xfs: load uncached unlinked inodes into memory on demand

shrikanth hegde reports that filesystems fail shortly after mount with
the following failure:

	WARNING: CPU: 56 PID: 12450 at fs/xfs/xfs_inode.c:1839 xfs_iunlink_lookup+0x58/0x80 [xfs]

This of course is the WARN_ON_ONCE in xfs_iunlink_lookup:

	ip = radix_tree_lookup(&pag->pag_ici_root, agino);
	if (WARN_ON_ONCE(!ip || !ip->i_ino)) { ... }

From diagnostic data collected by the bug reporters, it would appear
that we cleanly mounted a filesystem that contained unlinked inodes.
Unlinked inodes are only processed as a final step of log recovery,
which means that clean mounts do not process the unlinked list at all.

Prior to the introduction of the incore unlinked lists, this wasn't a
problem because the unlink code would (very expensively) traverse the
entire ondisk metadata iunlink chain to keep things up to date.
However, the incore unlinked list code complains when it realizes that
it is out of sync with the ondisk metadata and shuts down the fs, which
is bad.

Ritesh proposed to solve this problem by unconditionally parsing the
unlinked lists at mount time, but this imposes a mount time cost for
every filesystem to catch something that should be very infrequent.
Instead, let's target the places where we can encounter a next_unlinked
pointer that refers to an inode that is not in cache, and load it into
cache.

Note: This patch does not address the problem of iget loading an inode
from the middle of the iunlink list and needing to set i_prev_unlinked
correctly.

Reported-by: shrikanth hegde <sshegde@linux.vnet.ibm.com>
Triaged-by: Ritesh Harjani <ritesh.list@gmail.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

Author: Darrick J. Wong

fs/xfs/xfs_inode.c

@@ -1828,12 +1828,17 @@ xfs_iunlink_lookup(
 
 	rcu_read_lock();
 	ip = radix_tree_lookup(&pag->pag_ici_root, agino);
+	if (!ip) {
+		/* Caller can handle inode not being in memory. */
+		rcu_read_unlock();
+		return NULL;
+	}
 
 	/*
-	 * Inode not in memory or in RCU freeing limbo should not happen.
-	 * Warn about this and let the caller handle the failure.
+	 * Inode in RCU freeing limbo should not happen.  Warn about this and
+	 * let the caller handle the failure.
 	 */
-	if (WARN_ON_ONCE(!ip || !ip->i_ino)) {
+	if (WARN_ON_ONCE(!ip->i_ino)) {
 		rcu_read_unlock();
 		return NULL;
 	}
@@ -1842,7 +1847,10 @@ xfs_iunlink_lookup(
 	return ip;
 }
 
-/* Update the prev pointer of the next agino. */
+/*
+ * Update the prev pointer of the next agino.  Returns -ENOLINK if the inode
+ * is not in cache.
+ */
 static int
 xfs_iunlink_update_backref(
 	struct xfs_perag	*pag,
@@ -1857,7 +1865,8 @@ xfs_iunlink_update_backref(
 
 	ip = xfs_iunlink_lookup(pag, next_agino);
 	if (!ip)
-		return -EFSCORRUPTED;
+		return -ENOLINK;
+
 	ip->i_prev_unlinked = prev_agino;
 	return 0;
 }
@@ -1901,6 +1910,62 @@ xfs_iunlink_update_bucket(
 	return 0;
 }
 
