dax: Flush partial PMDs correctly

The radix tree would rewind the index in an iterator to the lowest index
of a multi-slot entry.  The XArray iterators instead leave the index
unchanged, but I overlooked that when converting DAX from the radix tree
to the XArray.  Adjust the index that we use for flushing to the start
of the PMD range.

Fixes: c1901cd3 ("page cache: Convert find_get_entries_tag to XArray")
Cc: <stable@vger.kernel.org>
Reported-by: NPiotr Balcer <piotr.balcer@intel.com>
Tested-by: NDan Williams <dan.j.williams@intel.com>
Reviewed-by: NJan Kara <jack@suse.cz>
Signed-off-by: NMatthew Wilcox <willy@infradead.org>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

fs/dax.c

@@ -843,9 +843,8 @@ static void dax_entry_mkclean(struct address_space *mapping, pgoff_t index,
 static int dax_writeback_one(struct xa_state *xas, struct dax_device *dax_dev,
 		struct address_space *mapping, void *entry)
 {
-	unsigned long pfn;
+	unsigned long pfn, index, count;
 	long ret = 0;
-	size_t size;
 
 	/*
 	 * A page got tagged dirty in DAX mapping? Something is seriously
@@ -894,17 +893,18 @@ static int dax_writeback_one(struct xa_state *xas, struct dax_device *dax_dev,
 	xas_unlock_irq(xas);
 
 	/*
-	 * Even if dax_writeback_mapping_range() was given a wbc->range_start
-	 * in the middle of a PMD, the 'index' we are given will be aligned to
-	 * the start index of the PMD, as will the pfn we pull from 'entry'.
+	 * If dax_writeback_mapping_range() was given a wbc->range_start
+	 * in the middle of a PMD, the 'index' we use needs to be
+	 * aligned to the start of the PMD.
 	 * This allows us to flush for PMD_SIZE and not have to worry about
 	 * partial PMD writebacks.
 	 */
 	pfn = dax_to_pfn(entry);
-	size = PAGE_SIZE << dax_entry_order(entry);
+	count = 1UL << dax_entry_order(entry);
+	index = xas->xa_index & ~(count - 1);
 
-	dax_entry_mkclean(mapping, xas->xa_index, pfn);
-	dax_flush(dax_dev, page_address(pfn_to_page(pfn)), size);
+	dax_entry_mkclean(mapping, index, pfn);
+	dax_flush(dax_dev, page_address(pfn_to_page(pfn)), count * PAGE_SIZE);
 	/*
 	 * After we have flushed the cache, we can clear the dirty tag. There
 	 * cannot be new dirty data in the pfn after the flush has completed as
@@ -917,8 +917,7 @@ static int dax_writeback_one(struct xa_state *xas, struct dax_device *dax_dev,
 	xas_clear_mark(xas, PAGECACHE_TAG_DIRTY);
 	dax_wake_entry(xas, entry, false);
 
-	trace_dax_writeback_one(mapping->host, xas->xa_index,
-			size >> PAGE_SHIFT);
+	trace_dax_writeback_one(mapping->host, index, count);
 	return ret;
 
  put_unlocked: