virtio/console: Allocate scatterlist according to the current pipe size

Allocate scatterlist according to the current pipe size.
This allows splicing bigger buffer if the pipe size has
been changed by fcntl.

Changes in v2:
 - Just a minor fix for avoiding a confliction with previous patch.
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Acked-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/char/virtio_console.c

@@ -229,7 +229,6 @@ struct port {
 	bool guest_connected;
 };
 
-#define MAX_SPLICE_PAGES	32
 /* This is the very early arch-specified put chars function. */
 static int (*early_put_chars)(u32, const char *, int);
 
@@ -482,15 +481,16 @@ struct buffer_token {
 		void *buf;
 		struct scatterlist *sg;
 	} u;
-	bool sgpages;
+	/* If sgpages == 0 then buf is used, else sg is used */
+	unsigned int sgpages;
 };
 
-static void reclaim_sg_pages(struct scatterlist *sg)
+static void reclaim_sg_pages(struct scatterlist *sg, unsigned int nrpages)
 {
 	int i;
 	struct page *page;
 
-	for (i = 0; i < MAX_SPLICE_PAGES; i++) {
+	for (i = 0; i < nrpages; i++) {
 		page = sg_page(&sg[i]);
 		if (!page)
 			break;
@@ -511,7 +511,7 @@ static void reclaim_consumed_buffers(struct port *port)
 	}
 	while ((tok = virtqueue_get_buf(port->out_vq, &len))) {
 		if (tok->sgpages)
-			reclaim_sg_pages(tok->u.sg);
+			reclaim_sg_pages(tok->u.sg, tok->sgpages);
 		else
 			kfree(tok->u.buf);
 		kfree(tok);
@@ -581,7 +581,7 @@ static ssize_t send_buf(struct port *port, void *in_buf, size_t in_count,
 	tok = kmalloc(sizeof(*tok), GFP_ATOMIC);
 	if (!tok)
 		return -ENOMEM;
-	tok->sgpages = false;
+	tok->sgpages = 0;
 	tok->u.buf = in_buf;
 
 	sg_init_one(sg, in_buf, in_count);
@@ -597,7 +597,7 @@ static ssize_t send_pages(struct port *port, struct scatterlist *sg, int nents,
 	tok = kmalloc(sizeof(*tok), GFP_ATOMIC);
 	if (!tok)
 		return -ENOMEM;
-	tok->sgpages = true;
+	tok->sgpages = nents;
 	tok->u.sg = sg;
 
 	return __send_to_port(port, sg, nents, in_count, tok, nonblock);
@@ -797,6 +797,7 @@ static ssize_t port_fops_write(struct file *filp, const char __user *ubuf,
 
 struct sg_list {
 	unsigned int n;
+	unsigned int size;
 	size_t len;
 	struct scatterlist *sg;
 };
@@ -807,7 +808,7 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 	struct sg_list *sgl = sd->u.data;
 	unsigned int offset, len;
 
-	if (sgl->n == MAX_SPLICE_PAGES)
+	if (sgl->n == sgl->size)
 		return 0;
 
 	/* Try lock this page */
@@ -868,12 +869,12 @@ static ssize_t port_fops_splice_write(struct pipe_inode_info *pipe,
 
 	sgl.n = 0;
 	sgl.len = 0;
-	sgl.sg = kmalloc(sizeof(struct scatterlist) * MAX_SPLICE_PAGES,
-			 GFP_KERNEL);
+	sgl.size = pipe->nrbufs;
+	sgl.sg = kmalloc(sizeof(struct scatterlist) * sgl.size, GFP_KERNEL);
 	if (unlikely(!sgl.sg))
 		return -ENOMEM;
 
-	sg_init_table(sgl.sg, MAX_SPLICE_PAGES);
+	sg_init_table(sgl.sg, sgl.size);
 	ret = __splice_from_pipe(pipe, &sd, pipe_to_sg);
 	if (likely(ret > 0))
 		ret = send_pages(port, sgl.sg, sgl.n, sgl.len, true);