[PATCH v3 3/3] dax: temporarily disable DAX PMD fault path

[Ross Zwisler <ross.zwisler@linux.intel.com>]

There is currently a deadlock in the DAX PMD fault path because it ends up
taking the same mapping->i_mmap_rwsem twice.  The first take is a
i_mmap_lock_read() near the beginning of __dax_pmd_fault(), and the second
is __dax_pmd_fault() -> unmap_mapping_range() -> i_mmap_lock_write().

We are re-working the way that DAX does its locking for v4.4, so for now
just disable DAX PMD faults and fall back to PAGE_SIZE faults to make sure
we don't hit this deadlock.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
---
 fs/dax.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/dax.c b/fs/dax.c
index f364c90..d42217d 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -534,6 +534,13 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
 	unsigned long pfn;
 	int result = 0;
 
+	/*
+	 * There is currently a deadlock in the DAX PMD fault path.  Disable
+	 * this path and fall back to the normal dax_fault() path for now
+	 * while we rework locking.
+	 */
+	return VM_FAULT_FALLBACK;
+
 	/* Fall back to PTEs if we're going to COW */
 	if (write && !(vma->vm_flags & VM_SHARED))
 		return VM_FAULT_FALLBACK;
-- 
2.1.0