[Xen-devel] [PATCH v3] xen/gntdev: Use mempolicy instead of VM_IO flag to avoid NUMA balancing

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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 don't use vm_flags. Instead, use mempolicy that prohibits
page migration (i.e. clear MPOL_F_MOF|MPOL_F_MORON) and make sure we
don't consult task's policy (which may include those flags) if vma
doesn't have one.

Reported-by: Olaf Hering <olaf@xxxxxxxxx>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
Changes in v3:
* Don't use __mpol_dup() and get_task_policy() which are not exported
  for use by drivers. Add vm_operations_struct.get_policy().
* Copy to stable


 drivers/xen/gntdev.c |   27 ++++++++++++++++++++++++++-
 1 files changed, 26 insertions(+), 1 deletions(-)

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index bb95212..632edd4 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>
@@ -433,10 +434,28 @@ static void gntdev_vma_close(struct vm_area_struct *vma)
        return map->pages[(addr - map->pages_vm_start) >> PAGE_SHIFT];
 }
 
+#ifdef CONFIG_NUMA
+/*
+ * We have this op to make sure callers (such as vma_policy_mof()) don't
+ * check current task's policy which may include migrate flags (MPOL_F_MOF
+ * or MPOL_F_MORON)
+ */
+static struct mempolicy *gntdev_vma_get_policy(struct vm_area_struct *vma,
+                                              unsigned long addr)
+{
+       if (mpol_needs_cond_ref(vma->vm_policy))
+               mpol_get(vma->vm_policy);
+       return vma->vm_policy;
+}
+#endif
+
 static const struct vm_operations_struct gntdev_vmops = {
        .open = gntdev_vma_open,
        .close = gntdev_vma_close,
        .find_special_page = gntdev_vma_find_special_page,
+#ifdef CONFIG_NUMA
+       .get_policy = gntdev_vma_get_policy,
+#endif
 };
 
 /* ------------------------------------------------------------------ */
@@ -1007,7 +1026,13 @@ 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);
+#endif
 
        if (use_ptemod)
                vma->vm_flags |= VM_DONTCOPY;
-- 
1.7.1


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