mm/vmalloc: modify struct vmap_area to reduce its size

Objective
---------

The current implementation of struct vmap_area wasted space.

After applying this commit, sizeof(struct vmap_area) has been
reduced from 11 words to 8 words.

Description
-----------

1) Pack "subtree_max_size", "vm" and "purge_list".  This is no problem
   because

A) "subtree_max_size" is only used when vmap_area is in "free" tree

B) "vm" is only used when vmap_area is in "busy" tree

C) "purge_list" is only used when vmap_area is in vmap_purge_list

2) Eliminate "flags".

;Since only one flag VM_VM_AREA is being used, and the same thing can be
done by judging whether "vm" is NULL, then the "flags" can be eliminated.

Link: http://lkml.kernel.org/r/20190716152656.12255-3-lpf.vector@gmail.com
Signed-off-by: Pengfei Li <lpf.vector@gmail.com>
Suggested-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/vmalloc.h

@@ -53,15 +53,21 @@ struct vmap_area {
 	unsigned long va_start;
 	unsigned long va_end;
 
-	/*
-	 * Largest available free size in subtree.
-	 */
-	unsigned long subtree_max_size;
-	unsigned long flags;
 	struct rb_node rb_node;         /* address sorted rbtree */
 	struct list_head list;          /* address sorted list */
-	struct llist_node purge_list;    /* "lazy purge" list */
-	struct vm_struct *vm;
+
+	/*
+	 * The following three variables can be packed, because
+	 * a vmap_area object is always one of the three states:
+	 *    1) in "free" tree (root is vmap_area_root)
+	 *    2) in "busy" tree (root is free_vmap_area_root)
+	 *    3) in purge list  (head is vmap_purge_list)
+	 */
+	union {
+		unsigned long subtree_max_size; /* in "free" tree */
+		struct vm_struct *vm;           /* in "busy" tree */
+		struct llist_node purge_list;   /* in purge list */
+	};
 };
 
 /*

mm/vmalloc.c

@@ -329,7 +329,6 @@ EXPORT_SYMBOL(vmalloc_to_pfn);
 #define DEBUG_AUGMENT_PROPAGATE_CHECK 0
 #define DEBUG_AUGMENT_LOWEST_MATCH_CHECK 0
 
-#define VM_VM_AREA	0x04
 
 static DEFINE_SPINLOCK(vmap_area_lock);
 /* Export for kexec only */
@@ -1115,7 +1114,7 @@ retry:
 
 	va->va_start = addr;
 	va->va_end = addr + size;
-	va->flags = 0;
+	va->vm = NULL;
 	insert_vmap_area(va, &vmap_area_root, &vmap_area_list);
 
 	spin_unlock(&vmap_area_lock);
@@ -1928,7 +1927,6 @@ void __init vmalloc_init(void)
 		if (WARN_ON_ONCE(!va))
 			continue;
 
-		va->flags = VM_VM_AREA;
 		va->va_start = (unsigned long)tmp->addr;
 		va->va_end = va->va_start + tmp->size;
 		va->vm = tmp;
@@ -2026,7 +2024,6 @@ static void setup_vmalloc_vm(struct vm_struct *vm, struct vmap_area *va,
 	vm->size = va->va_end - va->va_start;
 	vm->caller = caller;
 	va->vm = vm;
-	va->flags |= VM_VM_AREA;
 	spin_unlock(&vmap_area_lock);
 }
 
@@ -2131,10 +2128,10 @@ struct vm_struct *find_vm_area(const void *addr)
 	struct vmap_area *va;
 
 	va = find_vmap_area((unsigned long)addr);
-	if (va && va->flags & VM_VM_AREA)
-		return va->vm;
+	if (!va)
+		return NULL;
 
-	return NULL;
+	return va->vm;
 }
 
 /**
@@ -2155,11 +2152,10 @@ struct vm_struct *remove_vm_area(const void *addr)
 
 	spin_lock(&vmap_area_lock);
 	va = __find_vmap_area((unsigned long)addr);
-	if (va && va->flags & VM_VM_AREA) {
+	if (va && va->vm) {
 		struct vm_struct *vm = va->vm;
 
 		va->vm = NULL;
-		va->flags &= ~VM_VM_AREA;
 		spin_unlock(&vmap_area_lock);
 
 		kasan_free_shadow(vm);
@@ -2862,7 +2858,7 @@ long vread(char *buf, char *addr, unsigned long count)
 		if (!count)
 			break;
 
-		if (!(va->flags & VM_VM_AREA))
+		if (!va->vm)
 			continue;
 
 		vm = va->vm;
@@ -2942,7 +2938,7 @@ long vwrite(char *buf, char *addr, unsigned long count)
 		if (!count)
 			break;
 
-		if (!(va->flags & VM_VM_AREA))
+		if (!va->vm)
 			continue;
 
 		vm = va->vm;
@@ -3485,10 +3481,10 @@ static int s_show(struct seq_file *m, void *p)
 	va = list_entry(p, struct vmap_area, list);
 
 	/*
-	 * s_show can encounter race with remove_vm_area, !VM_VM_AREA on
-	 * behalf of vmap area is being tear down or vm_map_ram allocation.
+	 * s_show can encounter race with remove_vm_area, !vm on behalf
+	 * of vmap area is being tear down or vm_map_ram allocation.
 	 */
-	if (!(va->flags & VM_VM_AREA)) {
+	if (!va->vm) {
 		seq_printf(m, "0x%pK-0x%pK %7ld vm_map_ram\n",
 			(void *)va->va_start, (void *)va->va_end,
 			va->va_end - va->va_start);