ext2檔案系統super.c原始碼分析(Linux 2.6.24)

清風不及你@發表於2020-11-10

關於linux2.6.24版本的ext2檔案系統的原始碼,super.c也是第三大的檔案,歡迎指正!

/*
 *  linux/fs/ext2/super.c
 *
 * Copyright (C) 1992, 1993, 1994, 1995
 * Remy Card (card@masi.ibp.fr)
 * Laboratoire MASI - Institut Blaise Pascal
 * Universite Pierre et Marie Curie (Paris VI)
 *
 *  from
 *
 *  linux/fs/minix/inode.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *
 *  Big-endian to little-endian byte-swapping/bitmaps by
 *        David S. Miller (davem@caip.rutgers.edu), 1995
 */

#include <linux/module.h>
#include <linux/string.h>
#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/blkdev.h>
#include <linux/parser.h>
#include <linux/random.h>
#include <linux/buffer_head.h>
#include <linux/exportfs.h>
#include <linux/smp_lock.h>
#include <linux/vfs.h>
#include <linux/seq_file.h>
#include <linux/mount.h>
#include <linux/log2.h>
#include <asm/uaccess.h>
#include "ext2.h"
#include "xattr.h"
#include "acl.h"
#include "xip.h"
/*函式宣告,超級塊的同步函式,重新掛載檔案系統的函式以及得到檔案系統狀態的函式*/
static void ext2_sync_super(struct super_block *sb,
			    struct ext2_super_block *es);
static int ext2_remount (struct super_block * sb, int * flags, char * data);
static int ext2_statfs (struct dentry * dentry, struct kstatfs * buf);
/*ext2的列印錯誤資訊的函式,引數有可變引數*/
void ext2_error (struct super_block * sb, const char * function,
		 const char * fmt, ...)
{
	va_list args;
	/*記憶體裡存放的超級塊資訊結構體*/
	struct ext2_sb_info *sbi = EXT2_SB(sb);
	/*ext2硬碟上存放的超級塊,此時在塊快取中*/
	struct ext2_super_block *es = sbi->s_es;
	/*如果檔案系統不是隻讀的*/
	if (!(sb->s_flags & MS_RDONLY)) {
		/*出錯了,就在ext2_sb_info結構體的對應位上或上對應位,標記檔案系統有錯誤發生*/
		sbi->s_mount_state |= EXT2_ERROR_FS;
		/*ext2_super_block結構體上對應位置位,標識檔案系統有錯誤發生*/
		es->s_state =
			cpu_to_le16(le16_to_cpu(es->s_state) | EXT2_ERROR_FS);
	/*超級塊立即同步*/
		ext2_sync_super(sb, es);
	}
	/*列印出錯的資訊*/
	va_start(args, fmt);
	printk(KERN_CRIT "EXT2-fs error (device %s): %s: ",sb->s_id, function);
	vprintk(fmt, args);
	printk("\n");
	va_end(args);
	/*如果是嚴重錯誤,列印相應的資訊*/
	if (test_opt(sb, ERRORS_PANIC))
		panic("EXT2-fs panic from previous error\n");
	/*如果是隻讀錯誤,就列印應當把檔案系統重新只讀掛載*/
	if (test_opt(sb, ERRORS_RO)) {
		printk("Remounting filesystem read-only\n");
		/*超級塊對應位設定為只讀*/
		sb->s_flags |= MS_RDONLY;
	}
}
	/*列印警告資訊,錯誤不是很嚴重*/
void ext2_warning (struct super_block * sb, const char * function,
		   const char * fmt, ...)
{
	va_list args;
	/*僅僅是簡單的列印資訊*/
	va_start(args, fmt);
	printk(KERN_WARNING "EXT2-fs warning (device %s): %s: ",
	       sb->s_id, function);
	vprintk(fmt, args);
	printk("\n");
	va_end(args);
}
/*更新檔案系統為動態的inode的修改級別*/
void ext2_update_dynamic_rev(struct super_block *sb)
{
	struct ext2_super_block *es = EXT2_SB(sb)->s_es;
	/*如果檔案系統的修改級別已經是新的了,直接返回*/
	if (le32_to_cpu(es->s_rev_level) > EXT2_GOOD_OLD_REV)
		return;
	/*如果檔案系統不是新的修改級別,先警告,我們要修改了*/
	ext2_warning(sb, __FUNCTION__,
		     "updating to rev %d because of new feature flag, "
		     "running e2fsck is recommended",
		     EXT2_DYNAMIC_REV);
	/*設定第一個inode號,inode大小*/
	es->s_first_ino = cpu_to_le32(EXT2_GOOD_OLD_FIRST_INO);
	es->s_inode_size = cpu_to_le16(EXT2_GOOD_OLD_INODE_SIZE);
	/*新的修改級別*/
	es->s_rev_level = cpu_to_le32(EXT2_DYNAMIC_REV);
	/* 此處不處理 es->s_feature_*compat*/
	/* es->s_uuid會被e2fsck處理 */
 
