[PATCH 1/4] virtio_console: copy pipe buffers in splice_write()

["Kiryl Shutsemau (Meta)" <kas@kernel.org>]

pipe_to_sg() tries to steal the page of each pipe buffer and pass it to
the host as it is.  When the steal fails, it copies the data into a page
of its own.

Pages written into the pipe with write(), gifted user pages and page
cache pages can be stolen.  The ftrace ring buffer pages that
virtio-trace splices cannot, so that in-tree user always gets the copy.

Always copy.  The data that reaches the host does not change.

Preparation for removing ->try_steal() from pipe buffers.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 drivers/char/virtio_console.c | 39 +++++++++++++----------------------
 1 file changed, 14 insertions(+), 25 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 019bcae81af5..b1b1aaf43e07 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -891,45 +891,34 @@ 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;
+	struct page *page;
+	char *src;
 
 	if (sgl->n == sgl->size)
 		return 0;
 
-	/* Try lock this page */
-	if (pipe_buf_try_steal(pipe, buf)) {
-		/* Get reference and unlock page for moving */
-		get_page(buf->page);
-		unlock_page(buf->page);
+	page = alloc_page(GFP_KERNEL);
+	if (!page)
+		return -ENOMEM;
 
-		len = min(buf->len, sd->len);
-		sg_set_page(&(sgl->sg[sgl->n]), buf->page, len, buf->offset);
-	} else {
-		/* Failback to copying a page */
-		struct page *page = alloc_page(GFP_KERNEL);
-		char *src;
+	offset = sd->pos & ~PAGE_MASK;
 
-		if (!page)
-			return -ENOMEM;
+	len = sd->len;
+	if (len + offset > PAGE_SIZE)
+		len = PAGE_SIZE - offset;
 
-		offset = sd->pos & ~PAGE_MASK;
+	src = kmap_local_page(buf->page);
+	memcpy(page_address(page) + offset, src + buf->offset, len);
+	kunmap_local(src);
 
-		len = sd->len;
-		if (len + offset > PAGE_SIZE)
-			len = PAGE_SIZE - offset;
-
-		src = kmap_local_page(buf->page);
-		memcpy(page_address(page) + offset, src + buf->offset, len);
-		kunmap_local(src);
-
-		sg_set_page(&(sgl->sg[sgl->n]), page, len, offset);
-	}
+	sg_set_page(&sgl->sg[sgl->n], page, len, offset);
 	sgl->n++;
 	sgl->len += len;
 
 	return len;
 }
 
-/* Faster zero-copy write by splicing */
+/* Write by splicing, copying the data from the pipe */
 static ssize_t port_fops_splice_write(struct pipe_inode_info *pipe,
 				      struct file *filp, loff_t *ppos,
 				      size_t len, unsigned int flags)
-- 
2.54.0