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[23.128.96.18]) by mx.google.com with ESMTP id b2si6868516plh.76.2021.04.14.05.39.31; Wed, 14 Apr 2021 05:39:44 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=intel.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S245234AbhDNFpb (ORCPT + 99 others); Wed, 14 Apr 2021 01:45:31 -0400 Received: from mga18.intel.com ([134.134.136.126]:51742 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233412AbhDNFp0 (ORCPT ); Wed, 14 Apr 2021 01:45:26 -0400 IronPort-SDR: 2mJynyBiSH2AiUaTP4Z7jrPgmVOvO2dNwrf8uwDgagefztgIRHqYD9FdX8gAe+6tfxNJX+93dJ Cn7SyzJ110AA== X-IronPort-AV: E=McAfee;i="6200,9189,9953"; a="182080044" X-IronPort-AV: E=Sophos;i="5.82,221,1613462400"; d="scan'208";a="182080044" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 Apr 2021 22:45:05 -0700 IronPort-SDR: ulmcWhborXQ71mK476K7rpKfP5DpVpfMjsqGgE5QrwR3HFyMZA3ntRcRcgLRNeGzjBgwJCECKH XF4eD9K3expQ== X-IronPort-AV: E=Sophos;i="5.82,221,1613462400"; d="scan'208";a="424576449" 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 22:45:01 -0700 From: "Huang, Ying" To: Dennis Zhou Cc: Miaohe Lin , , , , , , , , , , , , , 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> <87tuo9sjpj.fsf@yhuang6-desk1.ccr.corp.intel.com> <877dl5seig.fsf@yhuang6-desk1.ccr.corp.intel.com> Date: Wed, 14 Apr 2021 13:44:58 +0800 In-Reply-To: (Dennis Zhou's message of "Wed, 14 Apr 2021 04:05:31 +0000") Message-ID: <87zgy1qv1h.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 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Dennis Zhou writes: > On Wed, Apr 14, 2021 at 11:59:03AM +0800, Huang, Ying wrote: >> Dennis Zhou writes: >> >> > Hello, >> > >> > On Wed, Apr 14, 2021 at 10:06:48AM +0800, Huang, Ying wrote: >> >> 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. >> > >> > I'm just catching up and following along a little bit. I apologize I >> > haven't read the swap code, but my understanding is you are trying to >> > narrow a race condition with swapoff. That makes sense to me. I'm not >> > sure I follow the need to race with reinitializing the ref though? Is it >> > not possible to wait out the dying swap info and then create a new one >> > rather than push acquire semantics? >> >> We want to check whether the swap entry is valid (that is, the swap >> device isn't swapped off now), prevent it from swapping off, then access >> the swap_info_struct data structure. When accessing swap_info_struct, >> we want to guarantee the ordering, so that we will not reference >> uninitialized fields of swap_info_struct. >> > > So in the normal context of percpu_ref, once someone can access it, the > elements that it is protecting are expected to be initialized. If we can make sure that all elements being initialized fully, why not just use percpu_ref_get() instead of percpu_ref_tryget*()? > In the basic case for swap off, I'm seeing the goal as to prevent > destruction until anyone currently accessing swap is done. In this > case wouldn't we always be protecting a live struct? > > I'm maybe not understanding what conditions you're trying to revive the > percpu_ref? A swap entry likes an indirect pointer to a swap device. We may hold a swap entry for long time, so that the swap device is swapoff/swapon. Then we need to make sure the swap device are fully initialized before accessing the swap device via the swap entry. Best Regards, Huang, Ying