 
	/* 超級塊的其他欄位應該是0,如果不是,說明他們正在被使用,我們可以接下來讓e2fsck來解決*/
	/* leave es->s_feature_*compat flags alone */
	/* es->s_uuid will be set by e2fsck if empty */

	/*
	 * The rest of the superblock fields should be zero, and if not it
	 * means they are likely already in use, so leave them alone.  We
	 * can leave it up to e2fsck to clean up any inconsistencies there.
	 */
}
/*釋放正在使用的超級塊*/
static void ext2_put_super (struct super_block * sb)
{
	int db_count;
	int i;
	struct ext2_sb_info *sbi = EXT2_SB(sb);
	/*釋放這個超級塊的塊裝置的所有在記憶體裡的緩衝區*/
	ext2_xattr_put_super(sb);
	/*如果檔案系統不是隻讀掛載的*/
	if (!(sb->s_flags & MS_RDONLY)) {
		struct ext2_super_block *es = sbi->s_es;
		/*同步檔案系統的掛載狀態*/
		es->s_state = cpu_to_le16(sbi->s_mount_state);
		ext2_sync_super(sb, es);
	}
	/*檔案系統的儲存組描述符塊的數目,不是組描述符的個數*/
	db_count = sbi->s_gdb_count;
	/*遍歷所有的塊,釋放所有的組描述符的緩衝區*/
	for (i = 0; i < db_count; i++)
		if (sbi->s_group_desc[i])
			brelse (sbi->s_group_desc[i]);
		/*釋放組描述符陣列和檔案系統負載陣列*/
	kfree(sbi->s_group_desc);
	kfree(sbi->s_debts);
	/*釋放者三個結構體*/
	percpu_counter_destroy(&sbi->s_freeblocks_counter);
	percpu_counter_destroy(&sbi->s_freeinodes_counter);
	percpu_counter_destroy(&sbi->s_dirs_counter);
	brelse (sbi->s_sbh);
	sb->s_fs_info = NULL;
	/*釋放ext2_sb_info結構體*/
	kfree(sbi);

	return;
}
/*用來存放ext2的inode的slab緩衝區*/
static struct kmem_cache * ext2_inode_cachep;
	/*ext2分配一個空的inode,先分配ext2_inode_info,再返回其中的inode物件*/
static struct inode *ext2_alloc_inode(struct super_block *sb)
{
	struct ext2_inode_info *ei;
	/*先從ext2_inode_cachep上分配記憶體對應的ext2_inode_info結構體*/
	ei = (struct ext2_inode_info *)kmem_cache_alloc(ext2_inode_cachep, GFP_KERNEL);
	if (!ei)
		return NULL;
	/*配置了acl,先初始化acl為沒有對映狀態*/
#ifdef CONFIG_EXT2_FS_POSIX_ACL
	ei->i_acl = EXT2_ACL_NOT_CACHED;
	ei->i_default_acl = EXT2_ACL_NOT_CACHED;
#endif/*版本號*/
	ei->i_block_alloc_info = NULL;
	ei->vfs_inode.i_version = 1;
	return &ei->vfs_inode;
}
	/*釋放一個inode磁碟記憶體結構*/
static void ext2_destroy_inode(struct inode *inode)
{	/*呼叫記憶體的緩衝釋放函式,從ext2_inode_cachep上釋放*/
	kmem_cache_free(ext2_inode_cachep, EXT2_I(inode));
}
/*初始化ext2_inode_info的指標,foo為ext2_inode_info*/
static void init_once(struct kmem_cache * cachep, void *foo)
{	/*先轉化獲得ext2_inode_info指標*/
	struct ext2_inode_info *ei = (struct ext2_inode_info *) foo;
	/*逐個初始化讀寫鎖,屬性鎖*/
	rwlock_init(&ei->i_meta_lock);
#ifdef CONFIG_EXT2_FS_XATTR
	init_rwsem(&ei->xattr_sem);
#endif
	/*初始化vfs層的inode*/
	mutex_init(&ei->truncate_mutex);
	inode_init_once(&ei->vfs_inode);
}
	/*初始化ext2_inode_cachep的緩衝區*/
static int init_inodecache(void)
{
	/*建立一塊可延展的記憶體緩衝區,並使用init_once函式初始化,
	然後賦值給ext2_inode_cachep,kmem_cache_create是記憶體子系統的函式,
	第一個引數是這個緩衝區的名字,第二個引數是緩衝區的大小,先設定為一個ext2_inode_info的大小,
	第三個引數是對齊,第四個引數flag,標誌這緩衝區可延展,就是可以向後繼續分配,並且可以重新設定大小,
	第五個引數是初始化函式*/

