dax: fix NULL pointer in __dax_pmd_fault()

Commit 46c043ede471 ("mm: take i_mmap_lock in unmap_mapping_range() for
DAX") moved some code in __dax_pmd_fault() that was responsible for
zeroing newly allocated PMD pages.  The new location didn't properly set
up 'kaddr', so when run this code resulted in a NULL pointer BUG.

Fix this by getting the correct 'kaddr' via bdev_direct_access().

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

diff --git a/fs/dax.c b/fs/dax.c
index 7ae6df7..bcfb14b 100644 (file)
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -569,8 +569,20 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
        if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE)
                goto fallback;
 
+       sector = bh.b_blocknr << (blkbits - 9);
+
        if (buffer_unwritten(&bh) || buffer_new(&bh)) {
                int i;
+
+               length = bdev_direct_access(bh.b_bdev, sector, &kaddr, &pfn,
+                                               bh.b_size);
+               if (length < 0) {
+                       result = VM_FAULT_SIGBUS;
+                       goto out;
+               }
+               if ((length < PMD_SIZE) || (pfn & PG_PMD_COLOUR))
+                       goto fallback;
+
                for (i = 0; i < PTRS_PER_PMD; i++)
                        clear_pmem(kaddr + i * PAGE_SIZE, PAGE_SIZE);
                wmb_pmem();
@@ -623,7 +635,6 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                result = VM_FAULT_NOPAGE;
                spin_unlock(ptl);
        } else {
-               sector = bh.b_blocknr << (blkbits - 9);
                length = bdev_direct_access(bh.b_bdev, sector, &kaddr, &pfn,
                                                bh.b_size);
                if (length < 0) {