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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id c17sm2560312qtq.42.2021.09.22.16.23.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 22 Sep 2021 16:23:43 -0700 (PDT) Date: Wed, 22 Sep 2021 16:23:41 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: Eric Dumazet cc: Andrew Morton , linux-kernel , linux-mm , Eric Dumazet , Hugh Dickins Subject: Re: [PATCH] mm: do not acquire zone lock in is_free_buddy_page() In-Reply-To: <20210922152833.4023972-1-eric.dumazet@gmail.com> Message-ID: <26db20dd-8e18-e68a-adb-adf171d62bf8@google.com> References: <20210922152833.4023972-1-eric.dumazet@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=Z72Fms7C; spf=pass (imf24.hostedemail.com: domain of hughd@google.com designates 209.85.222.172 as permitted sender) smtp.mailfrom=hughd@google.com; dmarc=pass (policy=reject) header.from=google.com X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: AE28BB000099 X-Stat-Signature: cmkt79trx6hk6w84j5uwspx3rrbhwuhr X-HE-Tag: 1632353024-667747 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, 22 Sep 2021, Eric Dumazet wrote: > From: Eric Dumazet > > 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 > Cc: Hugh Dickins Yes indeed, and you have already explained it well above: thanks. Acked-by: Hugh Dickins > --- > 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