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@xxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Reported-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
fs/dax.c | 7 +++++++
1 file changed, 7 insertions(+)
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++)
--
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