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=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 1872BC433ED for ; Tue, 13 Apr 2021 01:27:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E8680611F2 for ; Tue, 13 Apr 2021 01:27:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240276AbhDMB1x (ORCPT ); Mon, 12 Apr 2021 21:27:53 -0400 Received: from mga05.intel.com ([192.55.52.43]:3028 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237080AbhDMB1w (ORCPT ); Mon, 12 Apr 2021 21:27:52 -0400 IronPort-SDR: oarlFEOgE/Wl4aXkyADnwTm10sYT6+xy6PsNeP9n8mZpiHmKid17Fxu2GvZxkPsOZQ8ymXNFFg Uxm16i193rSw== X-IronPort-AV: E=McAfee;i="6200,9189,9952"; a="279617814" X-IronPort-AV: E=Sophos;i="5.82,216,1613462400"; d="scan'208";a="279617814" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by fmsmga105.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 12 Apr 2021 18:27:32 -0700 IronPort-SDR: 4bK8dtNIYAcL5D0+3xpb3eoVaW2oN7QyDqGY9Fcy+bUjAt+5yF+e0ZFlbpo3lxGBiyJXdvY6xC m0mXSysFT/CQ== X-IronPort-AV: E=Sophos;i="5.82,216,1613462400"; d="scan'208";a="424041399" 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; 12 Apr 2021 18:27:27 -0700 From: "Huang, Ying" To: Miaohe Lin Cc: , , , , , , , , , , , , , Matthew Wilcox Subject: Re: [PATCH 2/5] swap: fix do_swap_page() race with swapoff References: <20210408130820.48233-1-linmiaohe@huawei.com> <20210408130820.48233-3-linmiaohe@huawei.com> Date: Tue, 13 Apr 2021 09:27:25 +0800 In-Reply-To: <20210408130820.48233-3-linmiaohe@huawei.com> (Miaohe Lin's message of "Thu, 8 Apr 2021 09:08:17 -0400") Message-ID: <87o8ejug76.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 Miaohe Lin writes: > 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") This isn't the commit that introduces the race. You can use `git blame` find out the correct commit. For this it's commit 0bcac06f27d7 "mm, swap: skip swapcache for swapin of synchronous device". And I suggest to merge 1/5 and 2/5 to make it easy to get the full picture. > 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) > } > > I suggest to add comments here as follows (words copy from Matthew Wilcox) /* Prevent swapoff from happening to us */ > + si = get_swap_device(entry); > + /* In case we raced with swapoff. */ > + if (unlikely(!si)) > + goto out; > + Because we wrap the whole do_swap_page() with get/put_swap_device() now. We can remove several get/put_swap_device() for function called by do_swap_page(). That can be another optimization patch. > 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))) We can delete SWP_VALID, that is used together with RCU solution. > - 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; > } Best Regards, Huang, Ying 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 7E0F1C433B4 for ; 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Tue, 13 Apr 2021 01:27:37 +0000 (UTC) X-FDA: 78025606554.03.600D862 Received: from mga18.intel.com (mga18.intel.com [134.134.136.126]) by imf15.hostedemail.com (Postfix) with ESMTP id 73A8CA000381 for ; Tue, 13 Apr 2021 01:27:35 +0000 (UTC) IronPort-SDR: +xaO4LxrNbl9s1P1jxiUKSJRTyqQvnCauPEbM8b+PhAVGrS+0TzVYs9KKba2uoP0SKYdJ5MSnW RwIIRbKSmxPQ== X-IronPort-AV: E=McAfee;i="6200,9189,9952"; a="181832165" X-IronPort-AV: E=Sophos;i="5.82,216,1613462400"; d="scan'208";a="181832165" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 12 Apr 2021 18:27:33 -0700 IronPort-SDR: 4bK8dtNIYAcL5D0+3xpb3eoVaW2oN7QyDqGY9Fcy+bUjAt+5yF+e0ZFlbpo3lxGBiyJXdvY6xC m0mXSysFT/CQ== X-IronPort-AV: E=Sophos;i="5.82,216,1613462400"; d="scan'208";a="424041399" 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; 12 Apr 2021 18:27:27 -0700 From: "Huang, Ying" To: Miaohe Lin Cc: , , , , , , , , , , , , , Matthew Wilcox Subject: Re: [PATCH 2/5] swap: fix do_swap_page() race with swapoff References: <20210408130820.48233-1-linmiaohe@huawei.com> <20210408130820.48233-3-linmiaohe@huawei.com> Date: Tue, 13 Apr 2021 09:27:25 +0800 In-Reply-To: <20210408130820.48233-3-linmiaohe@huawei.com> (Miaohe Lin's message of "Thu, 8 Apr 2021 09:08:17 -0400") Message-ID: <87o8ejug76.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-Stat-Signature: hgqt9ft3nrztgddtraszr9d6bnsrsm11 X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 73A8CA000381 Received-SPF: none (intel.com>: No applicable sender policy available) receiver=imf15; identity=mailfrom; envelope-from=""; helo=mga18.intel.com; client-ip=134.134.136.126 X-HE-DKIM-Result: none/none X-HE-Tag: 1618277255-417652 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: > 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") This isn't the commit that introduces the race. You can use `git blame` find out the correct commit. For this it's commit 0bcac06f27d7 "mm, swap: skip swapcache for swapin of synchronous device". And I suggest to merge 1/5 and 2/5 to make it easy to get the full picture. > 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) > } > > I suggest to add comments here as follows (words copy from Matthew Wilcox) /* Prevent swapoff from happening to us */ > + si = get_swap_device(entry); > + /* In case we raced with swapoff. */ > + if (unlikely(!si)) > + goto out; > + Because we wrap the whole do_swap_page() with get/put_swap_device() now. We can remove several get/put_swap_device() for function called by do_swap_page(). That can be another optimization patch. > 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))) We can delete SWP_VALID, that is used together with RCU solution. > - 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; > } Best Regards, Huang, Ying