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=-10.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FORGED_MUA_MOZILLA,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 DD4E2C433E6 for ; Thu, 14 Jan 2021 21:51:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B094B23A5E for ; Thu, 14 Jan 2021 21:51:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730242AbhANVvZ (ORCPT ); Thu, 14 Jan 2021 16:51:25 -0500 Received: from a4-6.smtp-out.eu-west-1.amazonses.com ([54.240.4.6]:41485 "EHLO a4-6.smtp-out.eu-west-1.amazonses.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730236AbhANVvX (ORCPT ); Thu, 14 Jan 2021 16:51:23 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/simple; s=pqvuhxtqt36lwjpmqkszlz7wxaih4qwj; d=urbackup.org; t=1610661005; h=Subject:From:To:References:Message-ID:Date:MIME-Version:In-Reply-To:Content-Type:Content-Transfer-Encoding; bh=R8R4MphPd6WmnFqxAVEK8yhz00h89byOfXrnAMCkFMw=; b=ZgTQMyw+9oFC7Hzg8iUsIh6GDnTILOAUpv68f4bWs1kiDmo/8k40cGlq/ZzOd9j6 tp+cK0j0dTxKcl9IixPpxZirlEK//fnetFUqUzEjZXQCnQeblY8tcJOYktfTvpuGOa6 BoqinzkmRJA2ZrHJQh6GKNPIzIV6arb/EASYtbZc= DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/simple; s=shh3fegwg5fppqsuzphvschd53n6ihuv; d=amazonses.com; t=1610661005; h=Subject:From:To:References:Message-ID:Date:MIME-Version:In-Reply-To:Content-Type:Content-Transfer-Encoding:Feedback-ID; bh=R8R4MphPd6WmnFqxAVEK8yhz00h89byOfXrnAMCkFMw=; b=n5iL7DKYJirCEQcVapIvexBZst5HkeFvUe4jJdIjhNNrJT0tBS/s+oVvf1bXhial z4bmN1Caz/XGIScTIGwTTiPHlpmVbdGqlFvbmuBT6p6xsSyW0v4b6xdCZwDYmLKVQxH /4Bp4vIqLdv0wXgoSYK4vo+SbnyOsXyvynSUs6rA= Subject: Re: Fixed buffers have out-dated content From: Martin Raiber To: Pavel Begunkov , Jens Axboe , io-uring@vger.kernel.org References: <01020176e45e6c4d-c15dc1e2-6a6a-407c-a32d-24be51a1b3f8-000000@eu-west-1.amazonses.com> <8ba549a0-7724-a42f-bd11-3605ef0bd034@kernel.dk> <01020176e8159fa5-3f556133-fda7-451b-af78-94c712df611e-000000@eu-west-1.amazonses.com> <01020176ed350725-cc3c8fa7-7771-46c9-8fa9-af433acb2453-000000@eu-west-1.amazonses.com> Message-ID: <0102017702e086ca-cdb34993-86ad-4ec6-bea5-b6a5ad055a62-000000@eu-west-1.amazonses.com> Date: Thu, 14 Jan 2021 21:50:05 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.1 MIME-Version: 1.0 In-Reply-To: <01020176ed350725-cc3c8fa7-7771-46c9-8fa9-af433acb2453-000000@eu-west-1.amazonses.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit X-SES-Outgoing: 2021.01.14-54.240.4.6 Feedback-ID: 1.eu-west-1.zKMZH6MF2g3oUhhjaE2f3oQ8IBjABPbvixQzV8APwT0=:AmazonSES Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 10.01.2021 17:50 Martin Raiber wrote: > On 09.01.2021 21:32 Pavel Begunkov wrote: >> On 09/01/2021 16:58, Martin Raiber wrote: >>> On 09.01.2021 17:23 Jens Axboe wrote: >>>> On 1/8/21 4:39 PM, Martin Raiber wrote: >>>>> Hi, >>>>> >>>>> I have a gnarly issue with io_uring and fixed buffers (fixed >>>>> read/write). It seems the contents of those buffers contain old >>>>> data in >>>>> some rare cases under memory pressure after a read/during a write. >>>>> >>>>> Specifically I use io_uring with fuse and to confirm this is not some >>>>> user space issue let fuse print the unique id it adds to each >>>>> request. >>>>> Fuse adds this request data to a pipe, and when the pipe buffer is >>>>> later >>>>> copied to the io_uring fixed buffer it has the id of a fuse request >>>>> returned earlier using the same buffer while returning the size of >>>>> the >>>>> new request. Or I set the unique id in the buffer, write it to >>>>> fuse (via >>>>> writing to a pipe, then splicing) and then fuse returns with e.g. >>>>> ENOENT, because the unique id is not correct because in kernel it >>>>> reads >>>>> the id of the previous, already completed, request using this buffer. >>>>> >>>>> To make reproducing this faster running memtester (which mlocks a >>>>> configurable amount of memory) with a large amount of user memory >>>>> every >>>>> 30s helps. So it has something to do with swapping? It seems to not >>>>> occur if no swap space is active. Problem occurs without warning when >>>>> the kernel is build with KASAN and slab debugging. >>>>> >>>>> If I don't use the _FIXED opcodes (which is easy to do), the problem >>>>> does not occur. >>>>> >>>>> Problem occurs with 5.9.16 and 5.10.5. >>>> Can you mention more about what kind of IO you are doing, I'm assuming >>>> it's O_DIRECT? I'll see if I can reproduce this. >>> It's writing to/reading from pipes (nonblocking, no O_DIRECT). >> A blind guess, does it handle short reads and writes? If not, can you >> check whether they happen or not? > > Something like this was what I suspected at first as well. It does > check for short read/writes and I added (unnecessary -- because the > fuse request structure is 40 bytes and it does io in page sizes) code > for retrying short reads at some point. I also checked for the pipes > to be empty before they are used at some point and let the kernel log > allocation failures (idea was that it was short pipe read/writes > because of allocation failure or that something doesn't get rewound > properly in this case). Beyond that three things that make a user > space problem unlikely: > >  - occurs only when using fixed buffers and does not occur when > running same code without fixed buffer opcodes >  - doesn't occur when there is no memory pressure >  - I added print(k/f) logging that pointed me in this direction as well > >>> I can reproduce it with https://github.com/uroni/fuseuring on e.g. a >>> 2GB VPS. Modify bench.sh so that fio loops. Add swap, then run 1400M >>> memtester while it runs (so it swaps, I guess). I can try further >>> reducing the reproducer, but I wanted to avoid that work in case it >>> is something obvious. The next step would be to remove fuse from the >>> equation -- it does try to move the pages from the pipe when >>> splicing to it, for example. When I use 5.10.7 with 09854ba94c6aad7886996bfbee2530b3d8a7f4f4 ("mm: do_wp_page() simplification"), 1a0cf26323c80e2f1c58fc04f15686de61bfab0c ("mm/ksm: Remove reuse_ksm_page()") and be068f29034fb00530a053d18b8cf140c32b12b3 ("mm: fix misplaced unlock_page in do_wp_page()") reverted the issue doesn't seem to occur.