	ext2_inode_cachep = kmem_cache_create("ext2_inode_cache",
					     sizeof(struct ext2_inode_info),
					     0, (SLAB_RECLAIM_ACCOUNT|
						SLAB_MEM_SPREAD),
					     init_once);
	/*失敗了,就返回記憶體不夠的錯誤*/
	if (ext2_inode_cachep == NULL)
		return -ENOMEM;
	return 0;
}
	/*銷燬上個函式建立的ext2_inode_cachep*/
static void destroy_inodecache(void)
{
	/*記憶體子系統的記憶體銷燬函式*/
	kmem_cache_destroy(ext2_inode_cachep);
}

/*清理inode的額外的記憶體資源,一般是指acl結構體*/
static void ext2_clear_inode(struct inode *inode)
{
	struct ext2_block_alloc_info *rsv = EXT2_I(inode)->i_block_alloc_info;
	/*如果沒有配置acl,就不用釋放了*/
#ifdef CONFIG_EXT2_FS_POSIX_ACL
	struct ext2_inode_info *ei = EXT2_I(inode);
	/*如果i_acl不是未對映狀態就說明需要釋放*/
	if (ei->i_acl && ei->i_acl != EXT2_ACL_NOT_CACHED) {
		/*之前講過的函式,釋放acl結構體*/
		posix_acl_release(ei->i_acl);
		/*標記為未對映*/
		ei->i_acl = EXT2_ACL_NOT_CACHED;
	}
	/*如果i_default_acl不是未對映狀態就說明需要釋放*/
	if (ei->i_default_acl && ei->i_default_acl != EXT2_ACL_NOT_CACHED) {
	/*之前講過的函式,釋放acl結構體*/
		posix_acl_release(ei->i_default_acl);
	/*標記為未對映*/
		ei->i_default_acl = EXT2_ACL_NOT_CACHED;
	}
#endif
	ext2_discard_reservation(inode);
	EXT2_I(inode)->i_block_alloc_info = NULL;
	if (unlikely(rsv))
		kfree(rsv);
}
/*得到檔案系統的掛載選項,存放在seq裡*/
static int ext2_show_options(struct seq_file *seq, struct vfsmount *vfs)
{
	/*先得到超級塊的記憶體的資訊結構體,大部分資訊都在這個結構體裡放著*/
	struct super_block *sb = vfs->mnt_sb;
	struct ext2_sb_info *sbi = EXT2_SB(sb);
	struct ext2_super_block *es = sbi->s_es;
	unsigned long def_mount_opts;

	def_mount_opts = le32_to_cpu(es->s_default_mount_opts);

