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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 F20ECC48BDF for ; Wed, 16 Jun 2021 01:23:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DE8C46137D for ; Wed, 16 Jun 2021 01:23:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231860AbhFPBZt (ORCPT ); Tue, 15 Jun 2021 21:25:49 -0400 Received: from mail.kernel.org ([198.145.29.99]:40148 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231858AbhFPBZt (ORCPT ); Tue, 15 Jun 2021 21:25:49 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id EEF6561159; Wed, 16 Jun 2021 01:23:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1623806623; bh=JouAOaKCTdqY1DBVgWnTWERrrO0w/43jM+7ztemVfbM=; h=Date:From:To:Subject:In-Reply-To:From; b=QVjUdCIX3xbgtrEmftKbAt4lPXwC4Jo+fPJTEBRR+jiSmn0U9/61AStAo47JIEPPu 4bvF7yx84vd4cszrMY9BbsacWT8l8PGeJfCWvJXeGeDUZRd76kiECIE5sMJ0KIE7kI WVtMRrahERtXT+rXLsQ91LzSasNaB3HlWAnyNf4c= Date: Tue, 15 Jun 2021 18:23:42 -0700 From: Andrew Morton To: akpm@linux-foundation.org, gavin.dg@linux.alibaba.com, hughd@google.com, kirill.shutemov@linux.intel.com, linux-mm@kvack.org, mm-commits@vger.kernel.org, stable@vger.kernel.org, torvalds@linux-foundation.org, willy@infradead.org, xuyu@linux.alibaba.com Subject: [patch 10/18] mm, thp: use head page in __migration_entry_wait() Message-ID: <20210616012342.r-f9_qYFs%akpm@linux-foundation.org> In-Reply-To: <20210615182248.9a0ba90e8e66b9f4a53c0d23@linux-foundation.org> User-Agent: s-nail v14.8.16 Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org From: Xu Yu Subject: mm, thp: use head page in __migration_entry_wait() We notice that hung task happens in a corner 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. Link: https://lkml.kernel.org/r/b9836c1dd522e903891760af9f0c86a2cce987eb.1623144009.git.xuyu@linux.alibaba.com 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 Acked-by: Kirill A. Shutemov Acked-by: Hugh Dickins Cc: Matthew Wilcox Cc: Signed-off-by: Andrew Morton --- mm/migrate.c | 1 + 1 file changed, 1 insertion(+) --- a/mm/migrate.c~mm-thp-use-head-page-in-__migration_entry_wait +++ a/mm/migrate.c @@ -295,6 +295,7 @@ void __migration_entry_wait(struct mm_st goto out; page = migration_entry_to_page(entry); + page = compound_head(page); /* * Once page cache replacement of page migration started, page_count _