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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 529D6C433F5 for ; Fri, 24 Sep 2021 19:59:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 343A061107 for ; Fri, 24 Sep 2021 19:59:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348298AbhIXUBQ (ORCPT ); Fri, 24 Sep 2021 16:01:16 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:26654 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1348292AbhIXUBK (ORCPT ); Fri, 24 Sep 2021 16:01:10 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1632513576; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Fkb5V1jxKjOW0w30aVi0k+S3Ap2kOL1t5uX8+Txh3Lk=; b=R63vfxOSORznBydHfGObQmiNRjdGZ3lrMJ1u8Klj4ZacpHIc8qTo8hX9VSNl3tqjgS5Isq +ukse8Tsu8Mp0KqUWPXp+g/O37Yme+cwIvfWidbBngkWSe5xhH9pHBgLQRkpS6VeJhZPU1 Qg+uo/H4XDhDI0+otaQkZnbTFbouBSU= Received: from mail-qt1-f199.google.com (mail-qt1-f199.google.com [209.85.160.199]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-334-kV8i1DZXNWSrz6m5KXSgKw-1; Fri, 24 Sep 2021 15:59:35 -0400 X-MC-Unique: kV8i1DZXNWSrz6m5KXSgKw-1 Received: by mail-qt1-f199.google.com with SMTP id m26-20020ac87b5a000000b002a6b3744ae6so32362237qtu.22 for ; Fri, 24 Sep 2021 12:59:35 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=Fkb5V1jxKjOW0w30aVi0k+S3Ap2kOL1t5uX8+Txh3Lk=; b=XOBjLWJcEGo8DJFBwtHUKys8jun7xKT9KHXGheeZkDYG4hA6IduS1vD4oM/PC+A8dh fBXZuJeACnpRgMsOCjLiJ05xkB2N44KWCcFRZPwiiML3KsNQ4jft+u76qiHh0tN4cyJD DKesvHWqoAx2gHAp/xy+WNL0sznWmUHflN+9ktMJ6yrKE4JfaSmexiybarTEWIkyvJy/ lAWO14MTO7zfK69LsQoHoAWbZZQRSmRkJSTf958OU3lSVFG1cHKdx5weIsiulYWtgFxY MQk3Oh26Wzz8/fprIET4aIDSX5M3FEPg9m/cvJa8m1kvMSWLIeoNPXa2/obOleLD+47G ybPw== X-Gm-Message-State: AOAM532Yk4epi0hv6jhiuZX4wBQJmd8mAirKPd8L6r772oFe1XgIwAJY lLsWSl4b63iqTu5Jg2J+3lDqCrShR7NKXYVowNqcljtadeCBOAaAVgICbueDo6Bmn18hvqqb82j KkycOwvAC3KLMSb7gAUGtP4sD X-Received: by 2002:ad4:49c5:: with SMTP id j5mr4092373qvy.4.1632513574889; Fri, 24 Sep 2021 12:59:34 -0700 (PDT) X-Google-Smtp-Source: ABdhPJweeqq4+kBGkggxeqXp/1y2/jyMtv0/bMMsIQiahlx2ty6tYQUjL3Br/cz80YAVexQZo7pRQw== X-Received: by 2002:ad4:49c5:: with SMTP id j5mr4092318qvy.4.1632513574303; Fri, 24 Sep 2021 12:59:34 -0700 (PDT) Received: from t490s ([2607:fea8:56a2:9100::d3ec]) by smtp.gmail.com with ESMTPSA id v7sm7509431qkd.41.2021.09.24.12.59.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Sep 2021 12:59:33 -0700 (PDT) Date: Fri, 24 Sep 2021 15:59:32 -0400 From: Peter Xu To: Axel Rasmussen Cc: Linux MM , LKML , Andrew Morton , Andrea Arcangeli , Nadav Amit , Li Wang Subject: Re: [PATCH] mm/userfaultfd: selftests: Fix memory corruption with thp enabled Message-ID: References: <20210923232512.210092-1-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 24, 2021 at 10:21:30AM -0700, Axel Rasmussen wrote: > On Thu, Sep 23, 2021 at 4:25 PM Peter Xu wrote: > > > > In RHEL's gating selftests we've encountered memory corruption in the uffd > > event test even with upstream kernel: > > > > # ./userfaultfd anon 128 4 > > nr_pages: 32768, nr_pages_per_cpu: 32768 > > bounces: 3, mode: rnd racing read, userfaults: 6240 missing (6240) 14729 wp (14729) > > bounces: 2, mode: racing read, userfaults: 1444 missing (1444) 28877 wp (28877) > > bounces: 1, mode: rnd read, userfaults: 6055 missing (6055) 14699 wp (14699) > > bounces: 0, mode: read, userfaults: 82 missing (82) 25196 wp (25196) > > testing uffd-wp with pagemap (pgsize=4096): done > > testing uffd-wp with pagemap (pgsize=2097152): done > > testing events (fork, remap, remove): ERROR: nr 32427 memory corruption 0 1 (errno=0, line=963) > > ERROR: faulting process failed (errno=0, line=1117) > > > > It can be easily reproduced when global thp enabled, which is the default for > > RHEL. > > > > It's also known as a side effect of commit 0db282ba2c12 ("selftest: use mmap > > instead of posix_memalign to allocate memory", 2021-07-23), which is imho right > > itself on using mmap() to make sure the addresses will be untagged even on arm. > > > > The problem is, for each test we allocate buffers using two allocate_area() > > calls. We assumed these two buffers won't affect each other, however they > > could, because mmap() could have found that the two buffers are near each other > > and having the same VMA flags, so they got merged into one VMA. > > > > It won't be a big problem if thp is not enabled, but when thp is agressively > > enabled it means when initializing the src buffer it could accidentally setup > > part of the dest buffer too when there's a shared THP that overlaps the two > > regions. Then some of the dest buffer won't be able to be trapped by > > userfaultfd missing mode, then it'll cause memory corruption as described. > > > > To fix it, do release_pages() after initializing the src buffer. > > But, if I understand correctly, release_pages() will just free the > physical pages, but not touch the VMA(s). So, with the right > max_ptes_none setting, why couldn't khugepaged just decide to > re-collapse (with zero pages) immediately after we release the pages, > causing the same problem? It seems to me this change just > significantly narrows the race window (which explains why we see less > of the issue), but doesn't fix it fundamentally. Did you mean you can reproduce the issue even with this patch? It is a good point anyway, indeed I don't see anything stops it from happening. I wanted to prepare a v2 by releasing the pages after uffdio registration where we'll do the vma split, but it won't simply work because release_pages() will cause the process to hang death since that test registers with EVENT_REMOVE, and release_pages() upon the thp will trigger synchronous EVENT_REMOVE which cannot be handled by anyone. Another solution is to map some PROT_NONE regions between the buffers, to make sure they won't share a VMA. I'll need to think more about which is better.. -- Peter Xu