From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 32DF713C901 for ; Wed, 10 Apr 2024 07:58:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=45.249.212.187 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712735917; cv=none; b=aLHxyxZvZ9sqXkw5M8DBJuU0jyvHbsQ3q3dbbl/5ARuG/uf1Ebw9Ec5yRPSlLRcMuT0eIcK5MYAuOUdjTwCQ4JkPbjsIezp9EoiI7iyRrCQRTETFG9P8mpA5xELOZLe8RvaOoVuyoprO4ABdftZuay2e7OZYYWVslBopHWtGKEM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712735917; c=relaxed/simple; bh=nKJBodP5bAnf+nL27eO3VqssbnJkiUNxzOo7xkepETQ=; h=Subject:To:CC:References:From:Message-ID:Date:MIME-Version: In-Reply-To:Content-Type; b=DdTgNgPzK76qqceM5+zuPPpvZ6Sjrdl2caWD6D8Om/uhsJi/HyD3AZMjsEqNHjls4D/jA/PkjIfMHKD9YZnmzi2U+HA8V905GmwD3U0RcNxwJ0K8Ia83UirQnrmaX+3uZbfvKcbm3lADwXXFdZpZD7BHQw3725yWis9Ior5VxiA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; arc=none smtp.client-ip=45.249.212.187 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Received: from mail.maildlp.com (unknown [172.19.163.174]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4VDw9h2wwPzwRcF; Wed, 10 Apr 2024 15:55:36 +0800 (CST) Received: from canpemm500002.china.huawei.com (unknown [7.192.104.244]) by mail.maildlp.com (Postfix) with ESMTPS id BFAFB140159; Wed, 10 Apr 2024 15:58:31 +0800 (CST) Received: from [10.173.135.154] (10.173.135.154) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.1.2507.35; Wed, 10 Apr 2024 15:58:31 +0800 Subject: Re: [PATCH] mm,swap: add document about RCU read lock and swapoff interaction To: Huang Ying CC: , , Ryan Roberts , David Hildenbrand , Hugh Dickins , Minchan Kim , Andrew Morton References: <20240407065450.498821-1-ying.huang@intel.com> From: Miaohe Lin Message-ID: <62d09dae-39b9-f235-9786-6e12302425a0@huawei.com> Date: Wed, 10 Apr 2024 15:58:30 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20240407065450.498821-1-ying.huang@intel.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To canpemm500002.china.huawei.com (7.192.104.244) On 2024/4/7 14:54, Huang Ying wrote: > During reviewing a patch to fix the race condition between > free_swap_and_cache() and swapoff() [1], it was found that the > document about how to prevent racing with swapoff isn't clear enough. > Especially RCU read lock can prevent swapoff from freeing data > structures. So, the document is added as comments. > > [1] https://lore.kernel.org/linux-mm/c8fe62d0-78b8-527a-5bef-ee663ccdc37a@huawei.com/ > > Signed-off-by: "Huang, Ying" > Cc: Ryan Roberts > Cc: David Hildenbrand > Cc: Miaohe Lin > Cc: Hugh Dickins > Cc: Minchan Kim Thanks for your work. Reviewed-by: Miaohe Lin . > --- > mm/swapfile.c | 26 +++++++++++++------------- > 1 file changed, 13 insertions(+), 13 deletions(-) > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 4919423cce76..6925462406fa 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -1226,16 +1226,15 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p, > > /* > * When we get a swap entry, if there aren't some other ways to > - * prevent swapoff, such as the folio in swap cache is locked, page > - * table lock is held, etc., the swap entry may become invalid because > - * of swapoff. Then, we need to enclose all swap related functions > - * with get_swap_device() and put_swap_device(), unless the swap > - * functions call get/put_swap_device() by themselves. > + * prevent swapoff, such as the folio in swap cache is locked, RCU > + * reader side is locked, etc., the swap entry may become invalid > + * because of swapoff. Then, we need to enclose all swap related > + * functions with get_swap_device() and put_swap_device(), unless the > + * swap functions call get/put_swap_device() by themselves. > * > - * Note that when only holding the PTL, swapoff might succeed immediately > - * after freeing a swap entry. Therefore, immediately after > - * __swap_entry_free(), the swap info might become stale and should not > - * be touched without a prior get_swap_device(). > + * RCU reader side lock (including any spinlock) is sufficient to > + * prevent swapoff, because synchronize_rcu() is called in swapoff() > + * before freeing data structures. > * > * Check whether swap entry is valid in the swap device. If so, > * return pointer to swap_info_struct, and keep the swap entry valid > @@ -2495,10 +2494,11 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile) > > /* > * Wait for swap operations protected by get/put_swap_device() > - * to complete. > - * > - * We need synchronize_rcu() here to protect the accessing to > - * the swap cache data structure. > + * to complete. Because of synchronize_rcu() here, all swap > + * operations protected by RCU reader side lock (including any > + * spinlock) will be waited too. This makes it easy to > + * prevent folio_test_swapcache() and the following swap cache > + * operations from racing with swapoff. > */ > percpu_ref_kill(&p->users); > synchronize_rcu(); >