[PATCH net-next] net: reclaim skb->scm_io_uring bit

[Eric Dumazet <edumazet@google.com>]

Commit 0091bfc81741 ("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>
---
 include/linux/skbuff.h | 2 --
 include/net/af_unix.h  | 1 +
 io_uring/rsrc.c        | 3 +--
 net/unix/garbage.c     | 2 +-
 net/unix/scm.c         | 6 ++++++
 5 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index ff7ad331fb8259fd06c07913dfa197d7ac448390..fe661011644b8f468ff5e92075a6624f0557584c 100644
--- a/include/linux/skbuff.h
+++ b/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 */
diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 480fa579787e597f264ae26a32e519c235ce1d39..45ebde587138e59f8331d358420d3fca79d9ee66 100644
--- a/include/net/af_unix.h
+++ b/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);
diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index a59fc02de5983c4f789e9cdfea3a1376f578ebe6..27ceda3b50cf4e677cde5e2417a80adad66e162f 100644
--- a/io_uring/rsrc.c
+++ b/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);
 	}
 
diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index dc27635403932154f3dec069c2e10d2ae365d8cb..2405f0f9af31c0ccefe2aa404002cfab8583c090 100644
--- a/net/unix/garbage.c
+++ b/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);
 		}
diff --git a/net/unix/scm.c b/net/unix/scm.c
index aa27a02478dc1a7e4022f77e6ea7ac55f40b95c7..f9152881d77f636f9500d1b57fdda584df845fc2 100644
--- a/net/unix/scm.c
+++ b/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);
-- 
2.40.0.rc0.216.gc4246ad0f0-goog