virtio/console: Add a failback for unstealable pipe buffer

Add a failback memcpy path for unstealable pipe buffer.
If buf->ops->steal() fails, virtio-serial tries to
copy the page contents to an allocated page, instead
of just failing splice().

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>

1 files changed, 25 insertions, 3 deletions

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 730816cdeb45..22b737353017 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -794,7 +794,7 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
                         struct splice_desc *sd)
 {
         struct sg_list *sgl = sd->u.data;
-        unsigned int len = 0;
+        unsigned int offset, len;
 
         if (sgl->n == MAX_SPLICE_PAGES)
                 return 0;
@@ -807,9 +807,31 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 
                 len = min(buf->len, sd->len);
                 sg_set_page(&(sgl->sg[sgl->n]), buf->page, len, buf->offset);
-                sgl->n++;
-                sgl->len += len;
+        } else {
+                /* Failback to copying a page */
+                struct page *page = alloc_page(GFP_KERNEL);
+                char *src = buf->ops->map(pipe, buf, 1);
+                char *dst;
+
+                if (!page)
+                        return -ENOMEM;
+                dst = kmap(page);
+
+                offset = sd->pos & ~PAGE_MASK;
+
+                len = sd->len;
+                if (len + offset > PAGE_SIZE)
+                        len = PAGE_SIZE - offset;
+
+                memcpy(dst + offset, src + buf->offset, len);
+
+                kunmap(page);
+                buf->ops->unmap(pipe, buf, src);
+
+                sg_set_page(&(sgl->sg[sgl->n]), page, len, offset);
         }
+        sgl->n++;
+        sgl->len += len;
 
         return len;
 }