From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 232F7C433ED for ; Wed, 14 Apr 2021 02:07:00 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 9511F6120E for ; Wed, 14 Apr 2021 02:06:59 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9511F6120E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id BD3CB6B0070; Tue, 13 Apr 2021 22:06:58 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B83266B0071; Tue, 13 Apr 2021 22:06:58 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A24946B0072; Tue, 13 Apr 2021 22:06:58 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0196.hostedemail.com [216.40.44.196]) by kanga.kvack.org (Postfix) with ESMTP id 822FC6B0070 for ; Tue, 13 Apr 2021 22:06:58 -0400 (EDT) Received: from smtpin31.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 2F828181C443F for ; Wed, 14 Apr 2021 02:06:58 +0000 (UTC) X-FDA: 78029334516.31.EA98DEF Received: from mga06.intel.com (mga06.intel.com [134.134.136.31]) by imf17.hostedemail.com (Postfix) with ESMTP id C540F40002DA for ; Wed, 14 Apr 2021 02:06:54 +0000 (UTC) IronPort-SDR: ntwetp+bpGZuuL0m9jXn9l9Y0Qygzux67/i+v0eHUdofI0nL3lTcjI6S/SY8+bYEvlisn549TH XMugT2BQmiDQ== X-IronPort-AV: E=McAfee;i="6200,9189,9953"; a="255864351" X-IronPort-AV: E=Sophos;i="5.82,221,1613462400"; d="scan'208";a="255864351" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by orsmga104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 Apr 2021 19:06:55 -0700 IronPort-SDR: QpF2cIhyu3RV1SZE9v8ertWnSGe3LACqgteHd9KRS7D+pSj3EyBwZgmKMKyHb1sC4nnhRE64fd qxW2VTdd1uPQ== X-IronPort-AV: E=Sophos;i="5.82,221,1613462400"; d="scan'208";a="424512053" Received: from yhuang6-desk1.sh.intel.com (HELO yhuang6-desk1.ccr.corp.intel.com) ([10.239.13.1]) by orsmga008-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 Apr 2021 19:06:51 -0700 From: "Huang, Ying" To: Miaohe Lin Cc: , , , , , , , , , , , , Subject: Re: [PATCH 1/5] mm/swapfile: add percpu_ref support for swap References: <20210408130820.48233-1-linmiaohe@huawei.com> <20210408130820.48233-2-linmiaohe@huawei.com> <87fszww55d.fsf@yhuang6-desk1.ccr.corp.intel.com> <87zgy4ufr3.fsf@yhuang6-desk1.ccr.corp.intel.com> <46a51c49-2887-0c1a-bcf3-e1ebe9698ebf@huawei.com> <874kg9u0jo.fsf@yhuang6-desk1.ccr.corp.intel.com> <75e27441-7744-7a10-e709-c8cd00830099@huawei.com> Date: Wed, 14 Apr 2021 10:06:48 +0800 In-Reply-To: <75e27441-7744-7a10-e709-c8cd00830099@huawei.com> (Miaohe Lin's message of "Wed, 14 Apr 2021 09:58:33 +0800") Message-ID: <87tuo9sjpj.fsf@yhuang6-desk1.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii X-Rspamd-Queue-Id: C540F40002DA X-Stat-Signature: yxhya998tdw4rtfdopse77bf8zyhu6kj X-Rspamd-Server: rspam02 Received-SPF: none (intel.com>: No applicable sender policy available) receiver=imf17; identity=mailfrom; envelope-from=""; helo=mga06.intel.com; client-ip=134.134.136.31 X-HE-DKIM-Result: none/none X-HE-Tag: 1618366014-293258 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: Miaohe Lin writes: > On 2021/4/14 9:17, Huang, Ying wrote: >> Miaohe Lin writes: >> >>> On 2021/4/12 15:24, Huang, Ying wrote: >>>> "Huang, Ying" writes: >>>> >>>>> Miaohe Lin writes: >>>>> >>>>>> We will use percpu-refcount to serialize against concurrent swapoff. This >>>>>> patch adds the percpu_ref support for later fixup. >>>>>> >>>>>> Signed-off-by: Miaohe Lin >>>>>> --- >>>>>> include/linux/swap.h | 2 ++ >>>>>> mm/swapfile.c | 25 ++++++++++++++++++++++--- >>>>>> 2 files changed, 24 insertions(+), 3 deletions(-) >>>>>> >>>>>> diff --git a/include/linux/swap.h b/include/linux/swap.h >>>>>> index 144727041e78..849ba5265c11 100644 >>>>>> --- a/include/linux/swap.h >>>>>> +++ b/include/linux/swap.h >>>>>> @@ -240,6 +240,7 @@ struct swap_cluster_list { >>>>>> * The in-memory structure used to track swap areas. >>>>>> */ >>>>>> struct swap_info_struct { >>>>>> + struct percpu_ref users; /* serialization against concurrent swapoff */ >>>>>> unsigned long flags; /* SWP_USED etc: see above */ >>>>>> signed short prio; /* swap priority of this type */ >>>>>> struct plist_node list; /* entry in swap_active_head */ >>>>>> @@ -260,6 +261,7 @@ struct swap_info_struct { >>>>>> struct block_device *bdev; /* swap device or bdev of swap file */ >>>>>> struct file *swap_file; /* seldom referenced */ >>>>>> unsigned int old_block_size; /* seldom referenced */ >>>>>> + struct completion comp; /* seldom referenced */ >>>>>> #ifdef CONFIG_FRONTSWAP >>>>>> unsigned long *frontswap_map; /* frontswap in-use, one bit per page */ >>>>>> atomic_t frontswap_pages; /* frontswap pages in-use counter */ >>>>>> diff --git a/mm/swapfile.c b/mm/swapfile.c >>>>>> index 149e77454e3c..724173cd7d0c 100644 >>>>>> --- a/mm/swapfile.c >>>>>> +++ b/mm/swapfile.c >>>>>> @@ -39,6 +39,7 @@ >>>>>> #include >>>>>> #include >>>>>> #include >>>>>> +#include >>>>>> >>>>>> #include >>>>>> #include >>>>>> @@ -511,6 +512,15 @@ static void swap_discard_work(struct work_struct *work) >>>>>> spin_unlock(&si->lock); >>>>>> } >>>>>> >>>>>> +static void swap_users_ref_free(struct percpu_ref *ref) >>>>>> +{ >>>>>> + struct swap_info_struct *si; >>>>>> + >>>>>> + si = container_of(ref, struct swap_info_struct, users); >>>>>> + complete(&si->comp); >>>>>> + percpu_ref_exit(&si->users); >>>>> >>>>> Because percpu_ref_exit() is used, we cannot use percpu_ref_tryget() in >>>>> get_swap_device(), better to add comments there. >>>> >>>> I just noticed that the comments of percpu_ref_tryget_live() says, >>>> >>>> * This function is safe to call as long as @ref is between init and exit. >>>> >>>> While we need to call get_swap_device() almost at any time, so it's >>>> better to avoid to call percpu_ref_exit() at all. This will waste some >>>> memory, but we need to follow the API definition to avoid potential >>>> issues in the long term. >>> >>> I have to admit that I'am not really familiar with percpu_ref. So I read the >>> implementation code of the percpu_ref and found percpu_ref_tryget_live() could >>> be called after exit now. But you're right we need to follow the API definition >>> to avoid potential issues in the long term. >>> >>>> >>>> And we need to call percpu_ref_init() before insert the swap_info_struct >>>> into the swap_info[]. >>> >>> If we remove the call to percpu_ref_exit(), we should not use percpu_ref_init() >>> here because *percpu_ref->data is assumed to be NULL* in percpu_ref_init() while >>> this is not the case as we do not call percpu_ref_exit(). Maybe percpu_ref_reinit() >>> or percpu_ref_resurrect() will do the work. >>> >>> One more thing, how could I distinguish the killed percpu_ref from newly allocated one? >>> It seems percpu_ref_is_dying is only safe to call when @ref is between init and exit. >>> Maybe I could do this in alloc_swap_info()? >> >> Yes. In alloc_swap_info(), you can distinguish newly allocated and >> reused swap_info_struct. >> >>>> >>>>>> +} >>>>>> + >>>>>> static void alloc_cluster(struct swap_info_struct *si, unsigned long idx) >>>>>> { >>>>>> struct swap_cluster_info *ci = si->cluster_info; >>>>>> @@ -2500,7 +2510,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio, >>>>>> * Guarantee swap_map, cluster_info, etc. fields are valid >>>>>> * between get/put_swap_device() if SWP_VALID bit is set >>>>>> */ >>>>>> - synchronize_rcu(); >>>>>> + percpu_ref_reinit(&p->users); >>>>> >>>>> Although the effect is same, I think it's better to use >>>>> percpu_ref_resurrect() here to improve code readability. >>>> >>>> Check the original commit description for commit eb085574a752 "mm, swap: >>>> fix race between swapoff and some swap operations" and discussion email >>>> thread as follows again, >>>> >>>> https://lore.kernel.org/linux-mm/20171219053650.GB7829@linux.vnet.ibm.com/ >>>> >>>> I found that the synchronize_rcu() here is to avoid to call smp_rmb() or >>>> smp_load_acquire() in get_swap_device(). Now we will use >>>> percpu_ref_tryget_live() in get_swap_device(), so we will need to add >>>> the necessary memory barrier, or make sure percpu_ref_tryget_live() has >>>> ACQUIRE semantics. Per my understanding, we need to change >>>> percpu_ref_tryget_live() for that. >>>> >>> >>> Do you mean the below scene is possible? >>> >>> cpu1 >>> swapon() >>> ... >>> percpu_ref_init >>> ... >>> setup_swap_info >>> /* smp_store_release() is inside percpu_ref_reinit */ >>> percpu_ref_reinit >> >> spin_unlock() has RELEASE semantics already. >> >>> ... >>> >>> cpu2 >>> get_swap_device() >>> /* ignored smp_rmb() */ >>> percpu_ref_tryget_live >> >> Some kind of ACQUIRE is required here to guarantee the refcount is >> checked before fetching the other fields of swap_info_struct. I have >> sent out a RFC patch to mailing list to discuss this. > > Many thanks. > But We may still need to add a smp_rmb() in get_swap_device() in case > we can't add ACQUIRE for refcount. Yes. Best Regards, Huang, Ying