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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=no 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 0BCCFC43457 for ; Tue, 13 Oct 2020 05:31:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CB9A121BE5 for ; Tue, 13 Oct 2020 05:31:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732408AbgJMFbU (ORCPT ); Tue, 13 Oct 2020 01:31:20 -0400 Received: from out30-45.freemail.mail.aliyun.com ([115.124.30.45]:41788 "EHLO out30-45.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726963AbgJMFbU (ORCPT ); Tue, 13 Oct 2020 01:31:20 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R131e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=alimailimapcm10staff010182156082;MF=haoxu@linux.alibaba.com;NM=1;PH=DS;RN=4;SR=0;TI=SMTPD_---0UBttVd3_1602567077; Received: from B-25KNML85-0107.local(mailfrom:haoxu@linux.alibaba.com fp:SMTPD_---0UBttVd3_1602567077) by smtp.aliyun-inc.com(127.0.0.1); Tue, 13 Oct 2020 13:31:18 +0800 Subject: Re: Loophole in async page I/O To: Jens Axboe , Matthew Wilcox , io-uring@vger.kernel.org Cc: Johannes Weiner References: <20201012211355.GC20115@casper.infradead.org> <14d97ab3-edf7-c72a-51eb-d335e2768b65@kernel.dk> From: Hao_Xu Message-ID: <61743c36-ff6a-fce6-a3c4-55ec1c3f1cfa@linux.alibaba.com> Date: Tue, 13 Oct 2020 13:31:17 +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: <14d97ab3-edf7-c72a-51eb-d335e2768b65@kernel.dk> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 在 2020/10/13 上午6:08, Jens Axboe 写道: > On 10/12/20 3:13 PM, Matthew Wilcox wrote: >> 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. > > If NOWAIT isn't set, then the issue attempt is from the helper thread > already, and IOCB_WAITQ shouldn't be set either (the latter doesn't > matter for this discussion). So it's totally fine and expected to block > at that point. > > Hmm actually, I believe that: > > commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 > Author: Hao Xu > Date: Tue Sep 29 20:00:45 2020 +0800 > > io_uring: fix async buffered reads when readahead is disabled > > maybe messed up that case, so we could block off the retry-path. I'll > take a closer look, looks like that can be the case if read-ahead is > disabled. > > In general, we can only return -EIOCBQUEUED if the IO has been started > or is in progress already. That means we can safely rely on being told > when it's unlocked/done. If we need to block, we should be returning > -EAGAIN, which would punt to a worker thread. > Hi Jens, My undertanding of io_uring buffered reads process after the commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 has been merged is: the first io_uring IO try is with IOCB_NOWAIT, the second retry in the same context is with IOCB_WAITQ but without IOCB_NOWAIT. so in Matthew's case, lock_page_async() will be called after calling mapping->a_ops->readpage(), So it won't end up sleeping. Actually this case is what happens when readahead is disabled or somehow skipped for reasons like blk_cgroup_congested() returns true. And this case is my commit c8d317aa1887b40b188e for. Regards, Hao