xfs: add debug knob to slow down writeback for fun

Add a new error injection knob so that we can arbitrarily slow down
writeback 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
745b3f76d1c8 ("xfs: maintain a sequence count for inode fork
manipulations").

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

fs/xfs/libxfs/xfs_errortag.h

···
 #define XFS_ERRTAG_LARP					39
 #define XFS_ERRTAG_DA_LEAF_SPLIT			40
 #define XFS_ERRTAG_ATTR_LEAF_TO_NODE			41
-#define XFS_ERRTAG_MAX					42
+#define XFS_ERRTAG_WB_DELAY_MS				42
+#define XFS_ERRTAG_MAX					43
 
 /*
  * Random factors for above tags, 1 means always, 2 means 1/2 time, etc.
···
 #define XFS_RANDOM_LARP					1
 #define XFS_RANDOM_DA_LEAF_SPLIT			1
 #define XFS_RANDOM_ATTR_LEAF_TO_NODE			1
+#define XFS_RANDOM_WB_DELAY_MS				3000
 
 #endif /* __XFS_ERRORTAG_H_ */

fs/xfs/xfs_aops.c

···
 #include "xfs_bmap.h"
 #include "xfs_bmap_util.h"
 #include "xfs_reflink.h"
+#include "xfs_errortag.h"
+#include "xfs_error.h"
 
 struct xfs_writepage_ctx {
 	struct iomap_writepage_ctx ctx;
···
 	 * checked (and found nothing at this offset) could have added
 	 * overlapping blocks.
 	 */
-	if (XFS_WPC(wpc)->data_seq != READ_ONCE(ip->i_df.if_seq))
+	if (XFS_WPC(wpc)->data_seq != READ_ONCE(ip->i_df.if_seq)) {
+		trace_xfs_wb_data_iomap_invalid(ip, &wpc->iomap,
+				XFS_WPC(wpc)->data_seq, XFS_DATA_FORK);
 		return false;
+	}
 	if (xfs_inode_has_cow_data(ip) &&
-	    XFS_WPC(wpc)->cow_seq != READ_ONCE(ip->i_cowfp->if_seq))
+	    XFS_WPC(wpc)->cow_seq != READ_ONCE(ip->i_cowfp->if_seq)) {
+		trace_xfs_wb_cow_iomap_invalid(ip, &wpc->iomap,
+				XFS_WPC(wpc)->cow_seq, XFS_COW_FORK);
 		return false;
+	}
 	return true;
 }
 
···
 
 	if (xfs_is_shutdown(mp))
 		return -EIO;
+
+	XFS_ERRORTAG_DELAY(mp, XFS_ERRTAG_WB_DELAY_MS);
 
 	/*
 	 * COW fork blocks can overlap data fork blocks even if the blocks

fs/xfs/xfs_error.c

···
 	XFS_RANDOM_LARP,
 	XFS_RANDOM_DA_LEAF_SPLIT,
 	XFS_RANDOM_ATTR_LEAF_TO_NODE,
+	XFS_RANDOM_WB_DELAY_MS,
 };
 
 struct xfs_errortag_attr {
···
 XFS_ERRORTAG_ATTR_RW(larp,		XFS_ERRTAG_LARP);
 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);
 
 static struct attribute *xfs_errortag_attrs[] = {
 	XFS_ERRORTAG_ATTR_LIST(noerror),
···
 	XFS_ERRORTAG_ATTR_LIST(larp),
 	XFS_ERRORTAG_ATTR_LIST(da_leaf_split),
 	XFS_ERRORTAG_ATTR_LIST(attr_leaf_to_node),
+	XFS_ERRORTAG_ATTR_LIST(wb_delay_ms),
 	NULL,
 };
 ATTRIBUTE_GROUPS(xfs_errortag);
···
 	if (error_tag == XFS_ERRTAG_DROP_WRITES)
 		return false;
 	return true;
+}
+
+bool
+xfs_errortag_enabled(
+	struct xfs_mount	*mp,
+	unsigned int		tag)
+{
+	if (!mp->m_errortag)
+		return false;
+	if (!xfs_errortag_valid(tag))
+		return false;
+
+	return mp->m_errortag[tag] != 0;
 }
 
