In __dax_pmd_fault() we currently assume that get_block() will always set
bh.b_bdev and we unconditionally dereference it in __dax_dbg().  This
assumption isn't always true - when called for reads of holes
ext4_dax_mmap_get_block() returns a buffer head where bh->b_bdev is never
set.  I hit this BUG while testing the DAX PMD fault path.

Instead, initialize bh.b_bdev before passing bh into get_block().  It is
possible that the filesystem's get_block() will update bh.b_bdev, and this
is fine - we just want to initialize bh.b_bdev to something reasonable so
that the calls to __dax_dbg() work and print something useful.

Signed-off-by: Ross Zwisler <redacted>
Cc: Dan Williams <redacted>
---
 fs/dax.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/dax.c b/fs/dax.c
index 7af8797..513bba5 100644
--- a/fs/dax.c
+++ b/fs/dax.c

@@ -624,6 +624,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
 	}
 
 	memset(&bh, 0, sizeof(bh));
+	bh.b_bdev = inode->i_sb->s_bdev;
 	block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);
 
 	bh.b_size = PMD_SIZE;

-- 
2.5.0

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