn);
+
+		if (!(pfn & 63))
+			cond_resched_tasks_rcu_qs();
+
+		if (!page)
+			continue;
+
+		/* only scan pages belonging to this zone */
+		if (page_zone(page) != zone)
+			continue;
+		/* only scan if page is in use */
+		if (page_count(page) == 0)
+			continue;
+		if (scan_block(page, page + 1, NULL))
+			return 1;
+	}
+
+	return 0;
+}
+
+/*
  * Scan data sections and all the referenced memory blocks allocated via the
  * kernel's standard allocators. This function must be called with the
  * scan_mutex held.
@@ -1928,29 +1961,7 @@ static int __kmemleak_scan(bool full)
 	 */
 	get_online_mems();
 	for_each_populated_zone(zone) {
-		unsigned long start_pfn = zone->zone_start_pfn;
-		unsigned long end_pfn = zone_end_pfn(zone);
-		unsigned long pfn;
-
-		for (pfn = start_pfn; pfn < end_pfn; pfn++) {
-			struct page *page = pfn_to_online_page(pfn);
-
-			if (!(pfn & 63))
-				cond_resched_tasks_rcu_qs();
-
-			if (!page)
-				continue;
-
-			/* only scan pages belonging to this zone */
-			if (page_zone(page) != zone)
-				continue;
-			/* only scan if page is in use */
-			if (page_count(page) == 0)
-				continue;
-			stop = scan_block(page, page + 1, NULL);
-			if (stop)
-				break;
-		}
+		stop = scan_zone_pages(zone);
 		if (stop)
 			break;
 	}
_

Patches currently in -mm which might be from leitao@debian.org are