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=-13.6 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1, USER_IN_DEF_DKIM_WL 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 25404C433E2 for ; Mon, 31 Aug 2020 06:07:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 05ED420707 for ; Mon, 31 Aug 2020 06:07:12 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="cmF2xLSO" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726419AbgHaGHJ (ORCPT ); Mon, 31 Aug 2020 02:07:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52064 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726102AbgHaGHG (ORCPT ); Mon, 31 Aug 2020 02:07:06 -0400 Received: from mail-oi1-x241.google.com (mail-oi1-x241.google.com [IPv6:2607:f8b0:4864:20::241]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 115D0C061575 for ; Sun, 30 Aug 2020 23:07:05 -0700 (PDT) Received: by mail-oi1-x241.google.com with SMTP id j7so63014oij.9 for ; Sun, 30 Aug 2020 23:07:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:in-reply-to:message-id:references :user-agent:mime-version; bh=ztuJ7lCE7kvEopSezi/ervbTmQwS2BBYgYbiSUHWDXw=; b=cmF2xLSOc0U8iPN/z0M/2KlwqgwRrp6zq9MwpiRV27UnbWuS7mGffAS2K5qX6wGZDA vKV3JN+iiDvuhHUrUpKdss5ebLyYtzRN8/X6y16byabkAS2Nyq0e/ZjqlVt+h7dHjzAS 2+/a8PNuK2ZUVeGks4uD5ObjruiD14v/ThBq/Mb2pru7WMFg0ZkunMkPWRsdWw+d9xwh RLNt+mr1jdeBPF6KYi/e1cYn/Gbn+24t9EoWZgX7LTiPH8ccaSYYeTkNq0kAVW5A7TSN pz91sXm5kCQ8WCklfAXzQHteSKQsJjFIR3w7sUTaXaVWPcqay+RgSPuaiDr1sa+X689+ uXCg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:in-reply-to:message-id :references:user-agent:mime-version; bh=ztuJ7lCE7kvEopSezi/ervbTmQwS2BBYgYbiSUHWDXw=; b=Q6pj8ajKOoBt0bIq5j0/Lt6ornaybMw1CynA935N+o2zFK3DU9odFiyezFChTD97WE mFArjcfJFUceBvLJegrwQwYoKwCxfAbykHFVG5kz33JZCW5sqJgZFfSjyu5Dicik7T+i lAqRfMV2fef2eMYWcUu8v6d/x7e0nZHK1K4R7KIAhwg5CDCvR66Cvfw5yKG6lGkJ3r/8 4K66tAWKQLGqQpMNm4+uAkOGd7VczUqDmQdSgCn81jSt0jpiR/3Z+AQHU2owldYAINYT HNv7bwuE+mp1YdBe56AMMrpdFl38i8frCCN74O1zmr/PFe+XcsYSU7yZblGeJcESyGLw Yosw== X-Gm-Message-State: AOAM5309Wcj7hYmVkh79eGgRVHJfec5Arr8lGvnzodjmoCs7Y2xSev+r Zly8o+BTIwobTRFgikcRKgj9vw== X-Google-Smtp-Source: ABdhPJz/zU/QKQdg3JGY2l0l+MAtlTVJhSk3I1VuspUkRP/wEWRSujcUG5/U0enSatTYPzam6ne/kQ== X-Received: by 2002:aca:cd93:: with SMTP id d141mr14485oig.33.1598854024866; Sun, 30 Aug 2020 23:07:04 -0700 (PDT) Received: from eggly.attlocal.net (172-10-233-147.lightspeed.sntcca.sbcglobal.net. [172.10.233.147]) by smtp.gmail.com with ESMTPSA id 126sm597160oof.28.2020.08.30.23.07.02 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Sun, 30 Aug 2020 23:07:03 -0700 (PDT) Date: Sun, 30 Aug 2020 23:06:47 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Jann Horn cc: Andrew Morton , Linus Torvalds , Christoph Hellwig , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Alexander Viro , "Eric W . Biederman" , Oleg Nesterov Subject: Re: [PATCH v5 7/7] mm: Remove the now-unnecessary mmget_still_valid() hack In-Reply-To: <20200827114932.3572699-8-jannh@google.com> Message-ID: References: <20200827114932.3572699-1-jannh@google.com> <20200827114932.3572699-8-jannh@google.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, 27 Aug 2020, Jann Horn wrote: > The preceding patches have ensured that core dumping properly takes the > mmap_lock. Thanks to that, we can now remove mmget_still_valid() and all > its users. Hi Jann, while the only tears to be shed over losing mmget_still_valid() will be tears of joy, I think you need to explain why you believe it's safe to remove the instance in mm/khugepaged.c: which you'll have found I moved just recently, to cover an extra case (sorry for not Cc'ing you). > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -431,7 +431,7 @@ static void insert_to_mm_slots_hash(struct mm_struct *mm, > > static inline int khugepaged_test_exit(struct mm_struct *mm) > { > - return atomic_read(&mm->mm_users) == 0 || !mmget_still_valid(mm); > + return atomic_read(&mm->mm_users) == 0; > } > > static bool hugepage_vma_check(struct vm_area_struct *vma, The movement (which you have correctly followed) was in bbe98f9cadff ("khugepaged: khugepaged_test_exit() check mmget_still_valid()") but the "pmd .. physical page 0" issue is explained better in its parent 18e77600f7a1 ("khugepaged: retract_page_tables() remember to test exit") I think your core dumping is still reading the page tables without holding mmap_lock, so still vulnerable to that extra issue. It won't be as satisfying as removing all traces of mmget_still_valid(), but right now I think you should add an mm->core_state check there instead. (I do have a better solution in use, but it's a much larger patch, that will take a lot more effort to get in: checks in pte_offset_map_lock(), perhaps returning NULL when pmd is transitioning, requiring retry.) Or maybe it's me who has missed what you're doing instead. Hugh