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a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1712561013; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=bLI3llylebNmQVL3IpHbZ+qCV6kW6Buoji1qmehN80Y=; b=ZPcQD5cHIRfXIX+DeWwzZ6/+uthWEChtx88KbwN2QRY6gVKqk5SyMdeH5ii/mH6xk6DXAw KI9UXDER6dU1D1dKf3FFVcUPjGiDNR+w6fNHU/s7vQk5ZctZz9v1V4ewbD1wCPBnj62RWD p633IUA8gEtGdDNiE6FhbQVN56B79k8= 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 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 X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 5E27F16000D X-Stat-Signature: dtzusniba1541b4ktuhia5o395xmisgw X-HE-Tag: 1712561013-734580 X-HE-Meta: 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 dyfctF2E GKkJKCGpiUuzFj3OBxI6XMT8SsjTaTxlKRR2Fo5rnE+M+YtrMlKtJft7R8F6+Mns66gWJOk1M3Bw1V8y0JhEfWyepSCqQIa/KmLZDe60Z3CbrXPqYmbuo9t9/aR5MWIXSF9eAxPS/ucZcidldwU8ZAwHP7oeqO+urSr5gPrKESw9r42etg8BBwmX8I+ezgVDC+BmpBm8YtKML6bGilgLTCnvwUBu0i2Qfv900NAChTBIhkM6HaErfhEPXysv42MlNCrmubt+qmyMhwIzeSpGN3dsdvPRfdVeQXjMfDODLF87SXXtswxwb+UFZfKM9L11BsCnMB2R9+NgM9uiCU2PDmyVZUSw5EKxYTk+JaC3m7PaYaYA= 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: List-Subscribe: List-Unsubscribe: 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();