From: Eric Dumazet <edumazet@xxxxxxxxxx> 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/20210922152833.4023972-1-eric.dumazet@xxxxxxxxx Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/page_alloc.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) --- 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; } _