+/*
+ * Load the inode @next_agino into the cache and set its prev_unlinked pointer
+ * to @prev_agino.  Caller must hold the AGI to synchronize with other changes
+ * to the unlinked list.
+ */
+STATIC int
+xfs_iunlink_reload_next(
+	struct xfs_trans	*tp,
+	struct xfs_buf		*agibp,
+	xfs_agino_t		prev_agino,
+	xfs_agino_t		next_agino)
+{
+	struct xfs_perag	*pag = agibp->b_pag;
+	struct xfs_mount	*mp = pag->pag_mount;
+	struct xfs_inode	*next_ip = NULL;
+	xfs_ino_t		ino;
+	int			error;
+
+	ASSERT(next_agino != NULLAGINO);
+
+#ifdef DEBUG
+	rcu_read_lock();
+	next_ip = radix_tree_lookup(&pag->pag_ici_root, next_agino);
+	ASSERT(next_ip == NULL);
+	rcu_read_unlock();
+#endif
+
+	xfs_info_ratelimited(mp,
+ "Found unrecovered unlinked inode 0x%x in AG 0x%x.  Initiating recovery.",
+			next_agino, pag->pag_agno);
+
+	/*
+	 * Use an untrusted lookup just to be cautious in case the AGI has been
+	 * corrupted and now points at a free inode.  That shouldn't happen,
+	 * but we'd rather shut down now since we're already running in a weird
+	 * situation.
+	 */
+	ino = XFS_AGINO_TO_INO(mp, pag->pag_agno, next_agino);
+	error = xfs_iget(mp, tp, ino, XFS_IGET_UNTRUSTED, 0, &next_ip);
+	if (error)
+		return error;
+
+	/* If this is not an unlinked inode, something is very wrong. */
+	if (VFS_I(next_ip)->i_nlink != 0) {
+		error = -EFSCORRUPTED;
+		goto rele;
+	}
+
+	next_ip->i_prev_unlinked = prev_agino;
+	trace_xfs_iunlink_reload_next(next_ip);
+rele:
+	ASSERT(!(VFS_I(next_ip)->i_state & I_DONTCACHE));
+	xfs_irele(next_ip);
+	return error;
+}
+
 static int
 xfs_iunlink_insert_inode(
 	struct xfs_trans	*tp,
@@ -1932,6 +1997,8 @@ xfs_iunlink_insert_inode(
 	 * inode.
 	 */
 	error = xfs_iunlink_update_backref(pag, agino, next_agino);
+	if (error == -ENOLINK)
+		error = xfs_iunlink_reload_next(tp, agibp, agino, next_agino);
 	if (error)
 		return error;
 
@@ -2026,6 +2093,9 @@ xfs_iunlink_remove_inode(
 	 */
 	error = xfs_iunlink_update_backref(pag, ip->i_prev_unlinked,
 			ip->i_next_unlinked);
+	if (error == -ENOLINK)
+		error = xfs_iunlink_reload_next(tp, agibp, ip->i_prev_unlinked,
+				ip->i_next_unlinked);
 	if (error)
 		return error;
 

fs/xfs/xfs_trace.h

@@ -3824,6 +3824,31 @@ TRACE_EVENT(xfs_iunlink_update_dinode,
 		  __entry->new_ptr)
 );
 
+TRACE_EVENT(xfs_iunlink_reload_next,
+	TP_PROTO(struct xfs_inode *ip),
+	TP_ARGS(ip),
+	TP_STRUCT__entry(
+		__field(dev_t, dev)
+		__field(xfs_agnumber_t, agno)
+		__field(xfs_agino_t, agino)
+		__field(xfs_agino_t, prev_agino)
+		__field(xfs_agino_t, next_agino)
+	),
+	TP_fast_assign(
+		__entry->dev = ip->i_mount->m_super->s_dev;
+		__entry->agno = XFS_INO_TO_AGNO(ip->i_mount, ip->i_ino);
+		__entry->agino = XFS_INO_TO_AGINO(ip->i_mount, ip->i_ino);
+		__entry->prev_agino = ip->i_prev_unlinked;
+		__entry->next_agino = ip->i_next_unlinked;
+	),
+	TP_printk("dev %d:%d agno 0x%x agino 0x%x prev_unlinked 0x%x next_unlinked 0x%x",
+		  MAJOR(__entry->dev), MINOR(__entry->dev),
+		  __entry->agno,
+		  __entry->agino,
+		  __entry->prev_agino,
+		  __entry->next_agino)
+);
+
 DECLARE_EVENT_CLASS(xfs_ag_inode_class,
 	TP_PROTO(struct xfs_inode *ip),
 	TP_ARGS(ip),