From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932429AbcA0LbF (ORCPT ); Wed, 27 Jan 2016 06:31:05 -0500 Received: from mx2.suse.de ([195.135.220.15]:49606 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932403AbcA0LbA (ORCPT ); Wed, 27 Jan 2016 06:31:00 -0500 Subject: Re: [PATCH] mm, gup: introduce concept of "foreign" get_user_pages() To: Dave Hansen , linux-kernel@vger.kernel.org References: <20160122180219.164259F1@viggo.jf.intel.com> Cc: linux-mm@kvack.org, x86@kernel.org, dave.hansen@linux.intel.com, akpm@linux-foundation.org, kirill.shutemov@linux.intel.com, aarcange@redhat.com, n-horiguchi@ah.jp.nec.com, srikar@linux.vnet.ibm.com, jack@suse.cz From: Vlastimil Babka X-Enigmail-Draft-Status: N1110 Message-ID: <56A8AA6E.2080705@suse.cz> Date: Wed, 27 Jan 2016 12:30:54 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: <20160122180219.164259F1@viggo.jf.intel.com> Content-Type: text/plain; charset=iso-8859-2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/22/2016 07:02 PM, Dave Hansen wrote: > One of Vlastimil's comments made me go dig back in to the uprobes > code's use of get_user_pages(). I decided to change both of them > to be "foreign" accesses. > > This also fixes the nommu breakage that Vlastimil noted last time. > > Srikar, I'd appreciate if you can have a look at the uprobes.c > modifications, especially the comment. I don't think this will > change any behavior, but I want to make sure the comment is > accurate. > > --- > > From: Dave Hansen > > For protection keys, we need to understand whether protections > should be enforced in software or not. In general, we enforce > protections when working on our own task, but not when on others. > We call these "current" and "foreign" operations. > > This patch introduces a new get_user_pages() variant: > > get_user_pages_foreign() > > We modify the vanilla get_user_pages() so it can no longer be > used on mm/tasks other than 'current/current->mm', which is by > far the most common way it is called. Using it makes a few of > the call sites look a bit nicer. > > In other words, get_user_pages_foreign() is a replacement for > when get_user_pages() is called on non-current tsk/mm. > > This also switches get_user_pages_(un)locked() over to be like > get_user_pages() and not take a tsk/mm. There is no > get_user_pages_foreign_(un)locked(). If someone wants that > behavior they just have to use "__" variant and pass in > FOLL_FOREIGN explicitly. > > The uprobes is_trap_at_addr() location holds mmap_sem and > calls get_user_pages(current->mm) on an instruction address. This > makes it a pretty unique gup caller. Being an instruction access > and also really originating from the kernel (vs. the app), I opted > to consider this a 'foreign' access where protection keys will not > be enforced. > > Signed-off-by: Dave Hansen > Cc: Andrew Morton > Cc: Kirill A. Shutemov > Cc: Andrea Arcangeli > Cc: Naoya Horiguchi > Cc: Srikar Dronamraju > Cc: vbabka@suse.cz Acked-by: Vlastimil Babka But, > long __get_user_pages_unlocked(struct task_struct *tsk, struct mm_struct *mm, > unsigned long start, unsigned long nr_pages, > int write, int force, struct page **pages, > unsigned int gup_flags) > { > long ret; > - down_read(&mm->mmap_sem); > - ret = get_user_pages(tsk, mm, start, nr_pages, write, force, > - pages, NULL); > - up_read(&mm->mmap_sem); > + down_read(¤t->mm->mmap_sem); > + ret = get_user_pages(start, nr_pages, write, force, pages, NULL); > + up_read(¤t->mm->mmap_sem); I understand your reply to lkp report also means that this no longer locks current's mmap_sem? :) Vlastimil