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 <[email protected]> Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]> --- 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
