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

[David Howells]

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>
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;
 		pipe->bufs[head & mask] = *buf;
 		pipe->head = head + 1;
 		return buf->len;