[PATCH v2 2/4] xfs: add an error tag to inject a ->writeback_submit() failure

[Andrea Parri <parri.andrea@gmail.com>]

The only in-tree way for xfs_writeback_submit() to fail is a failing
xfs_reflink_convert_cow(), which requires a shared (reflinked) CoW
extent.  There was previously no way to exercise that failure path
from userspace, which made it hard to write an xfstest for it, e.g.
for "iomap: don't resubmit an ioend after ->writeback_submit()
failed".

Add XFS_ERRTAG_WB_COW_CONVERT_ERROR to force xfs_reflink_convert_cow()
to fail with -EIO, so that ->writeback_submit() failure and its
callers' error handling can be exercised with error injection alone,
without needing a corrupted or racy COW fork.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
---
 fs/xfs/libxfs/xfs_errortag.h | 6 ++++--
 fs/xfs/xfs_reflink.c         | 5 +++++
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_errortag.h b/fs/xfs/libxfs/xfs_errortag.h
index 6de207fed2d89..e6b0864051b0a 100644
--- a/fs/xfs/libxfs/xfs_errortag.h
+++ b/fs/xfs/libxfs/xfs_errortag.h
@@ -75,7 +75,8 @@
 #define XFS_ERRTAG_METAFILE_RESV_CRITICAL		45
 #define XFS_ERRTAG_FORCE_ZERO_RANGE			46
 #define XFS_ERRTAG_ZONE_RESET				47
-#define XFS_ERRTAG_MAX					48
+#define XFS_ERRTAG_WB_COW_CONVERT_ERROR		48
+#define XFS_ERRTAG_MAX					49
 
 /*
  * Random factors for above tags, 1 means always, 2 means 1/2 time, etc.
@@ -137,7 +138,8 @@ XFS_ERRTAG(WRITE_DELAY_MS,	write_delay_ms,		3000) \
 XFS_ERRTAG(EXCHMAPS_FINISH_ONE,	exchmaps_finish_one,	1) \
 XFS_ERRTAG(METAFILE_RESV_CRITICAL, metafile_resv_crit,	4) \
 XFS_ERRTAG(FORCE_ZERO_RANGE,	force_zero_range,	4) \
-XFS_ERRTAG(ZONE_RESET,		zone_reset,		1)
+XFS_ERRTAG(ZONE_RESET,		zone_reset,		1) \
+XFS_ERRTAG(WB_COW_CONVERT_ERROR, wb_cow_convert_error,	1)
 #endif /* XFS_ERRTAG */
 
 #endif /* __XFS_ERRORTAG_H_ */
diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
index 4801361366359..104542b10d9e0 100644
--- a/fs/xfs/xfs_reflink.c
+++ b/fs/xfs/xfs_reflink.c
@@ -34,6 +34,8 @@
 #include "xfs_rtalloc.h"
 #include "xfs_rtgroup.h"
 #include "xfs_metafile.h"
+#include "xfs_errortag.h"
+#include "xfs_error.h"
 
 /*
  * Copy on Write of Shared Blocks
@@ -346,6 +348,9 @@ xfs_reflink_convert_cow(
 
 	ASSERT(count != 0);
 
+	if (XFS_TEST_ERROR(mp, XFS_ERRTAG_WB_COW_CONVERT_ERROR))
+		return -EIO;
+
 	xfs_ilock(ip, XFS_ILOCK_EXCL);
 	error = xfs_reflink_convert_cow_locked(ip, offset_fsb, count_fsb);
 	xfs_iunlock(ip, XFS_ILOCK_EXCL);
-- 
2.53.0