tty: Halve flip buffer GFP_ATOMIC memory consumption

tty flip buffers use GFP_ATOMIC allocations for received data
which is to be processed by the line discipline. For each byte
received, an extra byte is used to indicate the error status of
that byte.

Instead, if the received data is error-free, encode the entire
buffer without status bytes.

Signed-off-by: Peter Hurley <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/tty_buffer.c

···
 	p->next = NULL;
 	p->commit = 0;
 	p->read = 0;
+	p->flags = 0;
 }
 
 /**
···
  *	tty_buffer_request_room		-	grow tty buffer if needed
  *	@tty: tty structure
  *	@size: size desired
+ *	@flags: buffer flags if new buffer allocated (default = 0)
  *
  *	Make at least size bytes of linear space available for the tty
  *	buffer. If we fail return the size we managed to find.
+ *
+ *	Will change over to a new buffer if the current buffer is encoded as
+ *	TTY_NORMAL (so has no flags buffer) and the new buffer requires
+ *	a flags buffer.
  */
-int tty_buffer_request_room(struct tty_port *port, size_t size)
+static int __tty_buffer_request_room(struct tty_port *port, size_t size,
+				     int flags)
 {
 	struct tty_bufhead *buf = &port->buf;
 	struct tty_buffer *b, *n;
-	int left;
+	int left, change;
 
 	b = buf->tail;
-	left = b->size - b->used;
+	if (b->flags & TTYB_NORMAL)
+		left = 2 * b->size - b->used;
+	else
+		left = b->size - b->used;
 
-	if (left < size) {
+	change = (b->flags & TTYB_NORMAL) && (~flags & TTYB_NORMAL);
+	if (change || left < size) {
 		/* This is the slow path - looking for new buffers to use */
 		if ((n = tty_buffer_alloc(port, size)) != NULL) {
+			n->flags = flags;
 			buf->tail = n;
 			b->commit = b->used;
 			smp_mb();
 			b->next = n;
-		} else
+		} else if (change)
+			size = 0;
+		else
 			size = left;
 	}
 	return size;
+}
+
+int tty_buffer_request_room(struct tty_port *port, size_t size)
+{
+	return __tty_buffer_request_room(port, size, 0);
 }
 EXPORT_SYMBOL_GPL(tty_buffer_request_room);
 
···
 	int copied = 0;
 	do {
 		int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
-		int space = tty_buffer_request_room(port, goal);
+		int flags = (flag == TTY_NORMAL) ? TTYB_NORMAL : 0;
+		int space = __tty_buffer_request_room(port, goal, flags);
 		struct tty_buffer *tb = port->buf.tail;
 		if (unlikely(space == 0))
 			break;
 		memcpy(char_buf_ptr(tb, tb->used), chars, space);
-		memset(flag_buf_ptr(tb, tb->used), flag, space);
+		if (~tb->flags & TTYB_NORMAL)
+			memset(flag_buf_ptr(tb, tb->used), flag, space);
 		tb->used += space;
 		copied += space;
 		chars += space;
···
 int tty_prepare_flip_string(struct tty_port *port, unsigned char **chars,
 		size_t size)
 {
-	int space = tty_buffer_request_room(port, size);
+	int space = __tty_buffer_request_room(port, size, TTYB_NORMAL);
 	if (likely(space)) {
 		struct tty_buffer *tb = port->buf.tail;
 		*chars = char_buf_ptr(tb, tb->used);
-		memset(flag_buf_ptr(tb, tb->used), TTY_NORMAL, space);
+		if (~tb->flags & TTYB_NORMAL)
+			memset(flag_buf_ptr(tb, tb->used), TTY_NORMAL, space);
 		tb->used += space;
 	}
 	return space;
···
 {
 	struct tty_ldisc *disc = tty->ldisc;
 	unsigned char *p = char_buf_ptr(head, head->read);
-	char	      *f = flag_buf_ptr(head, head->read);
+	char	      *f = NULL;
+
+	if (~head->flags & TTYB_NORMAL)
+		f = flag_buf_ptr(head, head->read);
 
 	if (disc->ops->receive_buf2)
 		count = disc->ops->receive_buf2(tty, p, f, count);

include/linux/tty.h

···
 	int size;
 	int commit;
 	int read;
+	int flags;
 	/* Data points here */
 	unsigned long data[0];
 };
+
+/* Values for .flags field of tty_buffer */
+#define TTYB_NORMAL	1	/* buffer has no flags buffer */
 
 static inline unsigned char *char_buf_ptr(struct tty_buffer *b, int ofs)
 {

include/linux/tty_flip.h

···
 					unsigned char ch, char flag)
 {
 	struct tty_buffer *tb = port->buf.tail;
-	if (tb && tb->used < tb->size) {
-		*flag_buf_ptr(tb, tb->used) = flag;
+	int change;
+
+	change = (tb->flags & TTYB_NORMAL) && (flag != TTY_NORMAL);
+	if (!change && tb->used < tb->size) {
+		if (~tb->flags & TTYB_NORMAL)
+			*flag_buf_ptr(tb, tb->used) = flag;
 		*char_buf_ptr(tb, tb->used++) = ch;
 		return 1;
 	}