	if (sbi->s_sb_block != 1)
		seq_printf(seq, ",sb=%lu", sbi->s_sb_block);
	if (test_opt(sb, MINIX_DF))
		seq_puts(seq, ",minixdf");
	if (test_opt(sb, GRPID))
		seq_puts(seq, ",grpid");
	if (!test_opt(sb, GRPID) && (def_mount_opts & EXT2_DEFM_BSDGROUPS))
		seq_puts(seq, ",nogrpid");
	if (sbi->s_resuid != EXT2_DEF_RESUID ||
	    le16_to_cpu(es->s_def_resuid) != EXT2_DEF_RESUID) {
		seq_printf(seq, ",resuid=%u", sbi->s_resuid);
	}
	if (sbi->s_resgid != EXT2_DEF_RESGID ||
	    le16_to_cpu(es->s_def_resgid) != EXT2_DEF_RESGID) {
		seq_printf(seq, ",resgid=%u", sbi->s_resgid);
	}
	if (test_opt(sb, ERRORS_CONT)) {
		int def_errors = le16_to_cpu(es->s_errors);

		if (def_errors == EXT2_ERRORS_PANIC ||
		    def_errors == EXT2_ERRORS_RO) {
			seq_puts(seq, ",errors=continue");
		}
	}
	if (test_opt(sb, ERRORS_RO))
		seq_puts(seq, ",errors=remount-ro");
	if (test_opt(sb, ERRORS_PANIC))
		seq_puts(seq, ",errors=panic");
	if (test_opt(sb, NO_UID32))
		seq_puts(seq, ",nouid32");
	if (test_opt(sb, DEBUG))
		seq_puts(seq, ",debug");
	if (test_opt(sb, OLDALLOC))
		seq_puts(seq, ",oldalloc");

#ifdef CONFIG_EXT2_FS_XATTR
	if (test_opt(sb, XATTR_USER))
		seq_puts(seq, ",user_xattr");
	if (!test_opt(sb, XATTR_USER) &&
	    (def_mount_opts & EXT2_DEFM_XATTR_USER)) {
		seq_puts(seq, ",nouser_xattr");
	}
#endif

#ifdef CONFIG_EXT2_FS_POSIX_ACL
	if (test_opt(sb, POSIX_ACL))
		seq_puts(seq, ",acl");
	if (!test_opt(sb, POSIX_ACL) && (def_mount_opts & EXT2_DEFM_ACL))
		seq_puts(seq, ",noacl");
#endif

	if (test_opt(sb, NOBH))
		seq_puts(seq, ",nobh");

#if defined(CONFIG_QUOTA)
	if (sbi->s_mount_opt & EXT2_MOUNT_USRQUOTA)
		seq_puts(seq, ",usrquota");

	if (sbi->s_mount_opt & EXT2_MOUNT_GRPQUOTA)
		seq_puts(seq, ",grpquota");
#endif

#if defined(CONFIG_EXT2_FS_XIP)
	if (sbi->s_mount_opt & EXT2_MOUNT_XIP)
		seq_puts(seq, ",xip");
#endif

	return 0;
}
/*配額的操作函式*/
#ifdef CONFIG_QUOTA
static ssize_t ext2_quota_read(struct super_block *sb, int type, char *data, size_t len, loff_t off);
static ssize_t ext2_quota_write(struct super_block *sb, int type, const char *data, size_t len, loff_t off);
#endif
/*超級塊的操作函式集合*/
static const struct super_operations ext2_sops = {
	.alloc_inode	= ext2_alloc_inode,
	.destroy_inode	= ext2_destroy_inode,
	.read_inode	= ext2_read_inode,
	.write_inode	= ext2_write_inode,
	.delete_inode	= ext2_delete_inode,
	.put_super	= ext2_put_super,
	.write_super	= ext2_write_super,
	.statfs		= ext2_statfs,
	.remount_fs	= ext2_remount,
	.clear_inode	= ext2_clear_inode,
	.show_options	= ext2_show_options,
#ifdef CONFIG_QUOTA
	.quota_read	= ext2_quota_read,
	.quota_write	= ext2_quota_write,
#endif
};
/*用在nfs和ext2交流上,sb是ext2的超級塊,vobjp是一個記憶體塊,第一個是inode號碼,第二個是ext2版本號,返回檔案的dentry結構體*/
static struct inode *ext2_nfs_get_inode(struct super_block *sb,
		u64 ino, u32 generation)
{
	struct inode *inode;
	if (ino < EXT2_FIRST_INO(sb) && ino != EXT2_ROOT_INO)
		return ERR_PTR(-ESTALE);
	if (ino > le32_to_cpu(EXT2_SB(sb)->s_es->s_inodes_count))
		return ERR_PTR(-ESTALE);

