From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 766952C19D for ; Mon, 8 Apr 2024 07:23:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712561015; cv=none; b=mtPOD+s5q69tRyHIE03NLFLuNY2337i3J5U4z+HyXvPFH3q6yNbxiTIg7XO6f6lYYG3M3MjLlnmgpQPlDXdrfhIJ61OEl5vK6qG4DBZ0k4wJ5tk/uJXJwfScdw/sf3NCzZ2m+apPzMZIWHDDZXi2n9OjeBQYBJWX2TpTz8vqCa8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712561015; c=relaxed/simple; bh=ZzRAcHxtSjjr3xSt8JnhXEEAZLntlCbosx2pWWo4Ink=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=bs+z1t13e6ysY7PRmu/6K14PoWelMIbdHOi02CClVjKZaSiZ+R3maW6x/V862CGUMO7/WdK3qEFyua+Y1GkR2z5+Hzymu1XoMew4c4xaRe9OCj38kumILC/qQe5OexytbocwFOd/BdvkyYhdnldtfFgFJ8+qwJhDO03ew5ArEcM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C91B51007; Mon, 8 Apr 2024 00:24:02 -0700 (PDT) Received: from [10.57.73.169] (unknown [10.57.73.169]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 29E063F64C; Mon, 8 Apr 2024 00:23:31 -0700 (PDT) Message-ID: <30f09196-5152-45db-85de-12ac89c974b1@arm.com> Date: Mon, 8 Apr 2024 08:23:29 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm,swap: add document about RCU read lock and swapoff interaction Content-Language: en-GB To: Huang Ying , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, David Hildenbrand , Miaohe Lin , Hugh Dickins , Minchan Kim References: <20240407065450.498821-1-ying.huang@intel.com> From: Ryan Roberts In-Reply-To: <20240407065450.498821-1-ying.huang@intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 07/04/2024 07:54, 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 LGTM! Reviewed-by: Ryan Roberts > --- > 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();