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Subject[PATCH] dax: fix NULL pointer in __dax_pmd_fault()
Date
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@linux.intel.com>
Reported-by: Dan Williams <dan.j.williams@intel.com>
---
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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