[PATCH 17/18] vfs: allow fsinfo to fetch the current state of s_wb_err [ver #21]

[David Howells <dhowells@redhat.com>]

From: Jeff Layton <jlayton@kernel.org>

Add a new "error_state" struct to fsinfo, and teach the kernel to fill
that out from sb->s_wb_err. There are two fields:

wb_error_last: the most recently recorded errno for the filesystem

wb_error_cookie: this value will change vs. the previously fetched
                 value if a new error was recorded since it was last
		 checked. Callers should treat this as an opaque value
		 that can be compared to earlier fetched values.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/fsinfo.c                 |   11 +++++++++++
 include/uapi/linux/fsinfo.h |   13 +++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/fs/fsinfo.c b/fs/fsinfo.c
index f230124ffdf5..ea9d9821d76b 100644
--- a/fs/fsinfo.c
+++ b/fs/fsinfo.c
@@ -274,6 +274,16 @@ static int fsinfo_generic_seq_read(struct path *path, struct fsinfo_context *ctx
 	return m.count + 1;
 }
 
+static int fsinfo_generic_error_state(struct path *path,
+				      struct fsinfo_context *ctx)
+{
+	struct fsinfo_error_state *es = ctx->buffer;
+
+	es->wb_error_cookie = errseq_scrape(&path->dentry->d_sb->s_wb_err);
+	es->wb_error_last = es->wb_error_cookie & MAX_ERRNO;
+	return sizeof(*es);
+}
+
 static const struct fsinfo_attribute fsinfo_common_attributes[] = {
 	FSINFO_VSTRUCT	(FSINFO_ATTR_STATFS,		fsinfo_generic_statfs),
 	FSINFO_VSTRUCT	(FSINFO_ATTR_IDS,		fsinfo_generic_ids),
@@ -286,6 +296,7 @@ static const struct fsinfo_attribute fsinfo_common_attributes[] = {
 	FSINFO_STRING	(FSINFO_ATTR_SOURCE,		fsinfo_generic_mount_source),
 	FSINFO_STRING	(FSINFO_ATTR_CONFIGURATION,	fsinfo_generic_seq_read),
 	FSINFO_STRING	(FSINFO_ATTR_FS_STATISTICS,	fsinfo_generic_seq_read),
+	FSINFO_VSTRUCT	(FSINFO_ATTR_ERROR_STATE,	fsinfo_generic_error_state),
 
 	FSINFO_LIST	(FSINFO_ATTR_FSINFO_ATTRIBUTES,	(void *)123UL),
 	FSINFO_VSTRUCT_N(FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO, (void *)123UL),
diff --git a/include/uapi/linux/fsinfo.h b/include/uapi/linux/fsinfo.h
index e40192d98648..dcd764771a7d 100644
--- a/include/uapi/linux/fsinfo.h
+++ b/include/uapi/linux/fsinfo.h
@@ -27,6 +27,7 @@
 #define FSINFO_ATTR_SOURCE		0x09	/* Superblock source/device name (string) */
 #define FSINFO_ATTR_CONFIGURATION	0x0a	/* Superblock configuration/options (string) */
 #define FSINFO_ATTR_FS_STATISTICS	0x0b	/* Superblock filesystem statistics (string) */
+#define FSINFO_ATTR_ERROR_STATE		0x0c	/* Superblock writeback error state */
 
 #define FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO 0x100	/* Information about attr N (for path) */
 #define FSINFO_ATTR_FSINFO_ATTRIBUTES	0x101	/* List of supported attrs (for path) */
@@ -328,4 +329,16 @@ struct fsinfo_afs_server_address {
 
 #define FSINFO_ATTR_AFS_SERVER_ADDRESSES__STRUCT struct fsinfo_afs_server_address
 
+/*
+ * Information struct for fsinfo(FSINFO_ATTR_ERROR_STATE).
+ *
+ * Retrieve the error state for a filesystem.
+ */
+struct fsinfo_error_state {
+	__u32		wb_error_cookie;	/* writeback error cookie */
+	__u32		wb_error_last;		/* latest writeback error */
+};
+
+#define FSINFO_ATTR_ERROR_STATE__STRUCT struct fsinfo_error_state
+
 #endif /* _UAPI_LINUX_FSINFO_H */