__init与__init_data、__exit、__exitdata段意义

 Linux在arch/$(ARCH)/kernel/vmlinux.lds中定义了.init段。__init和__initdata属性的数据都在这个段中，当内核启动完毕后，这个段中的内存会被释放掉供其他使用。

__init和__initdata宏定义如下：
/* include/linux/init.c */

#define __init __attribute__ ((__section__ (".init.text")))#define __initdata __attribute__ ((__section__ (".init.data")))

vmlinux.lds内容如下：
/* arch/arm/kernel/vmlinux.lds */

/* ld script to make ARM Linux kernel * taken from the i386 version by Russell King * Written by Martin Mares <[e-mail]> */#include <asm-generic/vmlinux.lds.h>#include <asm/thread_info.h>#include <asm/memory.h>    OUTPUT_ARCH(arm)ENTRY(stext)#ifndef __ARMEB__jiffies = jiffies_64;#elsejiffies = jiffies_64 + 4;#endifSECTIONS{#ifdef CONFIG_XIP_KERNEL    . = XIP_VIRT_ADDR(CONFIG_XIP_PHYS_ADDR);#else    . = PAGE_OFFSET + TEXT_OFFSET;#endif    .text.head : {        _stext = .;        _sinittext = .;        *(.text.head)    }    .init : {            /* Init code and data        */            INIT_TEXT        _einittext = .;        __proc_info_begin = .;            *(.proc.info.init)        __proc_info_end = .;        __arch_info_begin = .;            *(.arch.info.init)        __arch_info_end = .;        __tagtable_begin = .;            *(.taglist.init)        __tagtable_end = .;        . = ALIGN(16);        __setup_start = .;            *(.init.setup)        __setup_end = .;        __early_begin = .;            *(.early_param.init)        __early_end = .;        __initcall_start = .;            INITCALLS        __initcall_end = .;        __con_initcall_start = .;            *(.con_initcall.init)        __con_initcall_end = .;        __security_initcall_start = .;            *(.security_initcall.init)        __security_initcall_end = .;#ifdef CONFIG_BLK_DEV_INITRD        . = ALIGN(32);        __initramfs_start = .;            usr/built-in.o(.init.ramfs)        __initramfs_end = .;#endif        . = ALIGN(4096);        __per_cpu_load = .;        __per_cpu_start = .;            *(.data.percpu.page_aligned)            *(.data.percpu)            *(.data.percpu.shared_aligned)        __per_cpu_end = .;#ifndef CONFIG_XIP_KERNEL        __init_begin = _stext;        INIT_DATA        . = ALIGN(4096);        __init_end = .;#endif    }    /DISCARD/ : {            /* Exit code and data        */        EXIT_TEXT        EXIT_DATA        *(.exitcall.exit)        *(.ARM.exidx.exit.text)        *(.ARM.extab.exit.text)#ifndef CONFIG_MMU        *(.fixup)        *(__ex_table)#endif    }    .text : {            /* Real text segment        */        _text = .;        /* Text and read-only data    */            __exception_text_start = .;            *(.exception.text)            __exception_text_end = .;            TEXT_TEXT            SCHED_TEXT            LOCK_TEXT            KPROBES_TEXT#ifdef CONFIG_MMU            *(.fixup)#endif            *(.gnu.warning)            *(.rodata)            *(.rodata.*)            *(.glue_7)            *(.glue_7t)        *(.got)            /* Global offset table        */    }    RODATA    _etext = .;            /* End of text and rodata section */#ifdef CONFIG_ARM_UNWIND    /*     * Stack unwinding tables     */    . = ALIGN(8);    .ARM.unwind_idx : {        __start_unwind_idx = .;        *(.ARM.exidx*)        __stop_unwind_idx = .;    }    .ARM.unwind_tab : {        __start_unwind_tab = .;        *(.ARM.extab*)        __stop_unwind_tab = .;    }#endif#ifdef CONFIG_XIP_KERNEL    __data_loc = ALIGN(4);        /* location in binary */    . = PAGE_OFFSET + TEXT_OFFSET;#else    . = ALIGN(THREAD_SIZE);    __data_loc = .;#endif    .data : AT(__data_loc) {        _data = .;        /* address in memory */        /*         * first, the init task union, aligned         * to an 8192 byte boundary.         */        *(.data.init_task)#ifdef CONFIG_XIP_KERNEL        . = ALIGN(4096);        __init_begin = .;        INIT_DATA        . = ALIGN(4096);        __init_end = .;#endif        . = ALIGN(4096);        __nosave_begin = .;        *(.data.nosave)        . = ALIGN(4096);        __nosave_end = .;        /*         * then the cacheline aligned data         */        . = ALIGN(32);        *(.data.cacheline_aligned)        /*         * The exception fixup table (might need resorting at runtime)         */        . = ALIGN(32);        __start___ex_table = .;#ifdef CONFIG_MMU        *(__ex_table)#endif        __stop___ex_table = .;        /*         * and the usual data section         */        DATA_DATA        CONSTRUCTORS        _edata = .;    }    _edata_loc = __data_loc + SIZEOF(.data);    .bss : {        __bss_start = .;    /* BSS                */        *(.bss)        *(COMMON)        _end = .;    }                    /* Stabs debugging sections.    */    .stab 0 : { *(.stab) }    .stabstr 0 : { *(.stabstr) }    .stab.excl 0 : { *(.stab.excl) }    .stab.exclstr 0 : { *(.stab.exclstr) }    .stab.index 0 : { *(.stab.index) }    .stab.indexstr 0 : { *(.stab.indexstr) }    .comment 0 : { *(.comment) }}/* * These must never be empty * If you have to comment these two assert statements out, your * binutils is too old (for other reasons as well) */ASSERT((__proc_info_end - __proc_info_begin), "missing CPU support")ASSERT((__arch_info_end - __arch_info_begin), "no machine record defined")

可以发现__init对应的section(.init.text)和__initdata对应的section(.init.data)都在.init段中。同样,这里定义的其他一些section也都会在使用完后被释放，如.init.setup，.initcall1.init等。
释放memory的大小会在系统启动过程中打印出来：

eth0: link up

IP-Config: Complete:

device=eth0, addr=192.168.167.15, mask=255.255.255.0, gw=192.168.167.254,

host=192.168.167.15, domain=, nis-domain=(none),

bootserver=192.168.167.170, rootserver=192.168.167.170, rootpath=

Looking up port of RPC 100003/2 on 192.168.167.170

Looking up port of RPC 100005/1 on 192.168.167.170

VFS: Mounted root (nfs filesystem).

Freeing init memory: 128K