From 7331bdefda78d817f0724f19f989f6debd08fc70 Mon Sep 17 00:00:00 2001
From: Max Wash <mbswash@gmail.com>
Date: Thu, 2 Feb 2023 16:54:48 +0000
Subject: [PATCH] sandbox: vm: reduce vm_page order field size to 4 bits

---
 sandbox/vm/include/socks/vm.h | 27 ++++++++++++++++++++-------
 sandbox/vm/page.c             |  6 ++++++
 2 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/sandbox/vm/include/socks/vm.h b/sandbox/vm/include/socks/vm.h
index 23db87f..cc44b16 100644
--- a/sandbox/vm/include/socks/vm.h
+++ b/sandbox/vm/include/socks/vm.h
@@ -60,9 +60,15 @@ typedef enum vm_page_order {
 	VM_PAGE_32M,
 	VM_PAGE_64M,
 	VM_PAGE_128M,
+#if 0
+	/* vm_page_t only has 4 bits to store the page order with.
+	   the maximum order that can be stored in 4 bits is 15 (VM_PAGE_128M)
+	   to use any of the page orders listed here, this field
+	   will have to be expanded. */
 	VM_PAGE_256M,
 	VM_PAGE_512M,
 	VM_PAGE_1G,
+#endif
 	VM_PAGE_MIN_ORDER = VM_PAGE_4K,
 	VM_PAGE_MAX_ORDER = VM_PAGE_8M,
 } vm_page_order_t;
@@ -110,18 +116,25 @@ typedef enum vm_page_flags {
 } vm_page_flags_t;
 
 typedef struct vm_page {
+	/* order of the page block that this page belongs too */
+	uint16_t p_order : 4;
 	/* the id of the NUMA node that this page belongs to */
-	uint32_t p_node : 6;
+	uint16_t p_node : 6;
 	/* the id of the memory zone that this page belongs to */
-	uint32_t p_zone : 2;
-	/* vm_page_flags_t bitfields. */
-	uint32_t p_flags : 24;
+	uint16_t p_zone : 3;
+	/* some unused bits */
+	uint16_t p_reserved : 3;
 
-	/* buddy allocator free page list head (vm_zone_t->z_free_pages[p_order]) */
+	/* vm_page_flags_t bitfields. */
+	uint32_t p_flags;
+
+	/* multi-purpose list.
+	   the owner of the page can decide what to do with this.
+	   some examples:
+	     - the buddy allocator uses this to maintain its per-zone free-page lists.
+        */
 	queue_entry_t p_free_list;
 
-	/* order of the page block that this page belongs too */
-	unsigned char p_order;
 } __attribute__((aligned(2 * sizeof(unsigned long)))) vm_page_t;
 
 extern kern_status_t vm_bootstrap(const vm_zone_descriptor_t *zones, size_t nr_zones);
diff --git a/sandbox/vm/page.c b/sandbox/vm/page.c
index 40f402c..51ba4cf 100644
--- a/sandbox/vm/page.c
+++ b/sandbox/vm/page.c
@@ -28,9 +28,15 @@ static size_t page_order_bytes[] = {
 	[VM_PAGE_32M]  = 0x2000000,
 	[VM_PAGE_64M]  = 0x4000000,
 	[VM_PAGE_128M] = 0x8000000,
+#if 0
+	/* vm can support pages of this size, but
+	   vm_page_t only has 4 bits with which to store
+	   the page order, which cannot accomodate these
+	   larger order numbers */
 	[VM_PAGE_256M] = 0x10000000,
 	[VM_PAGE_512M] = 0x20000000,
 	[VM_PAGE_1G]   = 0x40000000,
+#endif
 };
 
 void vm_page_init_array()