+ dax-temporarily-disable-dax-pmd-fault-path.patch added to -mm tree

+ dax-temporarily-disable-dax-pmd-fault-path.patch added to -mm tree

[akpm]

The patch titled
     Subject: dax: temporarily disable DAX PMD fault path
has been added to the -mm tree.  Its filename is
     dax-temporarily-disable-dax-pmd-fault-path.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/dax-temporarily-disable-dax-pmd-fault-path.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/dax-temporarily-disable-dax-pmd-fault-path.patch

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------------------------------------------------------
From: Ross Zwisler <ross.zwisler@linux.intel.com>
Subject: 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 <ross.zwisler@linux.intel.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Matthew Wilcox <willy@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/dax.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff -puN fs/dax.c~dax-temporarily-disable-dax-pmd-fault-path fs/dax.c
--- a/fs/dax.c~dax-temporarily-disable-dax-pmd-fault-path
+++ a/fs/dax.c
@@ -534,6 +534,13 @@ int __dax_pmd_fault(struct vm_area_struc
 	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;
_

Patches currently in -mm which might be from ross.zwisler@linux.intel.com are

revert-mm-take-i_mmap_lock-in-unmap_mapping_range-for-dax.patch
revert-dax-fix-race-between-simultaneous-faults.patch
dax-temporarily-disable-dax-pmd-fault-path.patch