Give sendfile(2) a SF_SYNC flag which makes it wait until all mbufs

referencing the files VM pages are returned from the network stack,
making changes to the file safe.

This flag does not guarantee that the data has been transmitted to the
other end.

lib/libc/sys/sendfile.2

@@ -103,7 +103,9 @@ variable pointed to by
 .Pp
 The
 .Fa flags
-argument has one possible value:
+argument is a bitmap of these values:
+.Bl -item -offset indent 
+.It
 .Dv SF_NODISKIO .
 This flag causes any
 .Fn sendfile
@@ -112,6 +114,17 @@ return
 .Er EBUSY .
 Busy servers may benefit by transferring requests that would
 block to a separate I/O worker thread.
+.It
+.Dv SF_MNOWAIT .
+(description missing)
+.It
+.Dv SF_SYNC ,
+.Nm
+sleeps until the network stack no longer references the VM pages
+of the file, making subsequent modifications to it safe.
+Please note that this is not a guarantee that the data has actually
+been sent.
+.El
 .Pp
 When using a socket marked for non-blocking I/O,
 .Fn sendfile

sys/kern/uipc_syscalls.c

@@ -1660,6 +1660,14 @@ getsockaddr(namp, uaddr, len)
 	return (error);
 }
 
+#include <sys/condvar.h>
+
+struct sendfile_sync {
+	struct mtx	mtx;
+	struct cv	cv;
+	unsigned 	count;
+};
+
 /*
  * Detach mapped page and release resources back to the system.
  */
@@ -1667,6 +1675,7 @@ void
 sf_buf_mext(void *addr, void *args)
 {
 	vm_page_t m;
+	struct sendfile_sync *sfs;
 
 	m = sf_buf_page(args);
 	sf_buf_free(args);
@@ -1680,6 +1689,14 @@ sf_buf_mext(void *addr, void *args)
 	if (m->wire_count == 0 && m->object == NULL)
 		vm_page_free(m);
 	vm_page_unlock_queues();
+	if (addr == NULL)
+		return;
+	sfs = addr;
+	mtx_lock(&sfs->mtx);
+	KASSERT(sfs->count> 0, ("Sendfile sync botchup count == 0"));
+	if (--sfs->count == 0)
+		cv_signal(&sfs->cv);
+	mtx_unlock(&sfs->mtx);
 }
 
 /*
@@ -1767,6 +1784,7 @@ kern_sendfile(struct thread *td, struct sendfile_args *uap,
 	off_t off, xfsize, fsbytes = 0, sbytes = 0, rem = 0;
 	int error, hdrlen = 0, mnw = 0;
 	int vfslocked;
+	struct sendfile_sync *sfs = NULL;
 
 	/*
 	 * The file descriptor must be a regular file and have a
@@ -1829,6 +1847,13 @@ kern_sendfile(struct thread *td, struct sendfile_args *uap,
 	if (uap->flags & SF_MNOWAIT)
 		mnw = 1;
 
+	if (uap->flags & SF_SYNC) {
+		sfs = malloc(sizeof *sfs, M_TEMP, M_WAITOK);
+		memset(sfs, 0, sizeof *sfs);
+		mtx_init(&sfs->mtx, "sendfile", MTX_DEF, 0);
+		cv_init(&sfs->cv, "sendfile");
+	}
+
 #ifdef MAC
 	SOCK_LOCK(so);
 	error = mac_socket_check_send(td->td_ucred, so);
@@ -2099,7 +2124,7 @@ retry_space:
 				break;
 			}
 			MEXTADD(m0, sf_buf_kva(sf), PAGE_SIZE, sf_buf_mext,
-			    (void*)sf_buf_kva(sf), sf, M_RDONLY, EXT_SFBUF);
+			    sfs, sf, M_RDONLY, EXT_SFBUF);
 			m0->m_data = (char *)sf_buf_kva(sf) + pgoff;
 			m0->m_len = xfsize;
 
@@ -2112,6 +2137,12 @@ retry_space:
 			/* Keep track of bits processed. */
 			loopbytes += xfsize;
 			off += xfsize;
+
+			if (sfs != NULL) {
+				mtx_lock(&sfs->mtx);
+				sfs->count++;
+				mtx_unlock(&sfs->mtx);
+			}
 		}
 
 		/* Add the buffer chain to the socket buffer. */
@@ -2190,6 +2221,16 @@ out:
 	if (m)
 		m_freem(m);
 
+	if (sfs != NULL) {
+		mtx_lock(&sfs->mtx);
+		if (sfs->count != 0)
+			cv_wait(&sfs->cv, &sfs->mtx);
+		KASSERT(sfs->count == 0, ("sendfile sync still busy"));
+		cv_destroy(&sfs->cv);
+		mtx_destroy(&sfs->mtx);
+		free(sfs, M_TEMP);
+	}
+
 	if (error == ERESTART)
 		error = EINTR;
 

sys/sys/socket.h

@@ -609,6 +609,7 @@ struct sf_hdtr {
  */
 #define	SF_NODISKIO     0x00000001
 #define	SF_MNOWAIT	0x00000002
+#define	SF_SYNC		0x00000004
 #endif
 
 #ifndef	_KERNEL