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=-16.7 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,USER_AGENT_GIT 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 D3C90C433ED for ; Thu, 8 Apr 2021 13:09:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9DFE561074 for ; Thu, 8 Apr 2021 13:09:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231661AbhDHNJg (ORCPT ); Thu, 8 Apr 2021 09:09:36 -0400 Received: from szxga06-in.huawei.com ([45.249.212.32]:16416 "EHLO szxga06-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231527AbhDHNJb (ORCPT ); Thu, 8 Apr 2021 09:09:31 -0400 Received: from DGGEMS413-HUB.china.huawei.com (unknown [172.30.72.59]) by szxga06-in.huawei.com (SkyGuard) with ESMTP id 4FGM4K6dVwzkjbq; Thu, 8 Apr 2021 21:07:29 +0800 (CST) Received: from huawei.com (10.175.104.175) by DGGEMS413-HUB.china.huawei.com (10.3.19.213) with Microsoft SMTP Server id 14.3.498.0; Thu, 8 Apr 2021 21:09:09 +0800 From: Miaohe Lin To: CC: , , , , , , , , , , , , , Subject: [PATCH 2/5] swap: fix do_swap_page() race with swapoff Date: Thu, 8 Apr 2021 09:08:17 -0400 Message-ID: <20210408130820.48233-3-linmiaohe@huawei.com> X-Mailer: git-send-email 2.19.1 In-Reply-To: <20210408130820.48233-1-linmiaohe@huawei.com> References: <20210408130820.48233-1-linmiaohe@huawei.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7BIT Content-Type: text/plain; charset=US-ASCII X-Originating-IP: [10.175.104.175] X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When I was investigating the swap code, I found the below possible race window: CPU 1 CPU 2 ----- ----- do_swap_page synchronous swap_readpage alloc_page_vma swapoff release swap_file, bdev, or ... swap_readpage check sis->flags is ok access swap_file, bdev...[oops!] si->flags = 0 Using current get/put_swap_device() to guard against concurrent swapoff for swap_readpage() looks terrible because swap_readpage() may take really long time. And this race may not be really pernicious because swapoff is usually done when system shutdown only. To reduce the performance overhead on the hot-path as much as possible, it appears we can use the percpu_ref to close this race window(as suggested by Huang, Ying). Fixes: 235b62176712 ("mm/swap: add cluster lock") Signed-off-by: Miaohe Lin --- include/linux/swap.h | 2 +- mm/memory.c | 10 ++++++++++ mm/swapfile.c | 28 +++++++++++----------------- 3 files changed, 22 insertions(+), 18 deletions(-) diff --git a/include/linux/swap.h b/include/linux/swap.h index 849ba5265c11..9066addb57fd 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -513,7 +513,7 @@ 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/memory.c b/mm/memory.c index cc71a445c76c..8543c47b955c 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3311,6 +3311,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) { struct vm_area_struct *vma = vmf->vma; struct page *page = NULL, *swapcache; + struct swap_info_struct *si = NULL; swp_entry_t entry; pte_t pte; int locked; @@ -3339,6 +3340,11 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) } + si = get_swap_device(entry); + /* In case we raced with swapoff. */ + if (unlikely(!si)) + goto out; + delayacct_set_flag(DELAYACCT_PF_SWAPIN); page = lookup_swap_cache(entry, vma, vmf->address); swapcache = page; @@ -3514,6 +3520,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) unlock: pte_unmap_unlock(vmf->pte, vmf->ptl); out: + if (si) + put_swap_device(si); return ret; out_nomap: pte_unmap_unlock(vmf->pte, vmf->ptl); @@ -3525,6 +3533,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) unlock_page(swapcache); put_page(swapcache); } + if (si) + put_swap_device(si); return ret; } diff --git a/mm/swapfile.c b/mm/swapfile.c index 724173cd7d0c..01032c72ceae 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -1280,18 +1280,12 @@ 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() @@ -1319,21 +1313,21 @@ 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; + goto out; + if (!percpu_ref_tryget_live(&si->users)) + goto out; 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; } -- 2.19.1