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 6A706C433EF for ; Wed, 29 Dec 2021 11:01:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239823AbhL2LBX (ORCPT ); Wed, 29 Dec 2021 06:01:23 -0500 Received: from szxga02-in.huawei.com ([45.249.212.188]:17311 "EHLO szxga02-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239798AbhL2LBW (ORCPT ); Wed, 29 Dec 2021 06:01:22 -0500 Received: from dggpemm500024.china.huawei.com (unknown [172.30.72.57]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4JP7jM6zDdz9rvr; Wed, 29 Dec 2021 19:00:23 +0800 (CST) Received: from dggpemm500001.china.huawei.com (7.185.36.107) by dggpemm500024.china.huawei.com (7.185.36.203) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.20; Wed, 29 Dec 2021 19:01:20 +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 15.1.2308.20; Wed, 29 Dec 2021 19:01:19 +0800 Message-ID: <6cd5deb3-b71d-1058-f992-1c2f93c16ea4@huawei.com> Date: Wed, 29 Dec 2021 19:01:18 +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: Dave Hansen , Jonathan Corbet , Andrew Morton , , , , , , CC: Nicholas Piggin , Catalin Marinas , Will Deacon , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Michael Ellerman , "Benjamin Herrenschmidt" , Paul Mackerras , Christophe Leroy , 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> From: Kefeng Wang In-Reply-To: <31a75f95-6e6e-b640-2d95-08a95ea8cf51@intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.243] X-ClientProxiedBy: dggeme702-chm.china.huawei.com (10.1.199.98) 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 2021/12/29 0:14, Dave Hansen wrote: > On 12/28/21 2:26 AM, Kefeng Wang wrote: >>>> There are some disadvantages about this feature[2], one of the main >>>> concerns is the possible memory fragmentation/waste in some scenarios, >>>> also archs must ensure that any arch specific vmalloc allocations that >>>> require PAGE_SIZE mappings(eg, module alloc with STRICT_MODULE_RWX) >>>> use the VM_NO_HUGE_VMAP flag to inhibit larger mappings. >>> That just says that x86 *needs* PAGE_SIZE allocations.  But, what >>> happens if VM_NO_HUGE_VMAP is not passed (like it was in v1)?  Will the >>> subsequent permission changes just fragment the 2M mapping? >> Yes, without VM_NO_HUGE_VMAP, it could fragment the 2M mapping. >> >> When module alloc with STRICT_MODULE_RWX on x86, it calls >> __change_page_attr() >> >> from set_memory_ro/rw/nx which will split large page, so there is no >> need to make >> >> module alloc with HUGE_VMALLOC. > 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. They next > guy is going to forget, just like you did. > > Considering that this is not a hot path, a weak function would be a nice > choice: > > /* vmalloc() flags used for all module allocations. */ > unsigned long __weak arch_module_vm_flags() > { > /* > * Modules use a single, large vmalloc(). Different > * permissions are applied later and will fragment > * huge mappings. Avoid using huge pages for modules. > */ > return VM_NO_HUGE_VMAP; For x86, it only fragment, but for arm64, due to apply_to_page_range() in set_memory_*, without this flag maybe crash. Whatever, we need this flag for module. > } > > Stick that in some the common module code, next to: > >> void * __weak module_alloc(unsigned long size) >> { >> return __vmalloc_node_range(size, 1, VMALLOC_START, VMALLOC_END, > ... > > Then, put arch_module_vm_flags() in *all* of the module_alloc() > implementations, including the generic one. That way (even with a new > architecture) whoever copies-and-pastes their module_alloc() > implementation is likely to get it right. The next guy who just does a > "select HAVE_ARCH_HUGE_VMALLOC" will hopefully just work. OK， Let me check the VM_FLUSH_RESET_PERMS and try about this way. Thanks. > > VM_FLUSH_RESET_PERMS could probably be dealt with in the same way. > .