io_uring/zcrx: cache fallback availability in zcrx ctx

Store a flag in struct io_zcrx_ifq telling if the backing memory is
normal page or dmabuf based. It was looking it up from the area, however
it logically allocates from the zcrx ctx and not a particular area, and
once we add more than one area it'll become a mess.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://patch.msgid.link/65e75408a7758fe7e60fae89b7a8d5ae4857f515.1774261953.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/zcrx.c

@@ -423,8 +423,13 @@ static void io_zcrx_free_area(struct io_zcrx_ifq *ifq,
 static int io_zcrx_append_area(struct io_zcrx_ifq *ifq,
 				struct io_zcrx_area *area)
 {
+	bool kern_readable = !area->mem.is_dmabuf;
+
 	if (WARN_ON_ONCE(ifq->area))
 		return -EINVAL;
+	if (WARN_ON_ONCE(ifq->kern_readable != kern_readable))
+		return -EINVAL;
+
 	ifq->area = area;
 	return 0;
 }
@@ -882,6 +887,8 @@ int io_register_zcrx_ifq(struct io_ring_ctx *ctx,
 	if (ret)
 		goto err;
 
+	ifq->kern_readable = !(area.flags & IORING_ZCRX_AREA_DMABUF);
+
 	if (!(reg.flags & ZCRX_REG_NODEV)) {
 		ret = zcrx_register_netdev(ifq, &reg, &area);
 		if (ret)
@@ -1298,7 +1305,7 @@ static struct net_iov *io_alloc_fallback_niov(struct io_zcrx_ifq *ifq)
 	struct io_zcrx_area *area = ifq->area;
 	struct net_iov *niov = NULL;
 
-	if (area->mem.is_dmabuf)
+	if (!ifq->kern_readable)
 		return NULL;
 
 	scoped_guard(spinlock_bh, &area->freelist_lock)

io_uring/zcrx.h

@@ -54,6 +54,7 @@ struct io_zcrx_ifq {
 	unsigned			niov_shift;
 	struct user_struct		*user;
 	struct mm_struct		*mm_account;
+	bool				kern_readable;
 
 	struct zcrx_rq			rq ____cacheline_aligned_in_smp;