From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933377AbcAYSSS (ORCPT ); Mon, 25 Jan 2016 13:18:18 -0500 Received: from mx1.redhat.com ([209.132.183.28]:59328 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932792AbcAYSSR (ORCPT ); Mon, 25 Jan 2016 13:18:17 -0500 Date: Mon, 25 Jan 2016 19:18:12 +0100 From: Oleg Nesterov To: Srikar Dronamraju Cc: Dave Hansen , linux-kernel@vger.kernel.org, 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, vbabka@suse.cz, jack@suse.cz Subject: Re: [PATCH] mm, gup: introduce concept of "foreign" get_user_pages() Message-ID: <20160125181812.GA9050@redhat.com> References: <20160122180219.164259F1@viggo.jf.intel.com> <20160125131723.GB17206@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160125131723.GB17206@linux.vnet.ibm.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/25, Srikar Dronamraju wrote: > > > 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. Yes, in particular is_trap_at_addr() doesn't look really nice. But we need to read the insn under mmap_sem to avoid the race with unregister + register at the same address, so that we won't send the wrong SIGTRAP in this case. > Changes for uprobes.c looks good to me. > Acked-by: Srikar Dronamraju Agreed, the changes in uprobes.c look fine. > > @@ -1700,7 +1700,13 @@ static int is_trap_at_addr(struct mm_str > > if (likely(result == 0)) > > goto out; > > > > - result = get_user_pages(NULL, mm, vaddr, 1, 0, 1, &page, NULL); > > + /* > > + * The NULL 'tsk' here ensures that any faults that occur here > > + * will not be accounted to the task. 'mm' *is* current->mm, > > + * but we treat this as a 'foreign' access since it is > > + * essentially a kernel access to the memory. > > + */ > > + result = get_user_pages_foreign(NULL, mm, vaddr, 1, 0, 1, &page, NULL); > > if (result < 0) > > return result; Yes, but perhaps we should simply remove this get_user_pages_foreign() and just return -EFAULT if copy_from_user_inatomic() fails. This should be very unlikely case, I think it would be fine to restart this insn and take another bp hit to fault this page in. Srikar what do you think? IIRC, this get_user_pages() was needed before, when is_trap_at_addr() had other (non-restartable) callers with mm != current->mm. But again, I think this patch is fine, we can do this later. Oleg.