btrfs: introduce helper to grab an existing extent buffer from a page

This patch will extract the code to grab an extent buffer from a page
into a helper, grab_extent_buffer_from_page().

This reduces one indent level, and provides the work place for later
expansion for subapge support.

Reviewed-by: Johannes Thumshirn <[email protected]>
Signed-off-by: Qu Wenruo <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

fs/btrfs/extent_io.c

@@ -5247,6 +5247,29 @@ struct extent_buffer *alloc_test_extent_buffer(struct btrfs_fs_info *fs_info,
 }
 #endif
 
+static struct extent_buffer *grab_extent_buffer(struct page *page)
+{
+ struct extent_buffer *exists;
+
+ /* Page not yet attached to an extent buffer */
+ if (!PagePrivate(page))
+ return NULL;
+
+ /*
+ * We could have already allocated an eb for this page and attached one
+ * so lets see if we can get a ref on the existing eb, and if we can we
+ * know it's good and we can just return that one, else we know we can
+ * just overwrite page->private.
+ */
+ exists = (struct extent_buffer *)page->private;
+ if (atomic_inc_not_zero(&exists->refs))
+ return exists;
+
+ WARN_ON(PageDirty(page));
+ detach_page_private(page);
+ return NULL;
+}
+
 struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
  u64 start, u64 owner_root, int level)
 {
@@ -5292,26 +5315,13 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
  }
 
  spin_lock(&mapping->private_lock);
- if (PagePrivate(p)) {
- /*
- * We could have already allocated an eb for this page
- * and attached one so lets see if we can get a ref on
- * the existing eb, and if we can we know it's good and
- * we can just return that one, else we know we can just
- * overwrite page->private.
- */
- exists = (struct extent_buffer *)p->private;
- if (atomic_inc_not_zero(&exists->refs)) {
- spin_unlock(&mapping->private_lock);
- unlock_page(p);
- put_page(p);
- mark_extent_buffer_accessed(exists, p);
- goto free_eb;
- }
- exists = NULL;
-
- WARN_ON(PageDirty(p));
- detach_page_private(p);
+ exists = grab_extent_buffer(p);
+ if (exists) {
+ spin_unlock(&mapping->private_lock);
+ unlock_page(p);
+ put_page(p);
+ mark_extent_buffer_accessed(exists, p);
+ goto free_eb;
  }
  attach_extent_buffer_page(eb, p);
  spin_unlock(&mapping->private_lock);