ext4: deprecate max_writeback_mb_bump sysfs attribute

This attribute is now unused so deprecate it.  We still show the old
default value to keep some compatibility but we don't allow writing to
that attribute anymore.
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ext4.h

@@ -1241,7 +1241,6 @@ struct ext4_sb_info {
 	unsigned int s_mb_stats;
 	unsigned int s_mb_order2_reqs;
 	unsigned int s_mb_group_prealloc;
-	unsigned int s_max_writeback_mb_bump;
 	unsigned int s_max_dir_size_kb;
 	/* where last allocation was done - for stream allocation */
 	unsigned long s_mb_last_group;

fs/ext4/super.c

@@ -2380,7 +2380,10 @@ struct ext4_attr {
 	ssize_t (*show)(struct ext4_attr *, struct ext4_sb_info *, char *);
 	ssize_t (*store)(struct ext4_attr *, struct ext4_sb_info *,
 			 const char *, size_t);
-	int offset;
+	union {
+		int offset;
+		int deprecated_val;
+	} u;
 };
 
 static int parse_strtoull(const char *buf,
@@ -2449,7 +2452,7 @@ static ssize_t inode_readahead_blks_store(struct ext4_attr *a,
 static ssize_t sbi_ui_show(struct ext4_attr *a,
 			   struct ext4_sb_info *sbi, char *buf)
 {
-	unsigned int *ui = (unsigned int *) (((char *) sbi) + a->offset);
+	unsigned int *ui = (unsigned int *) (((char *) sbi) + a->u.offset);
 
 	return snprintf(buf, PAGE_SIZE, "%u\n", *ui);
 }
@@ -2458,7 +2461,7 @@ static ssize_t sbi_ui_store(struct ext4_attr *a,
 			    struct ext4_sb_info *sbi,
 			    const char *buf, size_t count)
 {
-	unsigned int *ui = (unsigned int *) (((char *) sbi) + a->offset);
+	unsigned int *ui = (unsigned int *) (((char *) sbi) + a->u.offset);
 	unsigned long t;
 	int ret;
 
@@ -2507,12 +2510,20 @@ static ssize_t trigger_test_error(struct ext4_attr *a,
 	return count;
 }
 
+static ssize_t sbi_deprecated_show(struct ext4_attr *a,
+				   struct ext4_sb_info *sbi, char *buf)
+{
+	return snprintf(buf, PAGE_SIZE, "%d\n", a->u.deprecated_val);
+}
+
 #define EXT4_ATTR_OFFSET(_name,_mode,_show,_store,_elname) \
 static struct ext4_attr ext4_attr_##_name = {			\
 	.attr = {.name = __stringify(_name), .mode = _mode },	\
 	.show	= _show,					\
 	.store	= _store,					\
-	.offset = offsetof(struct ext4_sb_info, _elname),	\
+	.u = {							\
+		.offset = offsetof(struct ext4_sb_info, _elname),\
+	},							\
 }
 #define EXT4_ATTR(name, mode, show, store) \
 static struct ext4_attr ext4_attr_##name = __ATTR(name, mode, show, store)
@@ -2523,6 +2534,14 @@ static struct ext4_attr ext4_attr_##name = __ATTR(name, mode, show, store)
 #define EXT4_RW_ATTR_SBI_UI(name, elname)	\
 	EXT4_ATTR_OFFSET(name, 0644, sbi_ui_show, sbi_ui_store, elname)
 #define ATTR_LIST(name) &ext4_attr_##name.attr
+#define EXT4_DEPRECATED_ATTR(_name, _val)	\
+static struct ext4_attr ext4_attr_##_name = {			\
+	.attr = {.name = __stringify(_name), .mode = 0444 },	\
+	.show	= sbi_deprecated_show,				\
+	.u = {							\
+		.deprecated_val = _val,				\
+	},							\
+}
 
 EXT4_RO_ATTR(delayed_allocation_blocks);
 EXT4_RO_ATTR(session_write_kbytes);
@@ -2537,7 +2556,7 @@ EXT4_RW_ATTR_SBI_UI(mb_min_to_scan, s_mb_min_to_scan);
 EXT4_RW_ATTR_SBI_UI(mb_order2_req, s_mb_order2_reqs);
 EXT4_RW_ATTR_SBI_UI(mb_stream_req, s_mb_stream_request);
 EXT4_RW_ATTR_SBI_UI(mb_group_prealloc, s_mb_group_prealloc);
-EXT4_RW_ATTR_SBI_UI(max_writeback_mb_bump, s_max_writeback_mb_bump);
+EXT4_DEPRECATED_ATTR(max_writeback_mb_bump, 128);
 EXT4_RW_ATTR_SBI_UI(extent_max_zeroout_kb, s_extent_max_zeroout_kb);
 EXT4_ATTR(trigger_fs_error, 0200, NULL, trigger_test_error);
 
@@ -3790,7 +3809,6 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	}
 
 	sbi->s_stripe = ext4_get_stripe_size(sbi);
-	sbi->s_max_writeback_mb_bump = 128;
 	sbi->s_extent_max_zeroout_kb = 32;
 
 	/*