Shrink struct vm_page size

struct vm_page is supposed to be a "small structure", but it takes up 96
bytes on x86_64 (to represent a 4k page). By utilizing bitfields and
strategically reordering members to avoid excessive padding, it can be
shrunk to 80 bytes.

- page_lock and unlock_request only need to store a bitmask of
  VM_PROT_READ, VM_PROT_WRITE, and VM_PROT_EXECUTE. Even though the
  special values VM_PROT_NO_CHANGE and VM_PROT_NOTIFY are defined, they
  are not used for the two struct vm_page members.

- type and seg_index both need to store one of the four possible values
  in the range from 0 to 3. Two bits are sufficient for this.

- order needs to store a number from 0 to VM_PAGE_NR_FREE_LISTS (which
  is 11), or a special value VM_PAGE_ORDER_UNLISTED. Four bits are
  sufficient for this.

No functional change.
Message-Id: <20230626112656.435622-2-bugaevc@gmail.com>

3 files changed, 15 insertions, 9 deletions

diff --git a/vm/vm_page.c b/vm/vm_page.c
index 50916b74..ce37c8aa 100644
--- a/vm/vm_page.c
+++ b/vm/vm_page.c
@@ -94,7 +94,7 @@ struct vm_page_cpu_pool {
  * Special order value for pages that aren't in a free list. Such pages are
  * either allocated, or part of a free block of pages but not the head page.
  */
-#define VM_PAGE_ORDER_UNLISTED ((unsigned short)-1)
+#define VM_PAGE_ORDER_UNLISTED (VM_PAGE_NR_FREE_LISTS + 1)
 
 /*
  * Doubly-linked list of free blocks.
@@ -1016,7 +1016,7 @@ vm_page_seg_balance_page(struct vm_page_seg *seg,
 
     vm_page_set_type(dest, 0, src->type);
     memcpy(&dest->vm_page_header, &src->vm_page_header,
-           sizeof(*dest) - VM_PAGE_HEADER_SIZE);
+           VM_PAGE_BODY_SIZE);
     vm_page_copy(src, dest);
 
     if (!src->dirty) {
diff --git a/vm/vm_page.h b/vm/vm_page.h
index d457f9a2..b2581d9e 100644
--- a/vm/vm_page.h
+++ b/vm/vm_page.h
@@ -79,9 +79,6 @@
 
 struct vm_page {
 	struct list node;		/* page queues or free list (P) */
-	unsigned short type;
-	unsigned short seg_index;
-	unsigned short order;
 	void *priv;
 
 	/*
@@ -95,7 +92,6 @@ struct vm_page {
 
 	/* We use an empty struct as the delimiter.  */
 	struct {} vm_page_header;
-#define VM_PAGE_HEADER_SIZE	offsetof(struct vm_page, vm_page_header)
 
 	vm_object_t	object;		/* which object am I in (O,P) */
 	vm_offset_t	offset;		/* offset into that object (O,P) */
@@ -126,10 +122,20 @@ struct vm_page {
 					 * without having data. (O)
 					 * [See vm_object_overwrite] */
 
-	vm_prot_t	page_lock;	/* Uses prohibited by data manager (O) */
-	vm_prot_t	unlock_request;	/* Outstanding unlock request (O) */
+	vm_prot_t	page_lock:3;	/* Uses prohibited by data manager (O) */
+	vm_prot_t	unlock_request:3;	/* Outstanding unlock request (O) */
+
+	struct {} vm_page_footer;
+
+	unsigned short type:2;
+	unsigned short seg_index:2;
+	unsigned short order:4;
 };
 
+#define VM_PAGE_BODY_SIZE					\
+		(offsetof(struct vm_page, vm_page_footer)	\
+		- offsetof(struct vm_page, vm_page_header))
+
 /*
  *	For debugging, this macro can be defined to perform
  *	some useful check on a page structure.
diff --git a/vm/vm_resident.c b/vm/vm_resident.c
index e0a03bf5..d4777e70 100644
--- a/vm/vm_resident.c
+++ b/vm/vm_resident.c
@@ -746,7 +746,7 @@ boolean_t vm_page_convert(struct vm_page **mp)
 
 	memcpy(&real_m->vm_page_header,
 	       &fict_m->vm_page_header,
-	       sizeof(*fict_m) - VM_PAGE_HEADER_SIZE);
+	       VM_PAGE_BODY_SIZE);
 	real_m->fictitious = FALSE;
 
 	vm_page_insert(real_m, object, offset);