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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,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 88EA6C10DCE for ; Thu, 12 Mar 2020 16:37:38 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 4AFF72067C for ; Thu, 12 Mar 2020 16:37:38 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=ziepe.ca header.i=@ziepe.ca header.b="KnteymGL" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4AFF72067C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ziepe.ca Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id D49786B0003; Thu, 12 Mar 2020 12:37:37 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id CF8D56B0006; Thu, 12 Mar 2020 12:37:37 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C0EA96B0007; Thu, 12 Mar 2020 12:37:37 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0078.hostedemail.com [216.40.44.78]) by kanga.kvack.org (Postfix) with ESMTP id A69706B0003 for ; Thu, 12 Mar 2020 12:37:37 -0400 (EDT) Received: from smtpin11.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 4CB0B8248D52 for ; Thu, 12 Mar 2020 16:37:37 +0000 (UTC) X-FDA: 76587266154.11.shake54_83bbc38aad1e X-HE-Tag: shake54_83bbc38aad1e X-Filterd-Recvd-Size: 5822 Received: from mail-qk1-f193.google.com (mail-qk1-f193.google.com [209.85.222.193]) by imf02.hostedemail.com (Postfix) with ESMTP for ; Thu, 12 Mar 2020 16:37:36 +0000 (UTC) Received: by mail-qk1-f193.google.com with SMTP id d8so7281785qka.2 for ; Thu, 12 Mar 2020 09:37:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ziepe.ca; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=fmA8zKRe76L9QKXynKJ5M/zisBlIkbsLwFQy3LQahYI=; b=KnteymGLpkCc/QfvlROTLY5FU+cvnMfHx3Z/DRw8BCFXZt7iuH6QM2L8dGPveQUUn3 Xlhpfnj22gbb00Ajojd+xRnUeKGKFC/Zglayy3ZU7Fb8m4VFbcrwr40XiWp3ov1/ykVn 50j0Xys2kPWj8lJQThzi6h+Abasy7fco7NKHepLokp4T3nUslp5eA0azM8UN5ShssXcR /coFE6WQnM7yvxRwRmdgJOv84KxclHIi1t/Vuh6HvLG48YWad9Scekhq9v5t4slQNOVw VUBakr2wwlLvqTGCCgx2gzI5gxsHdjkTDbdmt7DrBlCzDNkcudKItHAvui1LdFqzC7SL O7vA== 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:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=fmA8zKRe76L9QKXynKJ5M/zisBlIkbsLwFQy3LQahYI=; b=rhAQcdc7NEbvlXls7gWPySCcWMmm2jZs0R37MNkJtY+lG+C5XY3ykZiKWaLGrTmOjm Uvn9LeaTo364PXjdvdMPKUgKOCNe0Oq5Zd4ozGQ0P8d9QTdbOXxadj8Za9u1unjFbQUC yeZgfEn6yGJZGlbRln4hQTvB6AiipyzEdYITZ6CBbDFWPi5KMRuEfpgyFiJ0jsw193oC /M3tdrMaHtiCRXzjhaw/9V/149N89UJA9TwerhqCFpAL0MH91E7guIJppatN9fUONGoo txbwEmLVkAxU8Lmpo+3BrekcOPKTWl0YcVjJoajfLYm4DOcJbY2dYNIagxl2b9iOUel9 3ONg== X-Gm-Message-State: ANhLgQ2SRdVczwDcrV3uM+HPCJKSbXE7FsBMX11iWWtU1M8J1GOI4VIS wVUe1cRzsQYas0x7up/6LVbwvA== X-Google-Smtp-Source: ADFU+vsCZGS3KHARg6F0Q8AUmySkB/O1C49CFLK8V1S8E9P7Q1oeOBUV0BmgWYk1CW/U1GncetrzAg== X-Received: by 2002:ae9:e509:: with SMTP id w9mr8707944qkf.26.1584031055747; Thu, 12 Mar 2020 09:37:35 -0700 (PDT) Received: from ziepe.ca (hlfxns017vw-142-68-57-212.dhcp-dynamic.fibreop.ns.bellaliant.net. [142.68.57.212]) by smtp.gmail.com with ESMTPSA id f26sm12590660qkl.119.2020.03.12.09.37.35 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 12 Mar 2020 09:37:35 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1jCQpy-0008RM-Kv; Thu, 12 Mar 2020 13:37:34 -0300 Date: Thu, 12 Mar 2020 13:37:34 -0300 From: Jason Gunthorpe To: Steven Price Cc: Jerome Glisse , Ralph Campbell , "Felix.Kuehling@amd.com" , Philip Yang , John Hubbard , "amd-gfx@lists.freedesktop.org" , "linux-mm@kvack.org" , "dri-devel@lists.freedesktop.org" , Christoph Hellwig Subject: Re: [PATCH] mm/hmm: Simplify hmm_vma_walk_pud slightly Message-ID: <20200312163734.GR31668@ziepe.ca> References: <5bd778fa-51e5-3e0c-d9bb-b38539b03c8d@arm.com> <20200312102813.56699-1-steven.price@arm.com> <20200312142749.GM31668@ziepe.ca> <58e296a6-d32b-bb37-28ce-ade0f784454d@arm.com> <20200312151113.GO31668@ziepe.ca> <689d3c56-3d19-4655-21f5-f9aeab3089df@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <689d3c56-3d19-4655-21f5-f9aeab3089df@arm.com> User-Agent: Mutt/1.9.4 (2018-02-28) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Mar 12, 2020 at 04:16:33PM +0000, Steven Price wrote: > > Actually, while you are looking at this, do you think we should be > > adding at least READ_ONCE in the pagewalk.c walk_* functions? The > > multiple references of pmd, pud, etc without locking seems sketchy to > > me. > > I agree it seems worrying. I'm not entirely sure whether the holding of > mmap_sem is sufficient, I looked at this question, and at least for PMD, mmap_sem is not sufficient. I didn't easilly figure it out for the other ones I'm guessing if PMD is not safe then none of them are. > this isn't something that I changed so I've just > been hoping that it's sufficient since it seems to have been working > (whether that's by chance because the compiler didn't generate multiple > reads I've no idea). For walking the kernel's page tables the lack of > READ_ONCE is also not great, but at least for PTDUMP we don't care too much > about accuracy and it should be crash proof because there's no RCU grace > period. And again the code I was replacing didn't have any special > protection. > > I can't see any harm in updating the code to include READ_ONCE and I'm happy > to review a patch. The reason I ask is because hmm's walkers often have this pattern where they get the pointer and then de-ref it (again) then immediately have to recheck the 'again' conditions of the walker itself because the re-read may have given a different value. Having the walker deref the pointer and pass the value it into the ops for use rather than repeatedly de-refing an unlocked value seems like a much safer design to me. If this also implicitly relies on a RCU grace period then it is also missing RCU locking... I also didn't quite understand why walk_pte_range() skipped locking the pte in the no_vma case - I don't get why vma would be related to locking here. I also saw that hmm open coded the pte walk, presumably for performance, so I was thinking of adding some kind of pte_range() callback to avoid the expensive indirect function call per pte, but hmm also can't have the pmd locked... Jason