tcp: add TCP_ZEROCOPY_RECEIVE support for zerocopy receive

When adding tcp mmap() implementation, I forgot that socket lock
had to be taken before current->mm->mmap_sem. syzbot eventually caught
the bug.

Since we can not lock the socket in tcp mmap() handler we have to
split the operation in two phases.

1) mmap() on a tcp socket simply reserves VMA space, and nothing else.
  This operation does not involve any TCP locking.

2) getsockopt(fd, IPPROTO_TCP, TCP_ZEROCOPY_RECEIVE, ...) implements
 the transfert of pages from skbs to one VMA.
  This operation only uses down_read(&current->mm->mmap_sem) after
  holding TCP lock, thus solving the lockdep issue.

This new implementation was suggested by Andy Lutomirski with great details.

Benefits are :

- Better scalability, in case multiple threads reuse VMAS
   (without mmap()/munmap() calls) since mmap_sem wont be write locked.

- Better error recovery.
   The previous mmap() model had to provide the expected size of the
   mapping. If for some reason one part could not be mapped (partial MSS),
   the whole operation had to be aborted.
   With the tcp_zerocopy_receive struct, kernel can report how
   many bytes were successfuly mapped, and how many bytes should
   be read to skip the problematic sequence.

- No more memory allocation to hold an array of page pointers.
  16 MB mappings needed 32 KB for this array, potentially using vmalloc() :/

- skbs are freed while mmap_sem has been released

Following patch makes the change in tcp_mmap tool to demonstrate
one possible use of mmap() and setsockopt(... TCP_ZEROCOPY_RECEIVE ...)

Note that memcg might require additional changes.

Fixes: 93ab6cc691 ("tcp: implement mmap() for zero copy receive")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Suggested-by: Andy Lutomirski <luto@kernel.org>
Cc: linux-mm@kvack.org
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/tcp.h

@@ -122,6 +122,7 @@ enum {
 #define TCP_MD5SIG_EXT		32	/* TCP MD5 Signature with extensions */
 #define TCP_FASTOPEN_KEY	33	/* Set the key for Fast Open (cookie) */
 #define TCP_FASTOPEN_NO_COOKIE	34	/* Enable TFO without a TFO cookie */
+#define TCP_ZEROCOPY_RECEIVE	35
 
 struct tcp_repair_opt {
 	__u32	opt_code;
@@ -276,4 +277,11 @@ struct tcp_diag_md5sig {
 	__u8	tcpm_key[TCP_MD5SIG_MAXKEYLEN];
 };
 
+/* setsockopt(fd, IPPROTO_TCP, TCP_ZEROCOPY_RECEIVE, ...) */
+
+struct tcp_zerocopy_receive {
+	__u64 address;		/* in: address of mapping */
+	__u32 length;		/* in/out: number of bytes to map/mapped */
+	__u32 recv_skip_hint;	/* out: amount of bytes to skip */
+};
 #endif /* _UAPI_LINUX_TCP_H */