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=-13.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 457FCC433B4 for ; Wed, 21 Apr 2021 01:05:20 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 9B29B61418 for ; Wed, 21 Apr 2021 01:05:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9B29B61418 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 0C2026B006E; Tue, 20 Apr 2021 21:05:19 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 09B066B0070; Tue, 20 Apr 2021 21:05:19 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E549F6B0071; Tue, 20 Apr 2021 21:05:18 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0137.hostedemail.com [216.40.44.137]) by kanga.kvack.org (Postfix) with ESMTP id CA2D46B006E for ; Tue, 20 Apr 2021 21:05:18 -0400 (EDT) Received: from smtpin38.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 87CE218041D1D for ; Wed, 21 Apr 2021 01:05:18 +0000 (UTC) X-FDA: 78054580716.38.E38051C Received: from mga03.intel.com (mga03.intel.com [134.134.136.65]) by imf08.hostedemail.com (Postfix) with ESMTP id 504D280192EA for ; Wed, 21 Apr 2021 01:04:58 +0000 (UTC) IronPort-SDR: 5+tgAlOSZHzsZ8XiKuKksO8ge56J3Fb3c/U4sCXQ548dBfBtM9lyhztEN5BEWd7XVfQBXHSebi oifeeAE7gUOA== X-IronPort-AV: E=McAfee;i="6200,9189,9960"; a="195647955" X-IronPort-AV: E=Sophos;i="5.82,238,1613462400"; d="scan'208";a="195647955" Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 20 Apr 2021 18:05:15 -0700 IronPort-SDR: ofllOQnAqjGwclmN0l+mJiJRuoK9RnwgIKdVJf5xmnHS85bGQUHCejv2//dlEhk/CfD2v6qdl0 xSjfArYbKJkw== X-IronPort-AV: E=Sophos;i="5.82,238,1613462400"; d="scan'208";a="401322839" Received: from yhuang6-desk1.sh.intel.com (HELO yhuang6-desk1.ccr.corp.intel.com) ([10.239.13.1]) by orsmga002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 20 Apr 2021 18:05:11 -0700 From: "Huang, Ying" To: Miaohe Lin Cc: , , , , , , , , , , , , , Subject: Re: [PATCH v3 1/4] mm/swapfile: use percpu_ref to serialize against concurrent swapoff References: <20210420133048.6773-1-linmiaohe@huawei.com> <20210420133048.6773-2-linmiaohe@huawei.com> Date: Wed, 21 Apr 2021 09:05:08 +0800 In-Reply-To: <20210420133048.6773-2-linmiaohe@huawei.com> (Miaohe Lin's message of "Tue, 20 Apr 2021 09:30:45 -0400") Message-ID: <87wnsw1m7v.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-Server: rspam05 X-Rspamd-Queue-Id: 504D280192EA X-Stat-Signature: jcqj85z6og1nh76e8g94j3xumhrrfdcz Received-SPF: none (intel.com>: No applicable sender policy available) receiver=imf08; identity=mailfrom; envelope-from=""; helo=mga03.intel.com; client-ip=134.134.136.65 X-HE-DKIM-Result: none/none X-HE-Tag: 1618967098-304232 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: > Using current get/put_swap_device() to guard against concurrent swapoff > for some swap ops, e.g. swap_readpage(), looks terrible because they > might take really long time. This patch adds the percpu_ref support to > serialize against concurrent swapoff. Also we remove the SWP_VALID flag > because it's used together with RCU solution. > > Signed-off-by: Miaohe Lin > --- > include/linux/swap.h | 5 +-- > mm/swapfile.c | 79 +++++++++++++++++++++++++++----------------- > 2 files changed, 52 insertions(+), 32 deletions(-) > > diff --git a/include/linux/swap.h b/include/linux/swap.h > index 144727041e78..c9e7fea10b83 100644 > --- a/include/linux/swap.h > +++ b/include/linux/swap.h > @@ -177,7 +177,6 @@ enum { > SWP_PAGE_DISCARD = (1 << 10), /* freed swap page-cluster discards */ > SWP_STABLE_WRITES = (1 << 11), /* no overwrite PG_writeback pages */ > SWP_SYNCHRONOUS_IO = (1 << 12), /* synchronous IO is efficient */ > - SWP_VALID = (1 << 13), /* swap is valid to be operated on? */ > /* add others here before... */ > SWP_SCANNING = (1 << 14), /* refcount in scan_swap_map */ > }; > @@ -240,6 +239,7 @@ struct swap_cluster_list { > * The in-memory structure used to track swap areas. > */ > struct swap_info_struct { > + struct percpu_ref users; /* indicate and keep swap device valid. */ > 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 +260,7 @@ 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 */ > @@ -511,7 +512,7 @@ sector_t swap_page_sector(struct page *page); > > static inline void put_swap_device(struct swap_info_struct *si) > { > - rcu_read_unlock(); > + percpu_ref_put(&si->users); > } > > #else /* CONFIG_SWAP */ > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 149e77454e3c..a640fc84be5b 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -39,6 +39,7 @@ > #include > #include > #include > +#include > > #include > #include > @@ -511,6 +512,14 @@ 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); > +} > + > static