dax: fix __dax_pmd_fault crash

Since 4.3 introduced devm_memremap_pages() the pfns handled by DAX may
optionally have a struct page backing.  When a mapped pfn reaches
vmf_insert_pfn_pmd() it fails with a crash signature like the following:

 kernel BUG at mm/huge_memory.c:905!
 [..]
 Call Trace:
  [<ffffffff812a73ba>] __dax_pmd_fault+0x2ea/0x5b0
  [<ffffffffa01a4182>] xfs_filemap_pmd_fault+0x92/0x150 [xfs]
  [<ffffffff811fbe02>] handle_mm_fault+0x312/0x1b50

Fix this by falling back to 4K mappings in the pfn_valid() case.  Longer
term, vmf_insert_pfn_pmd() needs to grow support for architectures that
can provide a 'pmd_special' capability.

Cc: <stable@vger.kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/fs/dax.c b/fs/dax.c
index 131fd35ae39d..bff20cc56130 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -627,6 +627,13 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                 if ((length < PMD_SIZE) || (pfn & PG_PMD_COLOUR))
                         goto fallback;
 
+                /*
+                 * TODO: teach vmf_insert_pfn_pmd() to support
+                 * 'pte_special' for pmds
+                 */
+                if (pfn_valid(pfn))
+                        goto fallback;
+
                 if (buffer_unwritten(&bh) || buffer_new(&bh)) {
                         int i;
                         for (i = 0; i < PTRS_PER_PMD; i++)