diff --git a/sys/kern/kern_sendfile.c b/sys/kern/kern_sendfile.c
--- a/sys/kern/kern_sendfile.c
+++ b/sys/kern/kern_sendfile.c
@@ -698,10 +698,13 @@
 	 */
 	error = 0;
 	SOCK_SENDBUF_LOCK(so);
-	if (so->so_snd.sb_lowat < so->so_snd.sb_hiwat / 2)
-		so->so_snd.sb_lowat = so->so_snd.sb_hiwat / 2;
-	if (so->so_snd.sb_lowat < PAGE_SIZE && so->so_snd.sb_hiwat >= PAGE_SIZE)
-		so->so_snd.sb_lowat = PAGE_SIZE;
+	if (so->so_snd.sb_flags & SB_AUTOLOWAT) {
+		if (so->so_snd.sb_lowat < so->so_snd.sb_hiwat / 2)
+			so->so_snd.sb_lowat = so->so_snd.sb_hiwat / 2;
+		if (so->so_snd.sb_lowat < PAGE_SIZE &&
+		    so->so_snd.sb_hiwat >= PAGE_SIZE)
+			so->so_snd.sb_lowat = PAGE_SIZE;
+	}
 retry_space:
 	if (so->so_snd.sb_state & SBS_CANTSENDMORE) {
 		error = EPIPE;
diff --git a/sys/kern/uipc_sockbuf.c b/sys/kern/uipc_sockbuf.c
--- a/sys/kern/uipc_sockbuf.c
+++ b/sys/kern/uipc_sockbuf.c
@@ -779,6 +779,7 @@
 		 * high-water.
 		 */
 		*lowat = (cc > *hiwat) ? *hiwat : cc;
+		*flags &= ~SB_AUTOLOWAT;
 		break;
 	}
 
diff --git a/sys/kern/uipc_socket.c b/sys/kern/uipc_socket.c
--- a/sys/kern/uipc_socket.c
+++ b/sys/kern/uipc_socket.c
@@ -1211,7 +1211,8 @@
 	so->so_rcv.sb_timeo = head->sol_sbrcv_timeo;
 	so->so_snd.sb_timeo = head->sol_sbsnd_timeo;
 	so->so_rcv.sb_flags = head->sol_sbrcv_flags & SB_AUTOSIZE;
-	so->so_snd.sb_flags = head->sol_sbsnd_flags & SB_AUTOSIZE;
+	so->so_snd.sb_flags = head->sol_sbsnd_flags &
+	    (SB_AUTOSIZE | SB_AUTOLOWAT);
 	if ((so->so_proto->pr_flags & PR_SOCKBUF) == 0) {
 		so->so_snd.sb_mtx = &so->so_snd_mtx;
 		so->so_rcv.sb_mtx = &so->so_rcv_mtx;
@@ -4514,6 +4515,9 @@
 		SOCK_BUF_LOCK(so, which);
 		knlist_add(knl, kn, 1);
 		sb->sb_flags |= SB_KNOTE;
+		if ((kn->kn_sfflags & NOTE_LOWAT) &&
+		    (sb->sb_flags & SB_AUTOLOWAT))
+			sb->sb_flags &= ~SB_AUTOLOWAT;
 		SOCK_BUF_UNLOCK(so, which);
 	}
 	SOCK_UNLOCK(so);
diff --git a/sys/netinet/tcp_usrreq.c b/sys/netinet/tcp_usrreq.c
--- a/sys/netinet/tcp_usrreq.c
+++ b/sys/netinet/tcp_usrreq.c
@@ -164,7 +164,7 @@
 		goto out;
 
 	so->so_rcv.sb_flags |= SB_AUTOSIZE;
-	so->so_snd.sb_flags |= SB_AUTOSIZE;
+	so->so_snd.sb_flags |= (SB_AUTOLOWAT | SB_AUTOSIZE);
 	error = in_pcballoc(so, &V_tcbinfo);
 	if (error)
 		goto out;
diff --git a/sys/sys/sockbuf.h b/sys/sys/sockbuf.h
--- a/sys/sys/sockbuf.h
+++ b/sys/sys/sockbuf.h
@@ -40,7 +40,7 @@
 #define	SB_SEL		0x08		/* someone is selecting */
 #define	SB_ASYNC	0x10		/* ASYNC I/O, need signals */
 #define	SB_UPCALL	0x20		/* someone wants an upcall */
-/* was	SB_NOINTR	0x40		*/
+#define	SB_AUTOLOWAT	0x40		/* sendfile(2) may autotune sb_lowat */
 #define	SB_AIO		0x80		/* AIO operations queued */
 #define	SB_KNOTE	0x100		/* kernel note attached */
 #define	SB_NOCOALESCE	0x200		/* don't coalesce new data into existing mbufs */