xfs: add debug knob to slow down write for fun

Add a new error injection knob so that we can arbitrarily slow down
pagecache writes to test for race conditions and aberrant reclaim
behavior if the writeback mechanisms are slow to issue writeback. This
will enable functional testing for the ifork sequence counters
introduced in commit 304a68b9c63b ("xfs: use iomap_valid method to
detect stale cached iomaps") that fixes write racing with reclaim
writeback.

Signed-off-by: Darrick J. Wong <[e-mail]>
Reviewed-by: Dave Chinner <[e-mail]>

fs/xfs/libxfs/xfs_errortag.h

···
 #define XFS_ERRTAG_DA_LEAF_SPLIT			40
 #define XFS_ERRTAG_ATTR_LEAF_TO_NODE			41
 #define XFS_ERRTAG_WB_DELAY_MS				42
-#define XFS_ERRTAG_MAX					43
+#define XFS_ERRTAG_WRITE_DELAY_MS			43
+#define XFS_ERRTAG_MAX					44
 
 /*
  * Random factors for above tags, 1 means always, 2 means 1/2 time, etc.
···
 #define XFS_RANDOM_DA_LEAF_SPLIT			1
 #define XFS_RANDOM_ATTR_LEAF_TO_NODE			1
 #define XFS_RANDOM_WB_DELAY_MS				3000
+#define XFS_RANDOM_WRITE_DELAY_MS			3000
 
 #endif /* __XFS_ERRORTAG_H_ */

fs/xfs/xfs_error.c

···
 	XFS_RANDOM_DA_LEAF_SPLIT,
 	XFS_RANDOM_ATTR_LEAF_TO_NODE,
 	XFS_RANDOM_WB_DELAY_MS,
+	XFS_RANDOM_WRITE_DELAY_MS,
 };
 
 struct xfs_errortag_attr {
···
 XFS_ERRORTAG_ATTR_RW(da_leaf_split,	XFS_ERRTAG_DA_LEAF_SPLIT);
 XFS_ERRORTAG_ATTR_RW(attr_leaf_to_node,	XFS_ERRTAG_ATTR_LEAF_TO_NODE);
 XFS_ERRORTAG_ATTR_RW(wb_delay_ms,	XFS_ERRTAG_WB_DELAY_MS);
+XFS_ERRORTAG_ATTR_RW(write_delay_ms,	XFS_ERRTAG_WRITE_DELAY_MS);
 
 static struct attribute *xfs_errortag_attrs[] = {
 	XFS_ERRORTAG_ATTR_LIST(noerror),
···
 	XFS_ERRORTAG_ATTR_LIST(da_leaf_split),
 	XFS_ERRORTAG_ATTR_LIST(attr_leaf_to_node),
 	XFS_ERRORTAG_ATTR_LIST(wb_delay_ms),
+	XFS_ERRORTAG_ATTR_LIST(write_delay_ms),
 	NULL,
 };
 ATTRIBUTE_GROUPS(xfs_errortag);

fs/xfs/xfs_iomap.c

···
 #include "xfs_dquot_item.h"
 #include "xfs_dquot.h"
 #include "xfs_reflink.h"
+#include "xfs_error.h"
+#include "xfs_errortag.h"
 
 #define XFS_ALLOC_ALIGN(mp, off) \
 	(((off) >> mp->m_allocsize_log) << mp->m_allocsize_log)
···
 	struct inode		*inode,
 	const struct iomap	*iomap)
 {
-	return iomap->validity_cookie ==
-			xfs_iomap_inode_sequence(XFS_I(inode), iomap->flags);
+	struct xfs_inode	*ip = XFS_I(inode);
+
+	if (iomap->validity_cookie !=
+			xfs_iomap_inode_sequence(ip, iomap->flags)) {
+		trace_xfs_iomap_invalid(ip, iomap);
+		return false;
+	}
+
+	XFS_ERRORTAG_DELAY(ip->i_mount, XFS_ERRTAG_WRITE_DELAY_MS);
+	return true;
 }
 
 const struct iomap_page_ops xfs_iomap_page_ops = {

fs/xfs/xfs_trace.h

···
 DEFINE_WB_INVALID_EVENT(xfs_wb_cow_iomap_invalid);
 DEFINE_WB_INVALID_EVENT(xfs_wb_data_iomap_invalid);
 
+DECLARE_EVENT_CLASS(xfs_iomap_invalid_class,
+	TP_PROTO(struct xfs_inode *ip, const struct iomap *iomap),
+	TP_ARGS(ip, iomap),
+	TP_STRUCT__entry(
+		__field(dev_t, dev)
+		__field(xfs_ino_t, ino)
+		__field(u64, addr)
+		__field(loff_t, pos)
+		__field(u64, len)
+		__field(u64, validity_cookie)
+		__field(u64, inodeseq)
+		__field(u16, type)
+		__field(u16, flags)
+	),
+	TP_fast_assign(
+		__entry->dev = VFS_I(ip)->i_sb->s_dev;
+		__entry->ino = ip->i_ino;
+		__entry->addr = iomap->addr;
+		__entry->pos = iomap->offset;
+		__entry->len = iomap->length;
+		__entry->validity_cookie = iomap->validity_cookie;
+		__entry->type = iomap->type;
+		__entry->flags = iomap->flags;
+		__entry->inodeseq = xfs_iomap_inode_sequence(ip, iomap->flags);
+	),
+	TP_printk("dev %d:%d ino 0x%llx pos 0x%llx addr 0x%llx bytecount 0x%llx type 0x%x flags 0x%x validity_cookie 0x%llx inodeseq 0x%llx",
+		  MAJOR(__entry->dev), MINOR(__entry->dev),
+		  __entry->ino,
+		  __entry->pos,
+		  __entry->addr,
+		  __entry->len,
+		  __entry->type,
+		  __entry->flags,
+		  __entry->validity_cookie,
+		  __entry->inodeseq)
+);
+#define DEFINE_IOMAP_INVALID_EVENT(name) \
+DEFINE_EVENT(xfs_iomap_invalid_class, name, \
+	TP_PROTO(struct xfs_inode *ip, const struct iomap *iomap), \
+	TP_ARGS(ip, iomap))
+DEFINE_IOMAP_INVALID_EVENT(xfs_iomap_invalid);
+
 /* refcount/reflink tracepoint definitions */
 
 /* reflink tracepoints */