	/* iget isn't really right if the inode is currently unallocated!!
	 * ext2_read_inode currently does appropriate checks, but
	 * it might be "neater" to call ext2_get_inode first and check
	 * if the inode is valid.....
	 */
	inode = iget(sb, ino);
	if (inode == NULL)
		return ERR_PTR(-ENOMEM);
	if (is_bad_inode(inode) ||
	    (generation && inode->i_generation != generation)) {
		/* we didn't find the right inode.. */
		iput(inode);
		return ERR_PTR(-ESTALE);
	}
	return inode;
}

static struct dentry *ext2_fh_to_dentry(struct super_block *sb, struct fid *fid,
		int fh_len, int fh_type)
{
	return generic_fh_to_dentry(sb, fid, fh_len, fh_type,
				    ext2_nfs_get_inode);
}

static struct dentry *ext2_fh_to_parent(struct super_block *sb, struct fid *fid,
		int fh_len, int fh_type)
{
	return generic_fh_to_parent(sb, fid, fh_len, fh_type,
				    ext2_nfs_get_inode);
}

/* Yes, most of these are left as NULL!!
 * A NULL value implies the default, which works with ext2-like file
 * systems, but can be improved upon.
 * Currently only get_parent is required.
 *//* nfs檔案系統與作業系統進行交流的操作函式集合 */
static const struct export_operations ext2_export_ops = {
	.fh_to_dentry = ext2_fh_to_dentry,
	.fh_to_parent = ext2_fh_to_parent,
	.get_parent = ext2_get_parent,
};
 /*獲得超級塊塊號,引數data是掛載檔案系統的選項*/
static unsigned long get_sb_block(void **data)
{
	unsigned long 	sb_block;
	 /*先獲得選項字串*/
	char 		*options = (char *) *data;
 /*檢驗字串是不是sb=開頭的,不是就說明錯了*/
	if (!options || strncmp(options, "sb=", 3) != 0)
		return 1;	/* Default location */
	options += 3;/*繼續看後邊的*/
	/*sb_block是從引數得到超級塊塊號*/
	sb_block = simple_strtoul(options, &options, 0);
	/*後邊沒了,也不對*/
	if (*options && *options != ',') {
		printk("EXT2-fs: Invalid sb specification: %s\n",
		       (char *) *data);
		return 1;
	}
	if (*options == ',')
		options++;
	*data = (void *) options;
	return sb_block;
}
/*檔案系統的各種選項*/
enum {
	Opt_bsd_df, Opt_minix_df, Opt_grpid, Opt_nogrpid,
	Opt_resgid, Opt_resuid, Opt_sb, Opt_err_cont, Opt_err_panic,
	Opt_err_ro, Opt_nouid32, Opt_nocheck, Opt_debug,
	Opt_oldalloc, Opt_orlov, Opt_nobh, Opt_user_xattr, Opt_nouser_xattr,
	Opt_acl, Opt_noacl, Opt_xip, Opt_ignore, Opt_err, Opt_quota,
	Opt_usrquota, Opt_grpquota, Opt_reservation, Opt_noreservation
};
	/*檔案系統的選項和對應的引數字串*/
static match_table_t tokens = {
	{Opt_bsd_df, "bsddf"},
	{Opt_minix_df, "minixdf"},
	{Opt_grpid, "grpid"},
	{Opt_grpid, "bsdgroups"},
	{Opt_nogrpid, "nogrpid"},
	{Opt_nogrpid, "sysvgroups"},
	{Opt_resgid, "resgid=%u"},
	{Opt_resuid, "resuid=%u"},
	{Opt_sb, "sb=%u"},
	{Opt_err_cont, "errors=continue"},
	{Opt_err_panic, "errors=panic"},
	{Opt_err_ro, "errors=remount-ro"},
	{Opt_nouid32, "nouid32"},
	{Opt_nocheck, "check=none"},
	{Opt_nocheck, "nocheck"},
	{Opt_debug, "debug"},
	{Opt_oldalloc, "oldalloc"},
	{Opt_orlov, "orlov"},
	{Opt_nobh, "nobh"},
	{Opt_user_xattr, "user_xattr"},
	{Opt_nouser_xattr, "nouser_xattr"},
	{Opt_acl, "acl"},
	{Opt_noacl, "noacl"},
	{Opt_xip, "xip"},