 bool

fs/xfs/xfs_error.h

···
 		const char *file, int line, unsigned int error_tag);
 #define XFS_TEST_ERROR(expr, mp, tag)		\
 	((expr) || xfs_errortag_test((mp), #expr, __FILE__, __LINE__, (tag)))
+bool xfs_errortag_enabled(struct xfs_mount *mp, unsigned int tag);
+#define XFS_ERRORTAG_DELAY(mp, tag)		\
+	do { \
+		might_sleep(); \
+		if (!xfs_errortag_enabled((mp), (tag))) \
+			break; \
+		xfs_warn_ratelimited((mp), \
+"Injecting %ums delay at file %s, line %d, on filesystem \"%s\"", \
+				(mp)->m_errortag[(tag)], __FILE__, __LINE__, \
+				(mp)->m_super->s_id); \
+		mdelay((mp)->m_errortag[(tag)]); \
+	} while (0)
 
 extern int xfs_errortag_get(struct xfs_mount *mp, unsigned int error_tag);
 extern int xfs_errortag_set(struct xfs_mount *mp, unsigned int error_tag,
···
 #define xfs_errortag_init(mp)			(0)
 #define xfs_errortag_del(mp)
 #define XFS_TEST_ERROR(expr, mp, tag)		(expr)
+#define XFS_ERRORTAG_DELAY(mp, tag)		((void)0)
 #define xfs_errortag_set(mp, tag, val)		(ENOSYS)
 #define xfs_errortag_add(mp, tag)		(ENOSYS)
 #define xfs_errortag_clearall(mp)		(ENOSYS)

fs/xfs/xfs_trace.c

···
 #include "xfs_ag.h"
 #include "xfs_ag_resv.h"
 #include "xfs_error.h"
+#include <linux/iomap.h>
+#include "xfs_iomap.h"
 
 /*
  * We include this last to have the helpers above available for the trace

fs/xfs/xfs_trace.h

···
 	TP_PROTO(struct xfs_inode *ip, struct xfs_bmbt_irec *irec), \
 	TP_ARGS(ip, irec))
 
+/* inode iomap invalidation events */
+DECLARE_EVENT_CLASS(xfs_wb_invalid_class,
+	TP_PROTO(struct xfs_inode *ip, const struct iomap *iomap, unsigned int wpcseq, int whichfork),
+	TP_ARGS(ip, iomap, wpcseq, whichfork),
+	TP_STRUCT__entry(
+		__field(dev_t, dev)
+		__field(xfs_ino_t, ino)
+		__field(u64, addr)
+		__field(loff_t, pos)
+		__field(u64, len)
+		__field(u16, type)
+		__field(u16, flags)
+		__field(u32, wpcseq)
+		__field(u32, forkseq)
+	),
+	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->type = iomap->type;
+		__entry->flags = iomap->flags;
+		__entry->wpcseq = wpcseq;
+		__entry->forkseq = READ_ONCE(xfs_ifork_ptr(ip, whichfork)->if_seq);
+	),
+	TP_printk("dev %d:%d ino 0x%llx pos 0x%llx addr 0x%llx bytecount 0x%llx type 0x%x flags 0x%x wpcseq 0x%x forkseq 0x%x",
+		  MAJOR(__entry->dev), MINOR(__entry->dev),
+		  __entry->ino,
+		  __entry->pos,
+		  __entry->addr,
+		  __entry->len,
+		  __entry->type,
+		  __entry->flags,
+		  __entry->wpcseq,
+		  __entry->forkseq)
+);
+#define DEFINE_WB_INVALID_EVENT(name) \
+DEFINE_EVENT(xfs_wb_invalid_class, name, \
+	TP_PROTO(struct xfs_inode *ip, const struct iomap *iomap, unsigned int wpcseq, int whichfork), \
+	TP_ARGS(ip, iomap, wpcseq, whichfork))
+DEFINE_WB_INVALID_EVENT(xfs_wb_cow_iomap_invalid);
+DEFINE_WB_INVALID_EVENT(xfs_wb_data_iomap_invalid);
+
 /* refcount/reflink tracepoint definitions */
 
 /* reflink tracepoints */