DAX radix tree locking currently locks entries based on the unique
combination of the 'mapping' pointer and the pgoff_t 'index' for the entry.
This works for PTEs, but as we move to PMDs we will need to have all the
offsets within the range covered by the PMD to map to the same bit lock.
To accomplish this, for ranges covered by a PMD entry we will instead lock
based on the page offset of the beginning of the PMD entry.  The 'mapping'
pointer is still used in the same way.

Signed-off-by: Ross Zwisler <redacted>
---
 fs/dax.c            | 37 ++++++++++++++++++++++++-------------
 include/linux/dax.h |  2 +-
 mm/filemap.c        |  2 +-
 3 files changed, 26 insertions(+), 15 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index baef586..406feea 100644
--- a/fs/dax.c
+++ b/fs/dax.c

@@ -64,10 +64,17 @@ static int __init init_dax_wait_table(void)
 }
 fs_initcall(init_dax_wait_table);
 
+static pgoff_t dax_entry_start(pgoff_t index, void *entry)
+{
+	if (RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD)
+		index &= (PMD_MASK >> PAGE_SHIFT);

@@ -447,10 +457,11 @@ restart:
 	return entry;
 }
 
-void dax_wake_mapping_entry_waiter(struct address_space *mapping,
+void dax_wake_mapping_entry_waiter(void *entry, struct address_space *mapping,
 				   pgoff_t index, bool wake_all)