Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755239AbcKVD72 (ORCPT ); Mon, 21 Nov 2016 22:59:28 -0500 Received: from mail-pg0-f43.google.com ([74.125.83.43]:34825 "EHLO mail-pg0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755169AbcKVD7Z (ORCPT ); Mon, 21 Nov 2016 22:59:25 -0500 Date: Mon, 21 Nov 2016 19:59:17 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Boris Ostrovsky cc: david.vrabel@citrix.com, jgross@suse.com, xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, olaf@aepfle.de, hughd@google.com Subject: Re: [PATCH v4] xen/gntdev: Use VM_MIXEDMAP instead of VM_IO to avoid NUMA balancing In-Reply-To: <1479740166-3109-1-git-send-email-boris.ostrovsky@oracle.com> Message-ID: References: <1479740166-3109-1-git-send-email-boris.ostrovsky@oracle.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1715 Lines: 57 On Mon, 21 Nov 2016, Boris Ostrovsky wrote: > Commit 9c17d96500f7 ("xen/gntdev: Grant maps should not be subject to > NUMA balancing") set VM_IO flag to prevent grant maps from being > subjected to NUMA balancing. > > It was discovered recently that this flag causes get_user_pages() to > always fail with -EFAULT. > > check_vma_flags > __get_user_pages > __get_user_pages_locked > __get_user_pages_unlocked > get_user_pages_fast > iov_iter_get_pages > dio_refill_pages > do_direct_IO > do_blockdev_direct_IO > do_blockdev_direct_IO > ext4_direct_IO_read > generic_file_read_iter > aio_run_iocb > > (which can happen if guest's vdisk has direct-io-safe option). > > To avoid this let's use VM_MIXEDMAP flag instead --- it prevents > NUMA balancing just as VM_IO does and has no effect on > check_vma_flags(). > > Reported-by: Olaf Hering > Suggested-by: Hugh Dickins > Signed-off-by: Boris Ostrovsky Acked-by: Hugh Dickins But nicer with the addition of a code comment saying /* VM_MIXEDMAP to forbid NUMA balancing but permit get_user_pages() */ > --- > drivers/xen/gntdev.c | 2 +- > 1 files changed, 1 insertions(+), 1 deletions(-) > > diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c > index bb95212..2ef2b61 100644 > --- a/drivers/xen/gntdev.c > +++ b/drivers/xen/gntdev.c > @@ -1007,7 +1007,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma) > > vma->vm_ops = &gntdev_vmops; > > - vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP | VM_IO; > + vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP | VM_MIXEDMAP; > > if (use_ptemod) > vma->vm_flags |= VM_DONTCOPY; > -- > 1.7.1