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/20210922152833.4023972-1-eric.dumazet@gmail.com
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e4aef907abe9db5369416b71aa37b2fa9ede5b19..e891560e0a80c485c2b03587c6452fc8c3d087fa 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -9356,21 +9356,21 @@ 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;
 }