[PATCH] Allow truncate to discard unmapped buffers

The buffer state "uptodate, dirty, unmapped" is legal.  It occurs when
a mapped page with attached buffers which is over a hole is dirtied.

So discard_buffer() needs to be able to discard those buffers as well;
otherwise the page ends up clean, with dirty buffers and is unfreeable.

fs/buffer.c

@@ -1091,17 +1091,15 @@ EXPORT_SYMBOL(set_bh_page);
 /*
  * Called when truncating a buffer on a page completely.
  */
-static void discard_buffer(struct buffer_head * bh)
+static /* inline */ void discard_buffer(struct buffer_head * bh)
 {
-	if (buffer_mapped(bh)) {
-		clear_buffer_dirty(bh);
 	lock_buffer(bh);
+	clear_buffer_dirty(bh);
 	bh->b_bdev = NULL;
 	clear_buffer_mapped(bh);
 	clear_buffer_req(bh);
 	clear_buffer_new(bh);
 	unlock_buffer(bh);
-	}
 }
 
 /**

fs/jbd/transaction.c

@@ -1587,7 +1587,7 @@ static int __journal_try_to_free_buffer(struct buffer_head *bh,
 		goto out;
 	}
 
-	if (!buffer_uptodate(bh))
+	if (!buffer_uptodate(bh))	/* AKPM: why? */
 		goto out;
 
 	if (jh->b_next_transaction != 0)
@@ -1775,9 +1775,6 @@ static int journal_unmap_buffer(journal_t *journal, struct buffer_head *bh)
 
 	BUFFER_TRACE(bh, "entry");
 
-	if (!buffer_mapped(bh))
-		return 1;
-
 	/* It is safe to proceed here without the
 	 * journal_datalist_spinlock because the buffers cannot be
 	 * stolen by try_to_free_buffers as long as we are holding the