btrfs: Reword dodgy comments in alloc_extent_buffer

Commit eb14ab8e ("Btrfs: fix page->private races") fixed a genuine
race between extent buffer initialisation and btree_releasepage.
Unfortunately as the code has evolved the comments weren't changed which
made them slightly wrong and they weren't very clear in the fist place.
Fix this by (hopefully) rewording them in a more approachable manner.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -5009,8 +5009,11 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 			uptodate = 0;
 
 		/*
-		 * see below about how we avoid a nasty race with release page
-		 * and why we unlock later
+		 * We can't unlock the pages just yet since the extent buffer
+		 * hasn't been properly inserted in the radix tree, this
+		 * opens a race with btree_releasepage which can free a page
+		 * while we are still filling in all pages for the buffer and
+		 * we could crash.
 		 */
 	}
 	if (uptodate)
@@ -5039,13 +5042,9 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 	set_bit(EXTENT_BUFFER_IN_TREE, &eb->bflags);
 
 	/*
-	 * there is a race where release page may have
-	 * tried to find this extent buffer in the radix
-	 * but failed.  It will tell the VM it is safe to
-	 * reclaim the, and it will clear the page private bit.
-	 * We must make sure to set the page private bit properly
-	 * after the extent buffer is in the radix tree so
-	 * it doesn't get lost
+	 * Now it's safe to unlock the pages because any calls to
+	 * btree_releasepage will correctly detect that a page belongs to a
+	 * live buffer and won't free them prematurely.
 	 */
 	for (i = 0; i < num_pages; i++)
 		unlock_page(eb->pages[i]);