void alloc_cluster(struct swap_info_struct *si, unsigned long idx) > { > struct swap_cluster_info *ci = si->cluster_info; > @@ -1270,18 +1279,12 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p, > * via preventing the swap device from being swapoff, until > * put_swap_device() is called. Otherwise return NULL. > * > - * The entirety of the RCU read critical section must come before the > - * return from or after the call to synchronize_rcu() in > - * enable_swap_info() or swapoff(). So if "si->flags & SWP_VALID" is > - * true, the si->map, si->cluster_info, etc. must be valid in the > - * critical section. > - * > * Notice that swapoff or swapoff+swapon can still happen before the > - * rcu_read_lock() in get_swap_device() or after the rcu_read_unlock() > - * in put_swap_device() if there isn't any other way to prevent > - * swapoff, such as page lock, page table lock, etc. The caller must > - * be prepared for that. For example, the following situation is > - * possible. > + * percpu_ref_tryget_live() in get_swap_device() or after the > + * percpu_ref_put() in put_swap_device() if there isn't any other way > + * to prevent swapoff, such as page lock, page table lock, etc. The > + * caller must be prepared for that. For example, the following > + * situation is possible. > * > * CPU1 CPU2 > * do_swap_page() > @@ -1309,21 +1312,27 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry) > si = swp_swap_info(entry); > if (!si) > goto bad_nofile; > - > - rcu_read_lock(); > - if (data_race(!(si->flags & SWP_VALID))) > - goto unlock_out; > + if (!percpu_ref_tryget_live(&si->users)) > + goto out; > + /* > + * Guarantee the si->users are checked before accessing other > + * fields of swap_info_struct. > + * > + * Paired with the spin_unlock() after setup_swap_info() in > + * enable_swap_info(). > + */ > + smp_rmb(); > offset = swp_offset(entry); > if (offset >= si->max) > - goto unlock_out; > + goto put_out; > > return si; > bad_nofile: > pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val); > out: > return NULL; > -unlock_out: > - rcu_read_unlock(); > +put_out: > + percpu_ref_put(&si->users); > return NULL; > } > > @@ -2466,7 +2475,7 @@ static void setup_swap_info(struct swap_info_struct *p, int prio, > > static void _enable_swap_info(struct swap_info_struct *p) > { > - p->flags |= SWP_WRITEOK | SWP_VALID; > + p->flags |= SWP_WRITEOK; > atomic_long_add(p->pages, &nr_swap_pages); > total_swap_pages += p->pages; > > @@ -2497,10 +2506,9 @@ static void enable_swap_info(struct swap_info_struct *p, int prio, > spin_unlock(&p->lock); > spin_unlock(&swap_lock); > /* > - * Guarantee swap_map, cluster_info, etc. fields are valid > - * between get/put_swap_device() if SWP_VALID bit is set > + * Finished initialized swap device, now it's safe to reference it. s/initialized/initializing/ Otherwise looks good to me! Thanks! Reviewed-by: "Huang, Ying" > */ > - synchronize_rcu(); > + percpu_ref_resurrect(&p->users); > spin_lock(&swap_lock); > spin_lock(&p->lock); > _enable_swap_info(p); > @@ -2616,16 +2624,16 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile) > > reenable_swap_slots_cache_unlock(); > > - spin_lock(&swap_lock); > - spin_lock(&p->lock); > - p->flags &= ~SWP_VALID; /* mark swap device as invalid */ > - spin_unlock(&p->lock); > - spin_unlock(&swap_lock); > /* > - * wait for swap operations protected by get/put_swap_device() > - * to complete > + * 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. > */ > + percpu_ref_kill(&p->users); > synchronize_rcu(); > + wait_for_completion(&p->comp); > > flush_work(&p->discard_work); > > @@ -2857,6 +2865,12 @@ static struct swap_info_struct *alloc_swap_info(void) > if (!p) > return ERR_PTR(-ENOMEM); > > + if (percpu_ref_init(&p->users, swap_users_ref_free, > + PERCPU_REF_INIT_DEAD, GFP_KERNEL)) { > + kvfree(p); > + return ERR_PTR(-ENOMEM); > + } > + > spin_lock(&swap_lock); > for (type = 0; type < nr_swapfiles; type++) { > if (!(swap_info[type]->flags & SWP_USED)) > @@ -2864,6 +2878,7 @@ static struct swap_info_struct *alloc_swap_info(void) > } > if (type >= MAX_SWAPFILES) { > spin_unlock(&swap_lock); > + percpu_ref_exit(&p->users); > kvfree(p); > return ERR_PTR(-EPERM); > } > @@ -2891,9 +2906,13 @@ static struct swap_info_struct *alloc_swap_info(void) > plist_node_init(&p->avail_lists[i], 0); > p->flags = SWP_USED; > spin_unlock(&swap_lock); > - kvfree(defer); > + if (defer) { > + percpu_ref_exit(&defer->users); > + kvfree(defer); > + } > spin_lock_init(&p->lock); > spin_lock_init(&p->cont_lock); > + init_completion(&p->comp); > > return p; > }