net: reclaim skb->scm_io_uring bit

Commit 0091bfc ("io_uring/af_unix: defer registered
files gc to io_uring release") added one bit to struct sk_buff.

This structure is critical for networking, and we try very hard
to not add bloat on it, unless absolutely required.

For instance, we can use a specific destructor as a wrapper
around unix_destruct_scm(), to identify skbs that unix_gc()
has to special case.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Cc: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: edumazet

include/linux/skbuff.h

@@ -810,7 +810,6 @@ typedef unsigned char *sk_buff_data_t;
  *	@csum_level: indicates the number of consecutive checksums found in
  *		the packet minus one that have been verified as
  *		CHECKSUM_UNNECESSARY (max 3)
- *	@scm_io_uring: SKB holds io_uring registered files
  *	@dst_pending_confirm: need to confirm neighbour
  *	@decrypted: Decrypted SKB
  *	@slow_gro: state present at GRO time, slower prepare step required
@@ -989,7 +988,6 @@ struct sk_buff {
 #endif
 	__u8			slow_gro:1;
 	__u8			csum_not_inet:1;
-	__u8			scm_io_uring:1;
 
 #ifdef CONFIG_NET_SCHED
 	__u16			tc_index;	/* traffic control index */

include/net/af_unix.h

@@ -11,6 +11,7 @@
 void unix_inflight(struct user_struct *user, struct file *fp);
 void unix_notinflight(struct user_struct *user, struct file *fp);
 void unix_destruct_scm(struct sk_buff *skb);
+void io_uring_destruct_scm(struct sk_buff *skb);
 void unix_gc(void);
 void wait_for_unix_gc(void);
 struct sock *unix_get_socket(struct file *filp);

io_uring/rsrc.c

@@ -867,8 +867,7 @@ int __io_scm_file_account(struct io_ring_ctx *ctx, struct file *file)
 
 		UNIXCB(skb).fp = fpl;
 		skb->sk = sk;
-		skb->scm_io_uring = 1;
-		skb->destructor = unix_destruct_scm;
+		skb->destructor = io_uring_destruct_scm;
 		refcount_add(skb->truesize, &sk->sk_wmem_alloc);
 	}
 

net/unix/garbage.c

@@ -305,7 +305,7 @@ void unix_gc(void)
 	 * release.path eventually putting registered files.
 	 */
 	skb_queue_walk_safe(&hitlist, skb, next_skb) {
-		if (skb->scm_io_uring) {
+		if (skb->destructor == io_uring_destruct_scm) {
 			__skb_unlink(skb, &hitlist);
 			skb_queue_tail(&skb->sk->sk_receive_queue, skb);
 		}

net/unix/scm.c

@@ -152,3 +152,9 @@ void unix_destruct_scm(struct sk_buff *skb)
 	sock_wfree(skb);
 }
 EXPORT_SYMBOL(unix_destruct_scm);
+
+void io_uring_destruct_scm(struct sk_buff *skb)
+{
+	unix_destruct_scm(skb);
+}
+EXPORT_SYMBOL(io_uring_destruct_scm);