dax: Hash on XArray instead of mapping

Since the XArray is embedded in the struct address_space, its address
contains exactly as much entropy as the address of the mapping.  This
patch is purely preparatory for later patches which will simplify the
wait/wake interfaces.
Signed-off-by: Matthew Wilcox <willy@infradead.org>

fs/dax.c

@@ -116,7 +116,7 @@ static int dax_is_empty_entry(void *entry)
  * DAX page cache entry locking
  */
 struct exceptional_entry_key {
-	struct address_space *mapping;
+	struct xarray *xa;
 	pgoff_t entry_start;
 };
 
@@ -125,7 +125,7 @@ struct wait_exceptional_entry_queue {
 	struct exceptional_entry_key key;
 };
 
-static wait_queue_head_t *dax_entry_waitqueue(struct address_space *mapping,
+static wait_queue_head_t *dax_entry_waitqueue(struct xarray *xa,
 		pgoff_t index, void *entry, struct exceptional_entry_key *key)
 {
 	unsigned long hash;
@@ -138,21 +138,21 @@ static wait_queue_head_t *dax_entry_waitqueue(struct address_space *mapping,
 	if (dax_is_pmd_entry(entry))
 		index &= ~PG_PMD_COLOUR;
 
-	key->mapping = mapping;
+	key->xa = xa;
 	key->entry_start = index;
 
-	hash = hash_long((unsigned long)mapping ^ index, DAX_WAIT_TABLE_BITS);
+	hash = hash_long((unsigned long)xa ^ index, DAX_WAIT_TABLE_BITS);
 	return wait_table + hash;
 }
 
-static int wake_exceptional_entry_func(wait_queue_entry_t *wait, unsigned int mode,
-				       int sync, void *keyp)
+static int wake_exceptional_entry_func(wait_queue_entry_t *wait,
+		unsigned int mode, int sync, void *keyp)
 {
 	struct exceptional_entry_key *key = keyp;
 	struct wait_exceptional_entry_queue *ewait =
 		container_of(wait, struct wait_exceptional_entry_queue, wait);
 
-	if (key->mapping != ewait->key.mapping ||
+	if (key->xa != ewait->key.xa ||
 	    key->entry_start != ewait->key.entry_start)
 		return 0;
 	return autoremove_wake_function(wait, mode, sync, NULL);
@@ -163,13 +163,13 @@ static int wake_exceptional_entry_func(wait_queue_entry_t *wait, unsigned int mo
  * The important information it's conveying is whether the entry at
  * this index used to be a PMD entry.
  */
-static void dax_wake_mapping_entry_waiter(struct address_space *mapping,
+static void dax_wake_mapping_entry_waiter(struct xarray *xa,
 		pgoff_t index, void *entry, bool wake_all)
 {
 	struct exceptional_entry_key key;
 	wait_queue_head_t *wq;
 
-	wq = dax_entry_waitqueue(mapping, index, entry, &key);
+	wq = dax_entry_waitqueue(xa, index, entry, &key);
 
 	/*
 	 * Checking for locked entry and prepare_to_wait_exclusive() happens
@@ -248,7 +248,8 @@ static void *__get_unlocked_mapping_entry(struct address_space *mapping,
 			return entry;
 		}
 
-		wq = dax_entry_waitqueue(mapping, index, entry, &ewait.key);
+		wq = dax_entry_waitqueue(&mapping->i_pages, index, entry,
+				&ewait.key);
 		prepare_to_wait_exclusive(wq, &ewait.wait,
 					  TASK_UNINTERRUPTIBLE);
 		xa_unlock_irq(&mapping->i_pages);
@@ -289,7 +290,7 @@ static void unlock_mapping_entry(struct address_space *mapping, pgoff_t index)
 	}
 	unlock_slot(mapping, slot);
 	xa_unlock_irq(&mapping->i_pages);
-	dax_wake_mapping_entry_waiter(mapping, index, entry, false);
+	dax_wake_mapping_entry_waiter(&mapping->i_pages, index, entry, false);
 }
 
 static void put_locked_mapping_entry(struct address_space *mapping,
@@ -309,7 +310,7 @@ static void put_unlocked_mapping_entry(struct address_space *mapping,
 		return;
 
 	/* We have to wake up next waiter for the page cache entry lock */
-	dax_wake_mapping_entry_waiter(mapping, index, entry, false);
+	dax_wake_mapping_entry_waiter(&mapping->i_pages, index, entry, false);
 }
 
 static unsigned long dax_entry_size(void *entry)
@@ -578,8 +579,8 @@ static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index,
 			dax_disassociate_entry(entry, mapping, false);
 			radix_tree_delete(&mapping->i_pages, index);
 			mapping->nrexceptional--;
-			dax_wake_mapping_entry_waiter(mapping, index, entry,
-					true);
+			dax_wake_mapping_entry_waiter(&mapping->i_pages,
+					index, entry, true);
 		}
 
 		entry = dax_make_locked(0, size_flag | DAX_EMPTY);