linux-kernel - [PATCH 3.16 056/370] xen/gntdev: Use VM_MIXEDMAP instead of VM

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Message-ID: <lsq.1489146382.425121066@decadent.org.uk>
Date:   Fri, 10 Mar 2017 11:46:22 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Olaf Hering" <olaf@...fle.de>,
        "Juergen Gross" <jgross@...e.com>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        "Hugh Dickins" <hughd@...gle.com>
Subject: [PATCH 3.16 056/370] xen/gntdev: Use VM_MIXEDMAP instead of VM_IO
 to avoid NUMA balancing

3.16.42-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Boris Ostrovsky <boris.ostrovsky@...cle.com>

commit 30faaafdfa0c754c91bac60f216c9f34a2bfdf7e upstream.

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 <olaf@...fle.de>
Suggested-by: Hugh Dickins <hughd@...gle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Acked-by: Hugh Dickins <hughd@...gle.com>
Tested-by: Olaf Hering <olaf@...fle.de>
Signed-off-by: Juergen Gross <jgross@...e.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/xen/gntdev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -765,7 +765,7 @@ static int gntdev_mmap(struct file *flip
 
 	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;