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=-8.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,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 5340CC433E7 for ; Tue, 13 Oct 2020 05:13:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2614620878 for ; Tue, 13 Oct 2020 05:13:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732295AbgJMFNv (ORCPT ); Tue, 13 Oct 2020 01:13:51 -0400 Received: from out30-42.freemail.mail.aliyun.com ([115.124.30.42]:54986 "EHLO out30-42.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729939AbgJMFNv (ORCPT ); Tue, 13 Oct 2020 01:13:51 -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=e01e01424;MF=haoxu@linux.alibaba.com;NM=1;PH=DS;RN=4;SR=0;TI=SMTPD_---0UBuAlRH_1602566028; Received: from B-25KNML85-0107.local(mailfrom:haoxu@linux.alibaba.com fp:SMTPD_---0UBuAlRH_1602566028) by smtp.aliyun-inc.com(127.0.0.1); Tue, 13 Oct 2020 13:13:49 +0800 Subject: Re: Loophole in async page I/O To: Matthew Wilcox , io-uring@vger.kernel.org Cc: Johannes Weiner , Jens Axboe References: <20201012211355.GC20115@casper.infradead.org> From: Hao_Xu Message-ID: <6e341fd1-bd2a-7774-5323-41f3a0531295@linux.alibaba.com> Date: Tue, 13 Oct 2020 13:13:48 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.3.2 MIME-Version: 1.0 In-Reply-To: <20201012211355.GC20115@casper.infradead.org> Content-Type: text/plain; charset=gbk; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org ÔÚ 2020/10/13 ÉÏÎç5:13, Matthew Wilcox дµÀ: > This one's pretty unlikely, but there's a case in buffered reads where > an IOCB_WAITQ read can end up sleeping. > > generic_file_buffered_read(): > page = find_get_page(mapping, index); > ... > if (!PageUptodate(page)) { > ... > if (iocb->ki_flags & IOCB_WAITQ) { > ... > error = wait_on_page_locked_async(page, > iocb->ki_waitq); > wait_on_page_locked_async(): > if (!PageLocked(page)) > return 0; > (back to generic_file_buffered_read): > if (!mapping->a_ops->is_partially_uptodate(page, > offset, iter->count)) > goto page_not_up_to_date_locked; > > page_not_up_to_date_locked: > if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { > unlock_page(page); > put_page(page); > goto would_block; > } > ... > error = mapping->a_ops->readpage(filp, page); > (will unlock page on I/O completion) > if (!PageUptodate(page)) { > error = lock_page_killable(page); > > So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() > and wait for the I/O to complete. I can't quite figure out if this is > intentional -- I think not; if I understand the semantics right, we > should be returning -EIOCBQUEUED and punting to an I/O thread to > kick off the I/O and wait. > > I think the right fix is to return -EIOCBQUEUED from > wait_on_page_locked_async() if the page isn't locked. ie this: > > @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, > struct wait_page_queue *wait) > { > if (!PageLocked(page)) > - return 0; > + return -EIOCBQUEUED; > return __wait_on_page_locked_async(compound_head(page), wait, false); > } > > But as I said, I'm not sure what the semantics are supposed to be. > Hi Matthew, which kernel version are you use, I believe I've fixed this case in the commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 in this commit, I did the modification: diff --git a/mm/filemap.c b/mm/filemap.c index 1aaea26556cc..ea383478fc22 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2267,7 +2267,11 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, } if (!PageUptodate(page)) { - error = lock_page_killable(page); + if (iocb->ki_flags & IOCB_WAITQ) + error = lock_page_async(page, iocb->ki_waitq); + else + error = lock_page_killable(page); + if (unlikely(error)) goto readpage_error; if (!PageUptodate(page)) { lock_page_killable() won't be called in this case.