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=-16.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,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 EF3D8C47082 for ; Tue, 8 Jun 2021 09:22:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D267E6124B for ; Tue, 8 Jun 2021 09:22:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230160AbhFHJYk (ORCPT ); Tue, 8 Jun 2021 05:24:40 -0400 Received: from out30-56.freemail.mail.aliyun.com ([115.124.30.56]:44065 "EHLO out30-56.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229507AbhFHJYj (ORCPT ); Tue, 8 Jun 2021 05:24:39 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R201e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04400;MF=xuyu@linux.alibaba.com;NM=1;PH=DS;RN=5;SR=0;TI=SMTPD_---0UbkfDnd_1623144164; Received: from localhost(mailfrom:xuyu@linux.alibaba.com fp:SMTPD_---0UbkfDnd_1623144164) by smtp.aliyun-inc.com(127.0.0.1); Tue, 08 Jun 2021 17:22:45 +0800 From: Xu Yu To: linux-mm@kvack.org, linux-kernel@vger.kernel.org, hughd@google.com Cc: akpm@linux-foundation.org, gavin.dg@linux.alibaba.com Subject: [PATCH v2] mm, thp: use head page in __migration_entry_wait Date: Tue, 8 Jun 2021 17:22:39 +0800 Message-Id: X-Mailer: git-send-email 2.20.1.2432.ga663e714 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org We notice that hung task happens in a conner but practical scenario when CONFIG_PREEMPT_NONE is enabled, as follows. Process 0 Process 1 Process 2..Inf split_huge_page_to_list unmap_page split_huge_pmd_address __migration_entry_wait(head) __migration_entry_wait(tail) remap_page (roll back) remove_migration_ptes rmap_walk_anon cond_resched Where __migration_entry_wait(tail) is occurred in kernel space, e.g., copy_to_user in fstat, which will immediately fault again without rescheduling, and thus occupy the cpu fully. When there are too many processes performing __migration_entry_wait on tail page, remap_page will never be done after cond_resched. This makes __migration_entry_wait operate on the compound head page, thus waits for remap_page to complete, whether the THP is split successfully or roll back. Note that put_and_wait_on_page_locked helps to drop the page reference acquired with get_page_unless_zero, as soon as the page is on the wait queue, before actually waiting. So splitting the THP is only prevented for a brief interval. Fixes: ba98828088ad ("thp: add option to setup migration entries during PMD split") Suggested-by: Hugh Dickins Signed-off-by: Gang Deng Signed-off-by: Xu Yu --- mm/migrate.c | 1 + 1 file changed, 1 insertion(+) diff --git a/mm/migrate.c b/mm/migrate.c index b234c3f3acb7..41ff2c9896c4 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -295,6 +295,7 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep, goto out; page = migration_entry_to_page(entry); + page = compound_head(page); /* * Once page cache replacement of page migration started, page_count -- 2.20.1.2432.ga663e714