[PATCH] dax: fix NULL pointer in __dax_pmd_fault()

[Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>]

The following commit:

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', though, so when run this code resulted in a NULL pointer BUG.

Fix this by getting the correct 'kaddr' via bdev_direct_access(), and
only make the second call to bdev_direct_access() if we don't already
have a PFN from the first call.

Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Reported-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 fs/dax.c | 31 ++++++++++++++++++++++---------
 1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 7ae6df7..08ac2bd 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -532,7 +532,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
 	void __pmem *kaddr;
 	pgoff_t size, pgoff;
 	sector_t block, sector;
-	unsigned long pfn;
+	unsigned long pfn = 0;
 	int result = 0;
 
 	/* Fall back to PTEs if we're going to COW */
@@ -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();
@@ -623,15 +635,16 @@ 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) {
-			result = VM_FAULT_SIGBUS;
-			goto out;
+		if (pfn == 0) {
+			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;
 		}
-		if ((length < PMD_SIZE) || (pfn & PG_PMD_COLOUR))
-			goto fallback;
 
 		result |= vmf_insert_pfn_pmd(vma, address, pmd, pfn, write);
 	}
-- 
2.1.0

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