[PATCH] virtio-console: avoid DMA from vmalloc area

[Xianting Tian <xianting.tian@linux.alibaba.com>]

This patch is to optimize the commit c4baad5029:
	virtio-console: avoid DMA from stack

Commit c4baad5029(virtio-console: avoid DMA from stack) directly uses
kmemdup() to alloc new buffer from kmalloc area and do memcpy no matter
the buf is in kmalloc area or not.

This patch is to optimize the commit c4baad5029, use is_vmalloc_addr()
to judge the buf to avoid unnecessary memory alloc and copy.

Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
---
 drivers/char/virtio_console.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 7eaf303a7..106247903 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -1119,6 +1119,7 @@ static int put_chars(u32 vtermno, const char *buf, int count)
 	struct scatterlist sg[1];
 	void *data;
 	int ret;
+	bool vmalloc_addr = false;
 
 	if (unlikely(early_put_chars))
 		return early_put_chars(vtermno, buf, count);
@@ -1127,13 +1128,18 @@ static int put_chars(u32 vtermno, const char *buf, int count)
 	if (!port)
 		return -EPIPE;
 
-	data = kmemdup(buf, count, GFP_ATOMIC);
-	if (!data)
-		return -ENOMEM;
+	if (is_vmalloc_addr(buf)) {
+		data = kmemdup(buf, count, GFP_ATOMIC);
+		if (!data)
+			return -ENOMEM;
+		vmalloc_addr = true;
+	} else
+		data = (void *)buf;
 
 	sg_init_one(sg, data, count);
 	ret = __send_to_port(port, sg, 1, count, data, false);
-	kfree(data);
+	if (vmalloc_addr)
+		kfree(data);
 	return ret;
 }
 
-- 
2.17.1