	{Opt_grpquota, "grpquota"},
	{Opt_ignore, "noquota"},
	{Opt_quota, "quota"},
	{Opt_usrquota, "usrquota"},
	{Opt_reservation, "reservation"},
	{Opt_noreservation, "noreservation"},
	{Opt_err, NULL}
};
 /*轉化引數字串,變成ext2_sb_info結構體上相應的位*/
static int parse_options (char * options,
			  struct ext2_sb_info *sbi)
{
	char * p;
	substring_t args[MAX_OPT_ARGS];
	int option;
	/*字串不可以是NULL*/
	if (!options)
		return 1;
	/*大迴圈,一個一個轉換字串為ext2_sb_info的flag,每次讀取逗號之間的*/
	while ((p = strsep (&options, ",")) != NULL) {
		int token;
		/*空的,讀下一個*/
		if (!*p)
			continue;
		/*在tokens陣列裡尋找和p指標指向的字串選項匹配的選項*/
		token = match_token(p, tokens, args);
		/*在ext2_sb_info的相應位上設定*/
		switch (token) {
		case Opt_bsd_df:
			clear_opt (sbi->s_mount_opt, MINIX_DF);
			break;
		case Opt_minix_df:
			set_opt (sbi->s_mount_opt, MINIX_DF);
			break;
		case Opt_grpid:
			set_opt (sbi->s_mount_opt, GRPID);
			break;
		case Opt_nogrpid:
			clear_opt (sbi->s_mount_opt, GRPID);
			break;
		case Opt_resuid:
			if (match_int(&args[0], &option))
				return 0;
			sbi->s_resuid = option;
			break;
		case Opt_resgid:
			if (match_int(&args[0], &option))
				return 0;
			sbi->s_resgid = option;
			break;
		case Opt_sb:
			/* handled by get_sb_block() instead of here */
			/* *sb_block = match_int(&args[0]); */
			break;
		case Opt_err_panic:
			clear_opt (sbi->s_mount_opt, ERRORS_CONT);
			clear_opt (sbi->s_mount_opt, ERRORS_RO);
			set_opt (sbi->s_mount_opt, ERRORS_PANIC);
			break;
		case Opt_err_ro:
			clear_opt (sbi->s_mount_opt, ERRORS_CONT);
			clear_opt (sbi->s_mount_opt, ERRORS_PANIC);
			set_opt (sbi->s_mount_opt, ERRORS_RO);
			break;
		case Opt_err_cont:
			clear_opt (sbi->s_mount_opt, ERRORS_RO);
			clear_opt (sbi->s_mount_opt, ERRORS_PANIC);
			set_opt (sbi->s_mount_opt, ERRORS_CONT);
			break;
		case Opt_nouid32:
			set_opt (sbi->s_mount_opt, NO_UID32);
			break;
		case Opt_nocheck:
			clear_opt (sbi->s_mount_opt, CHECK);
			break;
		case Opt_debug:
			set_opt (sbi->s_mount_opt, DEBUG);
			break;
		case Opt_oldalloc:
			set_opt (sbi->s_mount_opt, OLDALLOC);
			break;
		case Opt_orlov:
			clear_opt (sbi->s_mount_opt, OLDALLOC);
			break;
		case Opt_nobh:
			set_opt (sbi->s_mount_opt, NOBH);
			break;
#ifdef CONFIG_EXT2_FS_XATTR
		case Opt_user_xattr:
			set_opt (sbi->s_mount_opt, XATTR_USER);
			break;
		case Opt_nouser_xattr:
			clear_opt (sbi->s_mount_opt, XATTR_USER);
			break;
#else
		case Opt_user_xattr:
		case Opt_nouser_xattr:
			printk("EXT2 (no)user_xattr options not supported\n");
			break;
#endif
#ifdef CONFIG_EXT2_FS_POSIX_ACL
		case Opt_acl:
			set_opt(sbi->s_mount_opt, POSIX_ACL);
			break;
		case Opt_noacl:
			clear_opt(sbi->s_mount_opt, POSIX_ACL);
			break;
#else
		case Opt_acl:
		case Opt_noacl:
			printk("EXT2 (no)acl options not supported\n");
			break;
#endif
		case Opt_xip:
#ifdef CONFIG_EXT2_FS_XIP
			set_opt (sbi->s_mount_opt, XIP);
#else
			printk("EXT2 xip option not supported\n");
#endif
			break;

