本文参考：内核解读之内存管理（9) 第一个mm_struct_init-mm.c_奇妙之二进制的博客-CSDN博客
本文使用的内核版本为6.1.0。
init-mm.c文件只有56行，内容相对比较简单，直接贴全部代码了。mm/init-mm.c内容如下：
// SPDX-License-Identifier: GPL-2.0
#include <linux/mm_types.h>
#include <linux/maple_tree.h>
#include <linux/rwsem.h>
#include <linux/spinlock.h>
#include <linux/list.h>
#include <linux/cpumask.h>
#include <linux/mman.h>
#include <linux/pgtable.h>
#include <linux/atomic.h>
#include <linux/user_namespace.h>
#include <linux/ioasid.h>
#include <asm/mmu.h>
#ifndef INIT_MM_CONTEXT
#define INIT_MM_CONTEXT(name)
#endif
/*
* 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)
};
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;
}
可以看到，init-mm.c中的代码分为了两段。先来看前一段。
每个进程或内核线程都由一个task_struct来管理，task_struct结构中有一个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。
init_mm是全系统第一个mm_struct，它记录了内核进程的内存信息。
struct mm_struct在中定义，如下：
struct mm_struct {
struct {
struct maple_tree mm_mt;
#ifdef CONFIG_MMU
unsigned long (*get_unmapped_area) (struct file *filp,
unsigned long addr, unsigned long len,
unsigned long pgoff, unsigned long flags);
#endif
unsigned long mmap_base; /* base of mmap area */
unsigned long mmap_legacy_base; /* base of mmap area in bottom-up allocations */
#ifdef CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES
/* Base addresses for compatible mmap() */
unsigned long mmap_compat_base;
unsigned long mmap_compat_legacy_base;
#endif
unsigned long task_size; /* size of task vm space */
pgd_t * pgd;
#ifdef CONFIG_MEMBARRIER
/**
* @membarrier_state: Flags controlling membarrier behavior.
*
* This field is close to @pgd to hopefully fit in the same
* cache-line, which needs to be touched by switch_mm().
*/
atomic_t membarrier_state;
#endif
/**
* @mm_users: The number of users including userspace.
*
* Use mmget()/mmget_not_zero()/mmput() to modify. When this
* drops to 0 (i.e. when the task exits and there are no other
* temporary reference holders), we also release a reference on
* @mm_count (which may then free the &struct mm_struct if
* @mm_count also drops to 0).
*/
atomic_t mm_users;
/**
* @mm_count: The number of references to &struct mm_struct
* (@mm_users count as 1).
*
* Use mmgrab()/mmdrop() to modify. When this drops to 0, the
* &struct mm_struct is freed.
*/
atomic_t mm_count;
#ifdef CONFIG_MMU
atomic_long_t pgtables_bytes; /* PTE page table pages */
#endif
int map_count; /* number of VMAs */
spinlock_t page_table_lock; /* Protects page tables and some
* counters
*/
/*
* With some kernel config, the current mmap_lock's offset
* inside 'mm_struct' is at 0x120, which is very optimal, as
* its two hot fields 'count' and 'owner' sit in 2 different
* cachelines, and when mmap_lock is highly contended, both
* of the 2 fields will be accessed frequently, current layout
* will help to reduce cache bouncing.
*
* So please be careful with adding new fields before
* mmap_lock, which can easily push the 2 fields into one
* cacheline.
*/
struct rw_semaphore mmap_lock;
struct list_head mmlist; /* List of maybe swapped mm's. These
* are globally strung together off
* init_mm.mmlist, and are protected
* by mmlist_lock
*/
unsigned long hiwater_rss; /* High-watermark of RSS usage */
unsigned long hiwater_vm; /* High-water virtual memory usage */
unsigned long total_vm; /* Total pages mapped */
unsigned long locked_vm; /* Pages that have PG_mlocked set */
atomic64_t pinned_vm; /* Refcount permanently increased */
unsigned long data_vm; /* VM_WRITE & ~VM_SHARED & ~VM_STACK */
unsigned long exec_vm; /* VM_EXEC & ~VM_WRITE & ~VM_STACK */
unsigned long stack_vm; /* VM_STACK */
unsigned long def_flags;
/**
* @write_protect_seq: Locked when any thread is write
* protecting pages mapped by this mm to enforce a later COW,
* for instance during page table copying for fork().
