linux-kernel - [PATCH] mm: do not acquire zone lock in is_free_buddy

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Message-Id: <20210922152833.4023972-1-eric.dumazet@gmail.com>
Date:   Wed, 22 Sep 2021 08:28:33 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: [PATCH] mm: do not acquire zone lock in is_free_buddy_page()

From: Eric Dumazet <edumazet@...gle.com>

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.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Hugh Dickins <hughd@...gle.com>
---
 mm/page_alloc.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e115e21524739341d409b28379942241ed403060..cd8a72372b047e55c4cde80fe6b7a428d7721027 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -9354,21 +9354,21 @@ void __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
 }
 #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;
 }
-- 
2.33.0.464.g1972c5931b-goog