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

[Ross Zwisler]

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.
+*/
+   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

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

[Ross Zwisler]

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.
+*/
+   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

--
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Please read the FAQ at  http://www.tux.org/lkml/