On Tue, Sep 22, 2015 at 2:17 PM, Ross Zwisler
[off-list ref] wrote:

On Tue, Sep 22, 2015 at 01:51:04PM -0700, Dan Williams wrote:

quoted

[ adding Andrew ]

On Tue, Sep 22, 2015 at 12:36 PM, Ross Zwisler
[off-list ref] wrote:

quoted

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().

Signed-off-by: Ross Zwisler <redacted>
Reported-by: Dan Williams <redacted>

Taking into account the comment below,

Reviewed-by: Dan Williams <redacted>

quoted

---
 fs/dax.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/fs/dax.c b/fs/dax.c
index 7ae6df7..bcfb14b 100644
--- 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;
+

Hmm, we don't need the PG_PMD_COLOUR check since we aren't using the
pfn in this path, right?

I think we care, because we'll end up bailing anyway at the later
PG_PMD_COLOUR check before we actually insert the pfn via
vmf_insert_pfn_pmd().  If we don't check the alignment we'll do 2 MiB worth of
zeroing to the media, then later fall back to PTE faults.

Ok, good point.

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