ocfs2: make metadata estimation accurate and clear

Current code assume that ::w_unwritten_list always has only one item on.
This is not right and hard to get understood.  So improve how to count
unwritten item.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Changwei Ge <[email protected]>
Reported-by: John Lightsey <[email protected]>
Tested-by: John Lightsey <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joseph Qi <[email protected]>
Cc: Junxiao Bi <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Changwei Ge <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/ocfs2/aops.c

@@ -797,6 +797,7 @@ struct ocfs2_write_ctxt {
 	struct ocfs2_cached_dealloc_ctxt w_dealloc;
 
 	struct list_head		w_unwritten_list;
+	unsigned int			w_unwritten_count;
 };
 
 void ocfs2_unlock_and_free_pages(struct page **pages, int num_pages)
@@ -1386,6 +1387,7 @@ static int ocfs2_unwritten_check(struct inode *inode,
 	desc->c_clear_unwritten = 0;
 	list_add_tail(&new->ue_ip_node, &oi->ip_unwritten_list);
 	list_add_tail(&new->ue_node, &wc->w_unwritten_list);
+	wc->w_unwritten_count++;
 	new = NULL;
 unlock:
 	spin_unlock(&oi->ip_lock);
@@ -2256,7 +2258,7 @@ static int ocfs2_dio_wr_get_block(struct inode *inode, sector_t iblock,
 		ue->ue_phys = desc->c_phys;
 
 		list_splice_tail_init(&wc->w_unwritten_list, &dwc->dw_zero_list);
-		dwc->dw_zero_count++;
+		dwc->dw_zero_count += wc->w_unwritten_count;
 	}
 
 	ret = ocfs2_write_end_nolock(inode->i_mapping, pos, len, len, wc);