dax: Remove i_mmap_lock protection

Currently faults are protected against truncate by filesystem specific
i_mmap_sem and page lock in case of hole page. Cow faults are protected
DAX radix tree entry locking. So there's no need for i_mmap_lock in DAX
code. Remove it.
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>

fs/dax.c

@@ -798,29 +798,19 @@ static int dax_insert_mapping(struct address_space *mapping,
 		.sector = to_sector(bh, mapping->host),
 		.size = bh->b_size,
 	};
-	int error;
 	void *ret;
 	void *entry = *entryp;
 
-	i_mmap_lock_read(mapping);
-
-	if (dax_map_atomic(bdev, &dax) < 0) {
-		error = PTR_ERR(dax.addr);
-		goto out;
-	}
+	if (dax_map_atomic(bdev, &dax) < 0)
+		return PTR_ERR(dax.addr);
 	dax_unmap_atomic(bdev, &dax);
 
 	ret = dax_insert_mapping_entry(mapping, vmf, entry, dax.sector);
-	if (IS_ERR(ret)) {
-		error = PTR_ERR(ret);
-		goto out;
-	}
+	if (IS_ERR(ret))
+		return PTR_ERR(ret);
 	*entryp = ret;
 
-	error = vm_insert_mixed(vma, vaddr, dax.pfn);
- out:
-	i_mmap_unlock_read(mapping);
-	return error;
+	return vm_insert_mixed(vma, vaddr, dax.pfn);
 }
 
 /**
@@ -1058,8 +1048,6 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
 		truncate_pagecache_range(inode, lstart, lend);
 	}
 
-	i_mmap_lock_read(mapping);
-
 	if (!write && !buffer_mapped(&bh)) {
 		spinlock_t *ptl;
 		pmd_t entry;
@@ -1148,8 +1136,6 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
 	}
 
  out:
-	i_mmap_unlock_read(mapping);
-
 	return result;
 
  fallback:

mm/memory.c

@@ -2453,8 +2453,6 @@ void unmap_mapping_range(struct address_space *mapping,
 	if (details.last_index < details.first_index)
 		details.last_index = ULONG_MAX;
 
-
-	/* DAX uses i_mmap_lock to serialise file truncate vs page fault */
 	i_mmap_lock_write(mapping);
 	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap)))
 		unmap_mapping_range_tree(&mapping->i_mmap, &details);