Implement POSIX.1b shared memory objects. In this implementation,

shared memory objects are regular files; the shm_open(3) routine
uses fcntl(2) to set a flag on the descriptor which tells mmap(2)
to automatically apply MAP_NOSYNC.

Not objected to by: bde, dillon, dufault, jasone

1 files changed, 10 insertions, 0 deletions

diff --git a/sys/vm/vm_mmap.c b/sys/vm/vm_mmap.c
index 53462f4..d617d0c 100644
--- a/sys/vm/vm_mmap.c
+++ b/sys/vm/vm_mmap.c
@@ -282,6 +282,16 @@ mmap(p, uap)
 			return (EBADF);
 		if (fp->f_type != DTYPE_VNODE)
 			return (EINVAL);
+		/*
+		 * POSIX shared-memory objects are defined to have
+		 * kernel persistence, and are not defined to support
+		 * read(2)/write(2) -- or even open(2).  Thus, we can
+		 * use MAP_ASYNC to trade on-disk coherence for speed.
+		 * The shm_open(3) library routine turns on the FPOSIXSHM
+		 * flag to request this behavior.
+		 */
+		if (fp->f_flag & FPOSIXSHM)
+			flags |= MAP_NOSYNC;
 		vp = (struct vnode *) fp->f_data;
 		if (vp->v_type != VREG && vp->v_type != VCHR)
 			return (EINVAL);