[PATCH] fix nr_unused accounting, and avoid recursing in iput with I_WILL_FREE set

 			list_move(&inode->i_list, &inode_in_use);
 		} else {
 			list_move(&inode->i_list, &inode_unused);
+			inodes_stat.nr_unused++;
 		}
 	}
 	wake_up_inode(inode);

Are you sure the above diff is correct? It was added somewhere between
2.6.5 and 2.6.8. I think it's wrong.

The only way I can imagine the i_count to be zero in the above path, is
that I_WILL_FREE is set.  And if I_WILL_FREE is set, then we must not
increase nr_unused.  So I believe the above change is buggy and it will
definitely overstate the number of unused inodes and it should be backed
out.

Note that __writeback_single_inode before calling __sync_single_inode, can
drop the spinlock and we can have both the dirty and locked bitflags clear
here:

		spin_unlock(&inode_lock);
		__wait_on_inode(inode);
		iput(inode);
XXXXXXX
		spin_lock(&inode_lock);
	}
	use inode again here

a construct like the above makes zero sense from a reference counting
standpoint.

Either we don't ever use the inode again after the iput, or the
inode_lock should be taken _before_ executing the iput (i.e. a __iput
would be required). Taking the inode_lock after iput means the iget was
useless if we keep using the inode after the iput.

So the only chance the 2.6 was safe to call __writeback_single_inode
with the i_count == 0, is that I_WILL_FREE is set (I_WILL_FREE will
prevent the VM to free the inode in XXXXX).

Potentially calling the above iput with I_WILL_FREE was also wrong
because it would recurse in iput_final (the second mainline bug).

The below (untested) patch fixes the nr_unused accounting, avoids recursing
in iput when I_WILL_FREE is set and makes sure (with the BUG_ON) that we
don't corrupt memory and that all holders that don't set I_WILL_FREE, keeps
a reference on the inode!

Signed-off-by: Andrea Arcangeli <andrea@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/fs-writeback.c

@@ -230,7 +230,6 @@ __sync_single_inode(struct inode *inode, struct writeback_control *wbc)
 			 * The inode is clean, unused
 			 */
 			list_move(&inode->i_list, &inode_unused);
-			inodes_stat.nr_unused++;
 		}
 	}
 	wake_up_inode(inode);
@@ -238,14 +237,20 @@ __sync_single_inode(struct inode *inode, struct writeback_control *wbc)
 }
 
 /*
- * Write out an inode's dirty pages.  Called under inode_lock.
+ * Write out an inode's dirty pages.  Called under inode_lock.  Either the
+ * caller has ref on the inode (either via __iget or via syscall against an fd)
+ * or the inode has I_WILL_FREE set (via generic_forget_inode)
  */
 static int
-__writeback_single_inode(struct inode *inode,
-			struct writeback_control *wbc)
+__writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
 {
 	wait_queue_head_t *wqh;
 
+	if (!atomic_read(&inode->i_count))
+		WARN_ON(!(inode->i_state & I_WILL_FREE));
+	else
+		WARN_ON(inode->i_state & I_WILL_FREE);
+
 	if ((wbc->sync_mode != WB_SYNC_ALL) && (inode->i_state & I_LOCK)) {
 		list_move(&inode->i_list, &inode->i_sb->s_dirty);
 		return 0;
@@ -259,11 +264,9 @@ __writeback_single_inode(struct inode *inode,
 
 		wqh = bit_waitqueue(&inode->i_state, __I_LOCK);
 		do {
-			__iget(inode);
 			spin_unlock(&inode_lock);
 			__wait_on_bit(wqh, &wq, inode_wait,
 							TASK_UNINTERRUPTIBLE);
-			iput(inode);
 			spin_lock(&inode_lock);
 		} while (inode->i_state & I_LOCK);
 	}
@@ -541,14 +544,15 @@ void sync_inodes(int wait)
 }
 
 /**
- *	write_inode_now	-	write an inode to disk
- *	@inode: inode to write to disk
- *	@sync: whether the write should be synchronous or not
+ * write_inode_now	-	write an inode to disk
+ * @inode: inode to write to disk
+ * @sync: whether the write should be synchronous or not
+ *
+ * This function commits an inode to disk immediately if it is dirty. This is
+ * primarily needed by knfsd.
  *
- *	This function commits an inode to disk immediately if it is
- *	dirty. This is primarily needed by knfsd.
+ * The caller must either have a ref on the inode or must have set I_WILL_FREE.
  */
- 
 int write_inode_now(struct inode *inode, int sync)
 {
 	int ret;

fs/inode.c

@@ -1088,6 +1088,7 @@ static void generic_forget_inode(struct inode *inode)
 	if (inode->i_data.nrpages)
 		truncate_inode_pages(&inode->i_data, 0);
 	clear_inode(inode);
+	wake_up_inode(inode);
 	destroy_inode(inode);
 }