vfs: track per-sb writeback errors and report them via fsinfo()

index 858dd2f3300c..5378a845ca01 100644
@@ -1446,6 +1446,9 @@ struct super_block {
 	/* Being remounted read-only */
 	int s_readonly_remount;
 
+	/* per-sb errseq_t for reporting writeback errors via syncfs */
+	errseq_t s_wb_err;
+
 	/* AIO completions deferred from interrupt context */
 	struct workqueue_struct *s_dio_done_wq;
 	struct hlist_head s_pins;

index 442977811b59..c1e36a7b44d2 100644
@@ -51,7 +51,10 @@ static inline void mapping_set_error(struct address_space *mapping, int error)
 		return;
 
 	/* Record in wb_err for checkers using errseq_t based tracking */
-	filemap_set_wb_err(mapping, error);
+	__filemap_set_wb_err(mapping, error);
+
+	/* Record it in superblock */
+	errseq_set(&mapping->host->i_sb->s_wb_err, error);
 
 	/* Record it in flags for now, for legacy callers */
 	if (error == -ENOSPC)

index fc2777770768..de165623fa86 100644
@@ -9,6 +9,7 @@ typedef u32	errseq_t;
 
 errseq_t errseq_set(errseq_t *eseq, int err);
 errseq_t errseq_sample(errseq_t *eseq);
+errseq_t errseq_scrape(errseq_t *eseq);
 int errseq_check(errseq_t *eseq, errseq_t since);
 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since);
 #endif

index 81f9e33aa7e7..8ded0920eed3 100644
@@ -108,7 +108,7 @@ errseq_t errseq_set(errseq_t *eseq, int err)
 EXPORT_SYMBOL(errseq_set);
 
 /**
- * errseq_sample() - Grab current errseq_t value.
+ * errseq_sample() - Grab current errseq_t value (or 0 if it hasn't been seen)
  * @eseq: Pointer to errseq_t to be sampled.
  *
  * This function allows callers to initialise their errseq_t variable.

@@ -117,7 +117,7 @@ EXPORT_SYMBOL(errseq_set);
  * see it the next time it checks for an error.
  *
  * Context: Any context.
- * Return: The current errseq value.
+ * Return: The current errseq value or 0 if it wasn't previously seen
  */
 errseq_t errseq_sample(errseq_t *eseq)
 {

@@ -130,6 +130,35 @@ errseq_t errseq_sample(errseq_t *eseq)
 }
 EXPORT_SYMBOL(errseq_sample);
 
+/**
+ * errseq_scrape() - Grab current errseq_t value
+ * @eseq: Pointer to errseq_t to be sampled.
+ *
+ * This function allows callers to scrape the current value of an errseq_t.
+ * Unlike errseq_sample, this will always return the current value with
+ * the SEEN flag unset, even when the value has not yet been seen.
+ *
+ * Context: Any context.
+ * Return: The current errseq value with ERRSEQ_SEEN masked off
+ */
+errseq_t errseq_scrape(errseq_t *eseq)
+{
+	errseq_t old = READ_ONCE(*eseq);
+
+	/*
+	 * For the common case of no errors ever having been set, we can skip
+	 * marking the SEEN bit. Once an error has been set, the value will
+	 * never go back to zero.
+	 */
+	if (old != 0) {
+		errseq_t new = old | ERRSEQ_SEEN;
+		if (old != new)
+			cmpxchg(eseq, old, new);
+	}
+	return old & ~ERRSEQ_SEEN;
+}
+EXPORT_SYMBOL(errseq_scrape);
+
 /**
  * errseq_check() - Has an error occurred since a particular sample point?
  * @eseq: Pointer to errseq_t value to be checked.

index fa6be965dce1..fb1f48eb381a 100644
@@ -576,6 +576,14 @@ static int fsinfo_generic_io_size(struct dentry *dentry,
 	return sizeof(*c);
 }
 
+static int fsinfo_generic_error_state(struct dentry *dentry,
+				  struct fsinfo_error_state *c)
+{
+	c->wb_error_cookie = errseq_scrape(&dentry->d_sb->s_wb_err);
+	c->wb_error_last = c->wb_error_cookie & MAX_ERRNO;
+	return sizeof(*c);
+}
+
 /*
  * Implement some queries generically from stuff in the superblock.
  */

@@ -594,6 +602,7 @@ int generic_fsinfo(struct dentry *dentry, struct fsinfo_kparams *params)
 	case _gen(volume_id);
 	case _gen(name_encoding);
 	case _gen(io_size);
+	case _gen(error_state);
 	default:
 		return -EOPNOTSUPP;
 	}

index f2bc5130544d..fbc5d840ea4e 100644
@@ -35,6 +35,7 @@ enum fsinfo_attribute {
 	fsinfo_attr_name_encoding	= 17,	/* Filename encoding (string) */
 	fsinfo_attr_name_codepage	= 18,	/* Filename codepage (string) */
 	fsinfo_attr_io_size		= 19,	/* Optimal I/O sizes */
+	fsinfo_attr_error_state		= 20,	/* Error state */
 	fsinfo_attr__nr
 };
 

@@ -211,6 +212,16 @@ struct fsinfo_server_address {
 	struct __kernel_sockaddr_storage 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 */
+};
+
 /*
  * Information struct for fsinfo(fsinfo_attr_io_size).
  *

index 9e9fa62a3b9f..01dd8c6d3c86 100644
@@ -60,6 +60,7 @@ static const __u8 fsinfo_buffer_sizes[fsinfo_attr__nr] = {
 	FSINFO_STRING		(name_encoding),
 	FSINFO_STRING		(name_codepage),
 	FSINFO_STRUCT		(io_size),
+	FSINFO_STRUCT		(error_state),
 };
 
 #define FSINFO_NAME(N) [fsinfo_attr_##N] = #N

@@ -84,6 +85,7 @@ static const char *fsinfo_attr_names[fsinfo_attr__nr] = {
 	FSINFO_NAME(name_encoding),
 	FSINFO_NAME(name_codepage),
 	FSINFO_NAME(io_size),
+	FSINFO_NAME(error_state),
 };
 
 union reply {

@@ -98,6 +100,7 @@ union reply {
 	struct fsinfo_volume_uuid uuid;
 	struct fsinfo_server_address srv_addr;
 	struct fsinfo_io_size io_size;
+	struct fsinfo_error_state error_state;
 };
 
 static void dump_hex(unsigned int *data, int from, int to)

@@ -304,6 +307,15 @@ static void dump_attr_io_size(union reply *r, int size)
 	printf("bs=%u\n", f->block_size);
 }
 
+static void dump_attr_error_state(union reply *r, int size)
+{
+	struct fsinfo_error_state *f = &r->error_state;
+
+	printf("err_cookie=0x%x err_last=%u\n",
+			f->wb_error_cookie,
+			f->wb_error_last);
+}
+
 /*
  *
  */

@@ -321,6 +333,7 @@ static const dumper_t fsinfo_attr_dumper[fsinfo_attr__nr] = {
 	FSINFO_DUMPER(volume_uuid),
 	FSINFO_DUMPER(server_address),
 	FSINFO_DUMPER(io_size),
+	FSINFO_DUMPER(error_state),
 };
 
 static void dump_fsinfo(enum fsinfo_attribute attr, __u8 about,