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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 21EFFC433FE for ; Wed, 19 Jan 2022 13:32:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1354840AbiASNcU (ORCPT ); Wed, 19 Jan 2022 08:32:20 -0500 Received: from szxga01-in.huawei.com ([45.249.212.187]:16724 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1354832AbiASNcR (ORCPT ); Wed, 19 Jan 2022 08:32:17 -0500 Received: from dggpemm500023.china.huawei.com (unknown [172.30.72.56]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4Jf60Y3G8kzZfHP; Wed, 19 Jan 2022 21:28:29 +0800 (CST) Received: from dggpemm500001.china.huawei.com (7.185.36.107) by dggpemm500023.china.huawei.com (7.185.36.83) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.21; Wed, 19 Jan 2022 21:32:15 +0800 Received: from [10.174.177.243] (10.174.177.243) by dggpemm500001.china.huawei.com (7.185.36.107) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256) id 15.1.2308.21; Wed, 19 Jan 2022 21:32:14 +0800 Message-ID: Date: Wed, 19 Jan 2022 21:32:13 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH v2 3/3] x86: Support huge vmalloc mappings Content-Language: en-US To: Nicholas Piggin , Andrew Morton , Jonathan Corbet , Dave Hansen , , , , , , CC: Benjamin Herrenschmidt , Borislav Petkov , Catalin Marinas , Christophe Leroy , Dave Hansen , "H. Peter Anvin" , Ingo Molnar , "Michael Ellerman" , Paul Mackerras , "Thomas Gleixner" , Will Deacon , Matthew Wilcox References: <20211227145903.187152-1-wangkefeng.wang@huawei.com> <20211227145903.187152-4-wangkefeng.wang@huawei.com> <70ff58bc-3a92-55c2-2da8-c5877af72e44@intel.com> <3858de1f-cdbc-ff52-2890-4254d0f48b0a@huawei.com> <31a75f95-6e6e-b640-2d95-08a95ea8cf51@intel.com> <1642472965.lgfksp6krp.astroid@bobo.none> <4488d39f-0698-7bfd-b81c-1e609821818f@intel.com> <1642565468.c0jax91tvn.astroid@bobo.none> From: Kefeng Wang In-Reply-To: <1642565468.c0jax91tvn.astroid@bobo.none> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.243] X-ClientProxiedBy: dggeme705-chm.china.huawei.com (10.1.199.101) To dggpemm500001.china.huawei.com (7.185.36.107) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/1/19 12:17, Nicholas Piggin wrote: > Excerpts from Dave Hansen's message of January 19, 2022 3:28 am: >> On 1/17/22 6:46 PM, Nicholas Piggin wrote: >>>> This all sounds very fragile to me. Every time a new architecture would >>>> get added for huge vmalloc() support, the developer needs to know to go >>>> find that architecture's module_alloc() and add this flag. >>> This is documented in the Kconfig. >>> >>> # >>> # Archs that select this would be capable of PMD-sized vmaps (i.e., >>> # arch_vmap_pmd_supported() returns true), and they must make no assumptions >>> # that vmalloc memory is mapped with PAGE_SIZE ptes. The VM_NO_HUGE_VMAP flag >>> # can be used to prohibit arch-specific allocations from using hugepages to >>> # help with this (e.g., modules may require it). >>> # >>> config HAVE_ARCH_HUGE_VMALLOC >>> depends on HAVE_ARCH_HUGE_VMAP >>> bool >>> >>> Is it really fair to say it's *very* fragile? Surely it's reasonable to >>> read the (not very long) documentation ad understand the consequences for >>> the arch code before enabling it. >> Very fragile or not, I think folks are likely to get it wrong. It would >> be nice to have it default *everyone* to safe and slow and make *sure* > It's not safe to enable though. That's the problem. If it was just > modules then you'd have a point but it could be anything. > >> they go look at the architecture modules code itself before enabling >> this for modules. > This is required not just for modules for the whole arch code, it > has to be looked at and decided this will work. > >> Just from that Kconfig text, I don't think I'd know off the top of my >> head what do do for x86, or what code I needed to go touch. > You have to make sure arch/x86 makes no assumptions that vmalloc memory > is backed by PAGE_SIZE ptes. If you can't do that then you shouldn't > enable the option. The option can not explain it any more because any > arch could do anything with its mappings. The module code is an example, > not the recipe. Hi Nick, Dave and Christophe，thanks for your review,  a little confused,   I think, 1) for ppc/arm64 module_alloc(),  it must set VM_NO_HUGE_VMAP because the arch's set_memory_* funcitons can only support PAGE_SIZE mapping, due to the limit of apply_to_page_range(). 2) but for x86's module_alloc(), add VM_NO_HUGE_VMAP is to avoid fragmentation, x86's __change_page_attr functions will split the huge mapping. this flags is not a must. and the behavior above occurred when STRICT_MODULE_RWX enabled, so 1) add a unified function to set vm flags(suggested by Dave ) or 2) add vm flags with some comments to per-arch's module_alloc() are both acceptable, for the way of unified function ,  we could make this a default recipe with STRICT_MODULE_RWX, also make two more vm flags into it, eg, +unsigned long module_alloc_vm_flags(bool need_flush_reset_perms) +{ +       unsigned long vm_flags = VM_DEFER_KMEMLEAK; + +       if (need_flush_reset_perms) +               vm_flags |= VM_FLUSH_RESET_PERMS; +       /* +        * Modules use a single, large vmalloc(). Different permissions +        * are applied later and will fragment huge mappings or even +        * fails in set_memory_* on some architectures. Avoid using +        * huge pages for modules. +        */ +       if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX)) +               vm_flags |= VM_NO_HUGE_VMAP; + +       return vm_flags; +} then called each arch's module_alloc(). Any suggestion, many thanks. > > Thanks, > Nick > .