Re: [RFC][PATCH] splice: Prevent gifting of multipage folios

[Matthew Wilcox]

On Mon, Feb 27, 2023 at 02:23:32PM +0000, David Howells wrote:
>     
> Don't let parts of multipage folios be gifted by (vm)splice into a pipe as
> the other end may only be expecting single-page gifts (fuse and virtio
> console for example).
> 
> replace_page_cache_folio(), for example, will do the wrong thing if it
> tries to replace a single paged folio with a multipage folio.
> 
> Try to avoid this by making add_to_pipe() remove the gift flag on multipage
> folios.
> 
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>

What should the Fixes: here be?  This was already possible with THPs
(both anon and tmpfs backed) long before I introduced folios.

> cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> cc: Miklos Szeredi <miklos@xxxxxxxxxx>
> cc: Amit Shah <amit@xxxxxxxxxx>
> cc: linux-fsdevel@xxxxxxxxxxxxxxx
> cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
> cc: linux-mm@xxxxxxxxx
> ---
>  fs/splice.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/splice.c b/fs/splice.c
> index 2e76dbb81a8f..33caa28a86e4 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -240,6 +240,8 @@ ssize_t add_to_pipe(struct pipe_inode_info *pipe, struct pipe_buffer *buf)
>  	} else if (pipe_full(head, tail, pipe->max_usage)) {
>  		ret = -EAGAIN;
>  	} else {
> +		if (folio_nr_pages(page_folio(buf->page)) > 1)
> +			buf->flags &= ~PIPE_BUF_FLAG_GIFT;

		if (PageCompound(buf->page))
			buf->flags &= ~PIPE_BUF_FLAG_GIFT;

would be simpler and more backportable.