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=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 D9CFFC433ED for ; Tue, 11 May 2021 01:40:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BAD01616E8 for ; Tue, 11 May 2021 01:40:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230399AbhEKBlo (ORCPT ); Mon, 10 May 2021 21:41:44 -0400 Received: from szxga05-in.huawei.com ([45.249.212.191]:2554 "EHLO szxga05-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230382AbhEKBlm (ORCPT ); Mon, 10 May 2021 21:41:42 -0400 Received: from DGGEMS411-HUB.china.huawei.com (unknown [172.30.72.60]) by szxga05-in.huawei.com (SkyGuard) with ESMTP id 4FfLCS4THPzkWVG; Tue, 11 May 2021 09:37:56 +0800 (CST) Received: from [10.174.176.232] (10.174.176.232) by DGGEMS411-HUB.china.huawei.com (10.3.19.211) with Microsoft SMTP Server id 14.3.498.0; Tue, 11 May 2021 09:40:33 +0800 Subject: Re: [PATCH] ksm: Revert "use GET_KSM_PAGE_NOLOCK to get ksm page in remove_rmap_item_from_tree()" To: Hugh Dickins CC: Andrew Morton , , References: <0e04877c-5b2d-b810-7464-108e793b84d3@huawei.com> From: Miaohe Lin Message-ID: Date: Tue, 11 May 2021 09:40:32 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.176.232] X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/5/11 7:42, Hugh Dickins wrote: > On Mon, 10 May 2021, Miaohe Lin wrote: >> On 2021/5/10 13:59, Hugh Dickins wrote: >>> This reverts commit 3e96b6a2e9ad929a3230a22f4d64a74671a0720b. >>> General Protection Fault in rmap_walk_ksm() under memory pressure: >>> remove_rmap_item_from_tree() needs to take page lock, of course. >>> >> >> I'am really sorry about it! And many thanks for this bugfix! >> It seems rmap_walk_ksm() relies on the page lock to protect against >> concurrent modifications to that page's node of the stable tree. >> Could you please add a comment in remove_rmap_item_from_tree() to >> clarify this in case similar trouble again? Many thanks! > > Sorry, no. Page lock is held by callers of stable_tree_append() when > adding an rmap_item to the tree, and held by callers of rmap_walk_ksm() > (see VM_BUG_ON_PAGE there) when walking the tree: you would surely > expect some kind of locking when removing an rmap_item from the tree, > and the appropriate page lock is what GET_KSM_PAGE_LOCK provided. > > I do not want us to go through the kernel source adding a comment > /* We really mean to take this lock: it protects against concurrency */ > every time we take a lock in the kernel: you should generally assume > that if a lock is taken, then the writer intended it to be taken. > > There are sure to be some exceptions, where a lock is taken pointlessly: > but please look deeper before assuming that is the case. > I see. I should have been more careful. Many thanks for your detailed explanation and sorry about trouble! > Hugh > >> >>> Signed-off-by: Hugh Dickins >>> --- >>> >>> mm/ksm.c | 3 ++- >>> 1 file changed, 2 insertions(+), 1 deletion(-) >>> >>> --- 5.13-rc1/mm/ksm.c 2021-05-09 17:03:44.010422188 -0700 >>> +++ linux/mm/ksm.c 2021-05-09 22:12:39.403008350 -0700 >>> @@ -776,11 +776,12 @@ static void remove_rmap_item_from_tree(s >>> struct page *page; >>> >>> stable_node = rmap_item->head; >>> - page = get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK); >>> + page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK); >>> if (!page) >>> goto out; >>> >>> hlist_del(&rmap_item->hlist); >>> + unlock_page(page); >>> put_page(page); >>> >>> if (!hlist_empty(&stable_node->hlist)) > . >