Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753981Ab3JBTge (ORCPT ); Wed, 2 Oct 2013 15:36:34 -0400 Received: from cantor2.suse.de ([195.135.220.15]:38434 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753385Ab3JBTgd (ORCPT ); Wed, 2 Oct 2013 15:36:33 -0400 Date: Wed, 2 Oct 2013 21:36:31 +0200 From: Jan Kara To: KOSAKI Motohiro Cc: Jan Kara , LKML , linux-mm@kvack.org Subject: Re: [PATCH 18/26] mm: Convert process_vm_rw_pages() to use get_user_pages_unlocked() Message-ID: <20131002193631.GB16998@quack.suse.cz> References: <1380724087-13927-1-git-send-email-jack@suse.cz> <1380724087-13927-19-git-send-email-jack@suse.cz> <524C4AA1.7000409@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <524C4AA1.7000409@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2176 Lines: 50 On Wed 02-10-13 12:32:33, KOSAKI Motohiro wrote: > (10/2/13 10:27 AM), Jan Kara wrote: > > Signed-off-by: Jan Kara > > --- > > mm/process_vm_access.c | 8 ++------ > > 1 file changed, 2 insertions(+), 6 deletions(-) > > > > diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c > > index fd26d0433509..c1bc47d8ed90 100644 > > --- a/mm/process_vm_access.c > > +++ b/mm/process_vm_access.c > > @@ -64,12 +64,8 @@ static int process_vm_rw_pages(struct task_struct *task, > > *bytes_copied = 0; > > > > /* Get the pages we're interested in */ > > - down_read(&mm->mmap_sem); > > - pages_pinned = get_user_pages(task, mm, pa, > > - nr_pages_to_copy, > > - vm_write, 0, process_pages, NULL); > > - up_read(&mm->mmap_sem); > > - > > + pages_pinned = get_user_pages_unlocked(task, mm, pa, nr_pages_to_copy, > > + vm_write, 0, process_pages); > > if (pages_pinned != nr_pages_to_copy) { > > rc = -EFAULT; > > goto end; > > This is wrong because original code is wrong. In this function, page may > be pointed to anon pages. Then, you should keep to take mmap_sem until > finish to copying. Otherwise concurrent fork() makes nasty COW issue. Hum, can you be more specific? I suppose you are speaking about situation when the remote task we are copying data from/to does fork while process_vm_rw_pages() runs. If we are copying data from remote task, I don't see how COW could cause any problem. If we are copying to remote task and fork happens after get_user_pages() but before copy_to_user() then I can see we might be having some trouble. copy_to_user() would then copy data into both original remote process and its child thus essentially bypassing COW. If the child process manages to COW some of the pages before copy_to_user() happens, it can even see only some of the pages. Is that what you mean? Honza -- Jan Kara SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/