Switch to use shared vnode locks for text files during image activation.

kern_execve() locks text vnode exclusive to be able to set and clear
VV_TEXT flag. VV_TEXT is mutually exclusive with the v_writecount > 0
condition.

The change removes VV_TEXT, replacing it with the condition
v_writecount <= -1, and puts v_writecount under the vnode interlock.
Each text reference decrements v_writecount.  To clear the text
reference when the segment is unmapped, it is recorded in the
vm_map_entry backed by the text file as MAP_ENTRY_VN_TEXT flag, and
v_writecount is incremented on the map entry removal

The operations like VOP_ADD_WRITECOUNT() and VOP_SET_TEXT() check that
v_writecount does not contradict the desired change.  vn_writecheck()
is now racy and its use was eliminated everywhere except access.
Atomic check for writeability and increment of v_writecount is
performed by the VOP.  vn_truncate() now increments v_writecount
around VOP_SETATTR() call, lack of which is arguably a bug on its own.

nullfs bypasses v_writecount to the lower vnode always, so nullfs
vnode has its own v_writecount correct, and lower vnode gets all
references, since object->handle is always lower vnode.

On the text vnode' vm object dealloc, the v_writecount value is reset
to zero, and deadfs vop_unset_text short-circuit the operation.
Reclamation of lowervp always reclaims all nullfs vnodes referencing
lowervp first, so no stray references are left.

Reviewed by:	markj, trasz
Tested by:	mjg, pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 month
Differential revision:	https://reviews.freebsd.org/D19923

1 files changed, 9 insertions, 0 deletions

diff --git a/sys/fs/deadfs/dead_vnops.c b/sys/fs/deadfs/dead_vnops.c
index aa2e3f8f6585..a3153aed326a 100644
--- a/sys/fs/deadfs/dead_vnops.c
+++ b/sys/fs/deadfs/dead_vnops.c
@@ -47,6 +47,7 @@ static vop_lookup_t	dead_lookup;
 static vop_open_t	dead_open;
 static vop_getwritemount_t dead_getwritemount;
 static vop_rename_t	dead_rename;
+static vop_unset_text_t	dead_unset_text;
 
 struct vop_vector dead_vnodeops = {
 	.vop_default =		&default_vnodeops,
@@ -76,6 +77,7 @@ struct vop_vector dead_vnodeops = {
 	.vop_setattr =		VOP_EBADF,
 	.vop_symlink =		VOP_PANIC,
 	.vop_vptocnp =		VOP_EBADF,
+	.vop_unset_text =	dead_unset_text,
 	.vop_write =		dead_write,
 };
 
@@ -148,3 +150,10 @@ dead_rename(struct vop_rename_args *ap)
 	vop_rename_fail(ap);
 	return (EXDEV);
 }
+
+static int
+dead_unset_text(struct vop_unset_text_args *ap)
+{
+
+	return (0);
+}