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,UNPARSEABLE_RELAY, URIBL_BLOCKED,USER_AGENT_SANE_1 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 62097C47082 for ; Tue, 8 Jun 2021 13:22:37 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id C69BF6128D for ; Tue, 8 Jun 2021 13:22:36 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C69BF6128D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 5DAE56B006C; Tue, 8 Jun 2021 09:22:36 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 58A966B006E; Tue, 8 Jun 2021 09:22:36 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 452766B0070; Tue, 8 Jun 2021 09:22:36 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0199.hostedemail.com [216.40.44.199]) by kanga.kvack.org (Postfix) with ESMTP id 1528D6B006C for ; Tue, 8 Jun 2021 09:22:36 -0400 (EDT) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 88B90180AD806 for ; Tue, 8 Jun 2021 13:22:35 +0000 (UTC) X-FDA: 78230621070.30.001749B Received: from out4436.biz.mail.alibaba.com (out4436.biz.mail.alibaba.com [47.88.44.36]) by imf14.hostedemail.com (Postfix) with ESMTP id DB5AAC00F785 for ; Tue, 8 Jun 2021 13:22:30 +0000 (UTC) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R181e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04420;MF=xuyu@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0Ublzg6m_1623158548; Received: from xuyu-mbp15.local(mailfrom:xuyu@linux.alibaba.com fp:SMTPD_---0Ublzg6m_1623158548) by smtp.aliyun-inc.com(127.0.0.1); Tue, 08 Jun 2021 21:22:28 +0800 Subject: Re: [PATCH v2] mm, thp: use head page in __migration_entry_wait To: "Kirill A. Shutemov" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, hughd@google.com, akpm@linux-foundation.org, gavin.dg@linux.alibaba.com References: <20210608120026.ugfh72ydjeba44bo@box.shutemov.name> From: Yu Xu Message-ID: <57e151a8-03b2-3458-0178-21edb4ce97d2@linux.alibaba.com> Date: Tue, 8 Jun 2021 21:22:28 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210608120026.ugfh72ydjeba44bo@box.shutemov.name> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Authentication-Results: imf14.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=alibaba.com; spf=pass (imf14.hostedemail.com: domain of xuyu@linux.alibaba.com designates 47.88.44.36 as permitted sender) smtp.mailfrom=xuyu@linux.alibaba.com X-Rspamd-Server: rspam02 X-Stat-Signature: 6dkygozqs4uyjx57gtkqn63aa7t4799u X-Rspamd-Queue-Id: DB5AAC00F785 X-HE-Tag: 1623158550-212249 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: On 6/8/21 8:00 PM, Kirill A. Shutemov wrote: > On Tue, Jun 08, 2021 at 05:22:39PM +0800, Xu Yu wrote: >> 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 > > Looks good to me: > > Acked-by: Kirill A. Shutemov > > But there's one quirk: if split succeed we effectively wait on wrong > page to be unlocked. And it may take indefinite time if split_huge_page() > was called on the head page. Inspired by you, I look into the codes, and have a new question (nothing to do with this patch). If we split_huge_page_to_list on *tail* page (in fact, I haven't seen that used yet), mm/huge_memory.c:2666 checks "VM_BUG_ON_PAGE(!PageLocked(head), head);" in split_huge_page_to_list(), while mm/huge_memory.c:2497 does "unlock_page(subpage)", where subpage can be head in this scenario, in __split_huge_page(). My confusion is 1) how the pin on the @subpage is got outside split_huge_page_to_list()? can we ever get tail page? 2) head page is locked outside split_huge_page_to_list(), but unlocked in __split_huge_page()? > > Maybe we should consider waking up head waiter on head page, even if it is > still locked after split? > > Something like this (untested): > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 63ed6b25deaa..f79a38e21e53 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -2535,6 +2535,9 @@ static void __split_huge_page(struct page *page, struct list_head *list, > */ > put_page(subpage); > } > + > + if (page == head) > + wake_up_page_bit(page, PG_locked); > } > > int total_mapcount(struct page *page) > -- Thanks, Yu