switch_mm中是进行页表的切换，即将下一个的pgd的开始物理地址放入CP15中的C2寄存器。进程的pgd的虚拟地址存放在task_struct结构中的pgd指针中，通过__virt_to_phys宏可以转变成成物理地址。
static inline void
switch_mm(struct mm_struct *prev, struct mm_struct *next,
struct task_struct *tsk, unsigned int cpu)
{
if (prev != next)
cpu_switch_mm(next->pgd, tsk);
}
#define cpu_switch_mm(pgd,tsk) cpu_set_pgd(__virt_to_phys((unsigned long)(pgd)))
#define cpu_get_pgd() ({ unsigned long pg; __asm__("mrc p15, 0, %0, c2, c0, 0" : "=r" (pg)); pg &= ~0x3fff; (pgd_t *)phys_to_virt(pg); })
switch_to()完成进程上下文的切换，通过调用汇编函数__switch_to完成，其实现比较简单，也就是保存prev进程的上下文信息，该上下文信息由context_save_struct结构描述，包括主要的寄存器，然后将next的上下文信息读出，信息保存在task_struct中的thread.save中TSS_SAVE标识了thread.save在task_struct中的位置。
/*
* Register switch for ARMv3 and ARMv4 processors
* r0 = previous, r1 = next, return previous.
* previous and next are guaranteed not to be the same.
*/
ENTRY(__switch_to)
stmfd sp!, {r4 - sl, fp, lr} @ Store most regs on stack
mrs ip, cpsr
str ip, [sp, #-4]! @ Save cpsr_SVC
str sp, [r0, #TSS_SAVE] @ Save sp_SVC
ldr sp, [r1, #TSS_SAVE] @ Get saved sp_SVC
ldr r2, [r1, #TSS_DOMAIN]
ldr ip, [sp], #4
mcr p15, 0, r2, c3, c0 @ Set domain register
msr spsr, ip @ Save tasks CPSR into SPSR for this return
ldmfd sp!, {r4 - sl, fp, pc}^ @ Load all regs saved previously
struct context_save_struct {
unsigned long cpsr;
unsigned long r4;
unsigned long r5;
unsigned long r6;
unsigned long r7;
unsigned long r8;
unsigned long r9;
unsigned long sl;
unsigned long fp;
unsigned long pc;
};
