内核源码解读之内存管理（9) 第一个mm_struct

Posted 2023-01-04 奇妙之二进制

每个进程或内核线程都由一个任务描述数据结构(task_struct)来管理，每个task_struct中有个struct mm_strcut数据结构指针，用来管理任务的虚拟地址空间。而内核本身作为一个进程，也有对应的mm_struct。

mm/init-mm.c

init_mm是全系统第一个mm_struct，它记录了内核进程的内存信息。

/*
 * For dynamically allocated mm_structs, there is a dynamically sized cpumask
 * at the end of the structure, the size of which depends on the maximum CPU
 * number the system can see. That way we allocate only as much memory for
 * mm_cpumask() as needed for the hundreds, or thousands of processes that
 * a system typically runs.
 *
 * Since there is only one init_mm in the entire system, keep it simple
 * and size this cpu_bitmask to NR_CPUS.
 */
struct mm_struct init_mm = 
	.mm_mt		= MTREE_INIT_EXT(mm_mt, MM_MT_FLAGS, init_mm.mmap_lock),
	.pgd		= swapper_pg_dir,
	.mm_users	= ATOMIC_INIT(2),
	.mm_count	= ATOMIC_INIT(1),
	.write_protect_seq = SEQCNT_ZERO(init_mm.write_protect_seq),
	MMAP_LOCK_INITIALIZER(init_mm)
	.page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
	.arg_lock	=  __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
	.user_ns	= &init_user_ns,
	.cpu_bitmap	= CPU_BITS_NONE,
#ifdef CONFIG_IOMMU_SVA
	.pasid		= INVALID_IOASID,
#endif
	INIT_MM_CONTEXT(init_mm)
;

上面有个重要成员pgd，它记录了全局页目录项的地址，全局页目录项占据了一个page，swapper_pg_dir是在内核镜像链接脚本vmlinux.ld.S里定义的。

----> setup_arch
      -----> setup_initial_init_mm

void setup_initial_init_mm(void *start_code, void *end_code,
			   void *end_data, void *brk)

	init_mm.start_code = (unsigned long)start_code;
	init_mm.end_code = (unsigned long)end_code;
	init_mm.end_data = (unsigned long)end_data;
	init_mm.brk = (unsigned long)brk;