[PATCH] xen/gntdev: Use mempolicy instead of VM_IO flag to avoid NUMA balancing

From: Boris Ostrovsky
Date: Wed Nov 16 2016 - 12:02:00 EST


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 this this flag may cause page allocation
failures with the following stack:

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, instead of setting VM_IO use mempolicy that prohibits page
migration (i.e. clear policy's MPOL_F_MOF|MPOL_F_MORON)

Reported-by: Olaf Hering <olaf@xxxxxxxxx>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
---

Unfortunately I haven't been able to trigger NUMA balancing
so while I tested this in general I am not sure I actually
exercised the code path.

drivers/xen/gntdev.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index bb95212..743e6f0 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -35,6 +35,7 @@
#include <linux/spinlock.h>
#include <linux/slab.h>
#include <linux/highmem.h>
+#include <linux/mempolicy.h>

#include <xen/xen.h>
#include <xen/grant_table.h>
@@ -1007,8 +1008,20 @@ 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;

+#ifdef CONFIG_NUMA
+ /* Prevent NUMA balancing */
+ if (vma->vm_policy)
+ vma->vm_policy->flags &= ~(MPOL_F_MOF | MPOL_F_MORON);
+ else {
+ struct mempolicy *pol = get_task_policy(current);
+
+ vma->vm_policy = mpol_dup(pol);
+ if (vma->vm_policy)
+ vma->vm_policy->flags &= ~(MPOL_F_MOF | MPOL_F_MORON);
+ }
+#endif
if (use_ptemod)
vma->vm_flags |= VM_DONTCOPY;

--
2.5.5