[112/262] mm/page_alloc.c: do not acquire zone lock in is_free_buddy_page()

Commit Message

Andrew Morton Nov. 5, 2021, 8:40 p.m. UTC

From: Eric Dumazet <[e-mail]>
Subject: mm/page_alloc.c: do not acquire zone lock in is_free_buddy_page()

Grabbing zone lock in is_free_buddy_page() gives a wrong sense of safety,
and has potential performance implications when zone is experiencing lock
contention.

In any case, if a caller needs a stable result, it should grab zone lock
before calling this function.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Eric Dumazet <[e-mail]>
Acked-by: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/page_alloc.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

Patch

--- a/mm/page_alloc.c~mm-do-not-acquire-zone-lock-in-is_free_buddy_page
+++ a/mm/page_alloc.c
@@ -9356,21 +9356,21 @@  void __offline_isolated_pages(unsigned l
 }
 #endif
 
+/*
+ * This function returns a stable result only if called under zone lock.
+ */
 bool is_free_buddy_page(struct page *page)
 {
-	struct zone *zone = page_zone(page);
 	unsigned long pfn = page_to_pfn(page);
-	unsigned long flags;
 	unsigned int order;
 
-	spin_lock_irqsave(&zone->lock, flags);
 	for (order = 0; order < MAX_ORDER; order++) {
 		struct page *page_head = page - (pfn & ((1 << order) - 1));
 
-		if (PageBuddy(page_head) && buddy_order(page_head) >= order)
+		if (PageBuddy(page_head) &&
+		    buddy_order_unsafe(page_head) >= order)
 			break;
 	}
-	spin_unlock_irqrestore(&zone->lock, flags);
 
 	return order < MAX_ORDER;
 }