On Wed, Jan 06, 2016 at 11:04:35AM -0800, Dan Williams wrote:

On Wed, Jan 6, 2016 at 10:00 AM, Ross Zwisler
[off-list ref] wrote:

quoted

When we get a DAX PMD fault for a write it is possible that there could be
some number of 4k zero pages already present for the same range that were
inserted to service reads from a hole.  These 4k zero pages need to be
unmapped from the VMAs and removed from the struct address_space radix tree
before the real DAX PMD entry can be inserted.

For PTE faults this same use case also exists and is handled by a
combination of unmap_mapping_range() to unmap the VMAs and
delete_from_page_cache() to remove the page from the address_space radix
tree.

For PMD faults we do have a call to unmap_mapping_range() (protected by a
buffer_new() check), but nothing clears out the radix tree entry.  The
buffer_new() check is also incorrect as the current ext4 and XFS filesystem
code will never return a buffer_head with BH_New set, even when allocating
new blocks over a hole.  Instead the filesystem will zero the blocks
manually and return a buffer_head with only BH_Mapped set.

Fix this situation by removing the buffer_new() check and adding a call to
truncate_inode_pages_range() to clear out the radix tree entries before we
insert the DAX PMD.

Signed-off-by: Ross Zwisler <redacted>

Replaced the current contents of v6 in -mm from next-20160106 with
this v7 set and it looks good.

Reported-by: Dan Williams <redacted>
Tested-by: Dan Williams <redacted>

One question below...

quoted

---
 fs/dax.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 03cc4a3..9dc0c97 100644
--- a/fs/dax.c
+++ b/fs/dax.c

@@ -594,6 +594,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
        bool write = flags & FAULT_FLAG_WRITE;
        struct block_device *bdev;
        pgoff_t size, pgoff;
+       loff_t lstart, lend;
        sector_t block;
        int result = 0;

@@ -647,15 +648,13 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                goto fallback;
        }

-       /*
-        * If we allocated new storage, make sure no process has any
-        * zero pages covering this hole
-        */
-       if (buffer_new(&bh)) {
-               i_mmap_unlock_read(mapping);
-               unmap_mapping_range(mapping, pgoff << PAGE_SHIFT, PMD_SIZE, 0);
-               i_mmap_lock_read(mapping);
-       }
+       /* make sure no process has any zero pages covering this hole */
+       lstart = pgoff << PAGE_SHIFT;
+       lend = lstart + PMD_SIZE - 1; /* inclusive */
+       i_mmap_unlock_read(mapping);
+       unmap_mapping_range(mapping, lstart, PMD_SIZE, 0);
+       truncate_inode_pages_range(mapping, lstart, lend);

Do we need to do both unmap and truncate given that
truncate_inode_page() optionally does an unmap_mapping_range()
internally?

Ah, indeed it does.  Sure, having just the call to truncate_inode_page() seems
cleaner.  I'll re-test and send this out in v8.

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