Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932234AbcKVFVT (ORCPT ); Tue, 22 Nov 2016 00:21:19 -0500 Received: from mx2.suse.de ([195.135.220.15]:40806 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750812AbcKVFVR (ORCPT ); Tue, 22 Nov 2016 00:21:17 -0500 Subject: Re: [PATCH v4] xen/gntdev: Use VM_MIXEDMAP instead of VM_IO to avoid NUMA balancing To: Hugh Dickins , Boris Ostrovsky References: <1479740166-3109-1-git-send-email-boris.ostrovsky@oracle.com> Cc: david.vrabel@citrix.com, xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, olaf@aepfle.de From: Juergen Gross Message-ID: Date: Tue, 22 Nov 2016 06:21:15 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1863 Lines: 65 On 22/11/16 04:59, Hugh Dickins wrote: > 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() */ I can fix that up when committing. Juergen > >> --- >> 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 >