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>

1 files changed, 18 insertions, 0 deletions

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 38bb59f3f2a..774c1eb7f1c 100644
--- a/fs/seq_file.c
+++ b/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;