tun: use netdev_alloc_frag() in tun_napi_alloc_frags()

In order to cook skbs in the same way than Ethernet drivers,
it is probably better to not use GFP_KERNEL, but rather
use the GFP_ATOMIC and PFMEMALLOC mechanisms provided by
netdev_alloc_frag().

This would allow to use tun driver even in memory stress
situations, especially if swap is used over this tun channel.

Fixes: 90e33d45 ("tun: enable napi_gro_frags() for TUN/TAP driver")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Petar Penkov <peterpenkov96@gmail.com>
Cc: Mahesh Bandewar <maheshb@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1478,23 +1478,22 @@ static struct sk_buff *tun_napi_alloc_frags(struct tun_file *tfile,
 	skb->truesize += skb->data_len;
 
 	for (i = 1; i < it->nr_segs; i++) {
-		struct page_frag *pfrag = &current->task_frag;
 		size_t fragsz = it->iov[i].iov_len;
+		struct page *page;
+		void *frag;
 
 		if (fragsz == 0 || fragsz > PAGE_SIZE) {
 			err = -EINVAL;
 			goto free;
 		}
-
-		if (!skb_page_frag_refill(fragsz, pfrag, GFP_KERNEL)) {
+		frag = netdev_alloc_frag(fragsz);
+		if (!frag) {
 			err = -ENOMEM;
 			goto free;
 		}
-
-		skb_fill_page_desc(skb, i - 1, pfrag->page,
-				   pfrag->offset, fragsz);
-		page_ref_inc(pfrag->page);
-		pfrag->offset += fragsz;
+		page = virt_to_head_page(frag);
+		skb_fill_page_desc(skb, i - 1, page,
+				   frag - page_address(page), fragsz);
 	}
 
 	return skb;