*/
seqcount_t write_protect_seq;
spinlock_t arg_lock; /* protect the below fields */
unsigned long start_code, end_code, start_data, end_data;
unsigned long start_brk, brk, start_stack;
unsigned long arg_start, arg_end, env_start, env_end;
unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
/*
* Special counters, in some configurations protected by the
* page_table_lock, in other configurations by being atomic.
*/
struct mm_rss_stat rss_stat;
struct linux_binfmt *binfmt;
/* Architecture-specific MM context */
mm_context_t context;
unsigned long flags; /* Must use atomic bitops to access */
#ifdef CONFIG_AIO
spinlock_t ioctx_lock;
struct kioctx_table __rcu *ioctx_table;
#endif
#ifdef CONFIG_MEMCG
/*
* "owner" points to a task that is regarded as the canonical
* user/owner of this mm. All of the following must be true in
* order for it to be changed:
*
* current == mm->owner
* current->mm != mm
* new_owner->mm == mm
* new_owner->alloc_lock is held
*/
struct task_struct __rcu *owner;
#endif
struct user_namespace *user_ns;
/* store ref to file /proc/<pid>/exe symlink points to */
struct file __rcu *exe_file;
#ifdef CONFIG_MMU_NOTIFIER
struct mmu_notifier_subscriptions *notifier_subscriptions;
#endif
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
pgtable_t pmd_huge_pte; /* protected by page_table_lock */
#endif
#ifdef CONFIG_NUMA_BALANCING
/*
* numa_next_scan is the next time that PTEs will be remapped
* PROT_NONE to trigger NUMA hinting faults; such faults gather
* statistics and migrate pages to new nodes if necessary.
*/
unsigned long numa_next_scan;
/* Restart point for scanning and remapping PTEs. */
unsigned long numa_scan_offset;
/* numa_scan_seq prevents two threads remapping PTEs. */
int numa_scan_seq;
#endif
/*
* An operation with batched TLB flushing is going on. Anything
* that can move process memory needs to flush the TLB when
* moving a PROT_NONE mapped page.
*/
atomic_t tlb_flush_pending;
#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
/* See flush_tlb_batched_pending() */
atomic_t tlb_flush_batched;
#endif
struct uprobes_state uprobes_state;
#ifdef CONFIG_PREEMPT_RT
struct rcu_head delayed_drop;
#endif
#ifdef CONFIG_HUGETLB_PAGE
atomic_long_t hugetlb_usage;
#endif
struct work_struct async_put_work;
#ifdef CONFIG_IOMMU_SVA
u32 pasid;
#endif
#ifdef CONFIG_KSM
/*
* Represent how many pages of this process are involved in KSM
* merging.
*/
unsigned long ksm_merging_pages;
/*
* Represent how many pages are checked for ksm merging
* including merged and not merged.
*/
unsigned long ksm_rmap_items;
#endif
#ifdef CONFIG_LRU_GEN
struct {
/* this mm_struct is on lru_gen_mm_list */
struct list_head list;
/*
* Set when switching to this mm_struct, as a hint of
* whether it has been used since the last time per-node
* page table walkers cleared the corresponding bits.
*/
unsigned long bitmap;
#ifdef CONFIG_MEMCG
/* points to the memcg of "owner" above */
struct mem_cgroup *memcg;
#endif
} lru_gen;
#endif /* CONFIG_LRU_GEN */
} __randomize_layout;
/*
* The mm_cpumask needs to be at the end of mm_struct, because it
* is dynamically sized based on nr_cpu_ids.
*/
unsigned long cpu_bitmap[];
};重点关注mm_struct结构中的pgd_t * pgd成员，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），即以下值：
.pgd = swapper_pg_dir,swapper_pg_dir是体系结构相关的，不同架构中其定义位置和值并不相同。
在x86中，swapper_pg_dir是一个宏定义，在arch/x86/include/asm/pgtable_64.h中：
#define swapper_pg_dir init_top_pgtinit_top_gpt的介绍在Documentation/admin-guide/kdump/vmcoreinfo.rst中：
init_top_pgt
------------
Used to walk through the whole page table and convert virtual addresses
to physical addresses. The init_top_pgt is somewhat similar to
swapper_pg_dir, but it is only used in x86_64.那么这个init_to_pgt到底是何方神圣，在它背后又蕴藏了哪些知识呢？且看下回分解。