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Subject[PATCH v3 3/3] dax: temporarily disable DAX PMD fault path
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 <>
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.
+ */
/* Fall back to PTEs if we're going to COW */
if (write && !(vma->vm_flags & VM_SHARED))

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