From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754312AbdBGOYT (ORCPT ); Tue, 7 Feb 2017 09:24:19 -0500 Received: from mx2.suse.de ([195.135.220.15]:46904 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753284AbdBGOYS (ORCPT ); Tue, 7 Feb 2017 09:24:18 -0500 Subject: Re: [RFC PATCH] mm, vmalloc: use __GFP_HIGHMEM implicitly To: Michal Hocko , linux-mm@kvack.org References: <20170201140530.1325-1-mhocko@kernel.org> Cc: Al Viro , David Rientjes , Cristopher Lameter , LKML , Michal Hocko From: Vlastimil Babka Message-ID: <772f9b64-8f03-a4ce-e56b-779d081ebb6d@suse.cz> Date: Tue, 7 Feb 2017 15:24:14 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0 MIME-Version: 1.0 In-Reply-To: <20170201140530.1325-1-mhocko@kernel.org> Content-Type: text/plain; charset=iso-8859-2; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/01/2017 03:05 PM, Michal Hocko wrote: > From: Michal Hocko > > __vmalloc* allows users to provide gfp flags for the underlying > allocation. This API is quite popular > $ git grep "=[[:space:]]__vmalloc\|return[[:space:]]*__vmalloc" | wc -l > 77 > > the only problem is that many people are not aware that they really want > to give __GFP_HIGHMEM along with other flags because there is really no > reason to consume precious lowmemory on CONFIG_HIGHMEM systems for pages > which are mapped to the kernel vmalloc space. About half of users don't > use this flag, though. This signals that we make the API unnecessarily > too complex. > > This patch simply uses __GFP_HIGHMEM implicitly when allocating pages to > be mapped to the vmalloc space. Current users which add __GFP_HIGHMEM > are simplified and drop the flag. > > Signed-off-by: Michal Hocko > --- > Hi, > this is based on top of [1]. I believe it was Al who has brought this > up quite some time ago (or maybe I just misremember). The explicit > usage of __GFP_HIGHMEM in __vmalloc* seems to be too much to ask from > users. I believe there is no user which doesn't want vmalloc pages be > in the highmem but I might be missing something. There is vmalloc_32* > API but that uses GFP_DMA* explicitly which overrides __GFP_HIGHMEM. So > all current users _should_ be safe to use __GFP_HIGHMEM unconditionally. > This patch should simplify things and fix many users which consume > lowmem for no good reason. > > I am sending this as an RFC to get some feedback, I even haven't compile > tested it yet. > > Any comments are welcome. The idea sounds good. What are the potential dangers? That somebody of the current callers without __GFP_HIGHMEM would take a physical address of the page and then tried to access it via direct mapping?