Rework the handling of the tmpfs node backing swap object and tmpfs

vnode v_object to avoid double-buffering.  Use the same object both as
the backing store for tmpfs node and as the v_object.

Besides reducing memory use up to 2x times for situation of mapping
files from tmpfs, it also makes tmpfs read and write operations copy
twice bytes less.

VM subsystem was already slightly adapted to tolerate OBJT_SWAP object
as v_object. Now the vm_object_deallocate() is modified to not
reinstantiate OBJ_ONEMAPPING flag and help the VFS to correctly handle
VV_TEXT flag on the last dereference of the tmpfs backing object.

Reviewed by:	alc
Tested by:	pho, bf
MFC after:	1 month

2 files changed, 34 insertions, 1 deletions

diff --git a/sys/vm/vm_object.c b/sys/vm/vm_object.c
index 1f7cb78..ce81ad5 100644
--- a/sys/vm/vm_object.c
+++ b/sys/vm/vm_object.c
@@ -505,6 +505,7 @@ void
 vm_object_deallocate(vm_object_t object)
 {
 	vm_object_t temp;
+	struct vnode *vp;
 
 	while (object != NULL) {
 		VM_OBJECT_WLOCK(object);
@@ -527,15 +528,36 @@ vm_object_deallocate(vm_object_t object)
 			VM_OBJECT_WUNLOCK(object);
 			return;
 		} else if (object->ref_count == 1) {
+			if (object->type == OBJT_SWAP &&
+			    (object->flags & OBJ_TMPFS) != 0) {
+				vp = object->un_pager.swp.swp_tmpfs;
+				vhold(vp);
+				VM_OBJECT_WUNLOCK(object);
+				vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
+				vdrop(vp);
+				VM_OBJECT_WLOCK(object);
+				if (object->type == OBJT_DEAD) {
+					VM_OBJECT_WUNLOCK(object);
+					VOP_UNLOCK(vp, 0);
+					return;
+				} else if ((object->flags & OBJ_TMPFS) != 0) {
+					if (object->ref_count == 1)
+						VOP_UNSET_TEXT(vp);
+					VOP_UNLOCK(vp, 0);
+				}
+			}
 			if (object->shadow_count == 0 &&
 			    object->handle == NULL &&
 			    (object->type == OBJT_DEFAULT ||
-			     object->type == OBJT_SWAP)) {
+			    (object->type == OBJT_SWAP &&
+			    (object->flags & OBJ_TMPFS) == 0))) {
 				vm_object_set_flag(object, OBJ_ONEMAPPING);
 			} else if ((object->shadow_count == 1) &&
 			    (object->handle == NULL) &&
 			    (object->type == OBJT_DEFAULT ||
 			     object->type == OBJT_SWAP)) {
+				KASSERT((object->flags & OBJ_TMPFS) == 0,
+				    ("Shadowed tmpfs v_object"));
 				vm_object_t robject;
 
 				robject = LIST_FIRST(&object->shadow_head);
diff --git a/sys/vm/vm_object.h b/sys/vm/vm_object.h
index 2f2c961..cf54663 100644
--- a/sys/vm/vm_object.h
+++ b/sys/vm/vm_object.h
@@ -154,11 +154,21 @@ struct vm_object {
 		/*
 		 * Swap pager
 		 *
+		 *	swp_tmpfs - back-pointer to the tmpfs vnode,
+		 *		     if any, which uses the vm object
+		 *		     as backing store.  The handle
+		 *		     cannot be reused for linking,
+		 *		     because the vnode can be
+		 *		     reclaimed and recreated, making
+		 *		     the handle changed and hash-chain
+		 *		     invalid.
+		 *
 		 *	swp_bcount - number of swap 'swblock' metablocks, each
 		 *		     contains up to 16 swapblk assignments.
 		 *		     see vm/swap_pager.h
 		 */
 		struct {
+			void *swp_tmpfs;
 			int swp_bcount;
 		} swp;
 	} un_pager;
@@ -179,6 +189,7 @@ struct vm_object {
 #define	OBJ_COLORED	0x1000		/* pg_color is defined */
 #define	OBJ_ONEMAPPING	0x2000		/* One USE (a single, non-forked) mapping flag */
 #define	OBJ_DISCONNECTWNT 0x4000	/* disconnect from vnode wanted */
+#define	OBJ_TMPFS	0x8000
 
 #define IDX_TO_OFF(idx) (((vm_ooffset_t)(idx)) << PAGE_SHIFT)
 #define OFF_TO_IDX(off) ((vm_pindex_t)(((vm_ooffset_t)(off)) >> PAGE_SHIFT))