new helper: single_open_size()

Same as single_open(), but preallocates the buffer of given size.
Doesn't make any sense for sizes up to PAGE_SIZE and doesn't make
sense if output of show() exceeds PAGE_SIZE only rarely - seq_read()
will take care of growing the buffer and redoing show().  If you
_know_ that it will be large, it might make more sense to look into
saner iterator, rather than go with single-shot one.  If that's
impossible, single_open_size() might be for you.

Again, don't use that without a good reason; occasionally that's really
the best way to go, but very often there are better solutions.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/seq_file.c

@@ -599,6 +599,24 @@ int single_open(struct file *file, int (*show)(struct seq_file *, void *),
 }
 EXPORT_SYMBOL(single_open);
 
+int single_open_size(struct file *file, int (*show)(struct seq_file *, void *),
+		void *data, size_t size)
+{
+	char *buf = kmalloc(size, GFP_KERNEL);
+	int ret;
+	if (!buf)
+		return -ENOMEM;
+	ret = single_open(file, show, data);
+	if (ret) {
+		kfree(buf);
+		return ret;
+	}
+	((struct seq_file *)file->private_data)->buf = buf;
+	((struct seq_file *)file->private_data)->size = size;
+	return 0;
+}
+EXPORT_SYMBOL(single_open_size);
+
 int single_release(struct inode *inode, struct file *file)
 {
 	const struct seq_operations *op = ((struct seq_file *)file->private_data)->op;

include/linux/seq_file.h

@@ -123,6 +123,7 @@ static inline int seq_nodemask_list(struct seq_file *m, nodemask_t *mask)
 }
 
 int single_open(struct file *, int (*)(struct seq_file *, void *), void *);
+int single_open_size(struct file *, int (*)(struct seq_file *, void *), void *, size_t);
 int single_release(struct inode *, struct file *);
 void *__seq_open_private(struct file *, const struct seq_operations *, int);
 int seq_open_private(struct file *, const struct seq_operations *, int);