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

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Date:	Mon,  5 Oct 2015 14:32:54 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	linux-kernel@...r.kernel.org
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-nvdimm@...ts.01.org
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 <ross.zwisler@...ux.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

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