#if defined(CONFIG_QUOTA)
		case Opt_quota:
		case Opt_usrquota:
			set_opt(sbi->s_mount_opt, USRQUOTA);
			break;

		case Opt_grpquota:
			set_opt(sbi->s_mount_opt, GRPQUOTA);
			break;
#else
		case Opt_quota:
		case Opt_usrquota:
		case Opt_grpquota:
			printk(KERN_ERR
				"EXT2-fs: quota operations not supported.\n");

			break;
#endif

		case Opt_reservation:
			set_opt(sbi->s_mount_opt, RESERVATION);
			printk("reservations ON\n");
			break;
		case Opt_noreservation:
			clear_opt(sbi->s_mount_opt, RESERVATION);
			printk("reservations OFF\n");
			break;
		case Opt_ignore:
			break;
		default:
			return 0;
		}
	}
	return 1;
}
/*ext2的初始化超級塊,read_only標誌檔案系統是不是隻讀掛載*/
static int ext2_setup_super (struct super_block * sb,
			      struct ext2_super_block * es,
			      int read_only)
{
	int res = 0;
	struct ext2_sb_info *sbi = EXT2_SB(sb);
	/*檢查修改級別的引數是不是合法*/
	if (le32_to_cpu(es->s_rev_level) > EXT2_MAX_SUPP_REV) {
		printk ("EXT2-fs warning: revision level too high, "
			"forcing read-only mode\n");
		res = MS_RDONLY;
	}
	/*只讀掛載就不用繼續了*/
	if (read_only)
		return res;
	/*合法位沒有置位,說明檔案系統沒有經過檢查,應當檢查一下*/
	if (!(sbi->s_mount_state & EXT2_VALID_FS))
		printk ("EXT2-fs warning: mounting unchecked fs, "
			"running e2fsck is recommended\n");
	/*如果掛在的時候有錯誤,列印資訊*/
	else if ((sbi->s_mount_state & EXT2_ERROR_FS))
		printk ("EXT2-fs warning: mounting fs with errors, "
			"running e2fsck is recommended\n");
	/*檔案系統最大掛載次數大於等於0,並且當前已經掛載超過最大次數了,列印警告資訊*/
	else if ((__s16) le16_to_cpu(es->s_max_mnt_count) >= 0 &&
		 le16_to_cpu(es->s_mnt_count) >=
		 (unsigned short) (__s16) le16_to_cpu(es->s_max_mnt_count))
		printk ("EXT2-fs warning: maximal mount count reached, "
			"running e2fsck is recommended\n");
	/*檢查是不是很長時間沒有進行檢查了,如果是,列印資訊*/
	else if (le32_to_cpu(es->s_checkinterval) &&
		(le32_to_cpu(es->s_lastcheck) + le32_to_cpu(es->s_checkinterval) <= get_seconds()))
		printk ("EXT2-fs warning: checktime reached, "
			"running e2fsck is recommended\n");
	/*如果檔案系統的最大掛載次數字段為0,設定這個變數為預設的EXT2_DFL_MAX_MNT_COUNT*/
	if (!le16_to_cpu(es->s_max_mnt_count))
		es->s_max_mnt_count = cpu_to_le16(EXT2_DFL_MAX_MNT_COUNT);
	/*當前掛載的次數加一*/
	es->s_mnt_count=cpu_to_le16(le16_to_cpu(es->s_mnt_count) + 1);
	ext2_write_super(sb);/*把這個超級塊寫入到裝置上*/
	if (test_opt (sb, DEBUG))/*除錯資訊的列印*/
		printk ("[EXT II FS %s, %s, bs=%lu, fs=%lu, gc=%lu, "
			"bpg=%lu, ipg=%lu, mo=%04lx]\n",
			EXT2FS_VERSION, EXT2FS_DATE, sb->s_blocksize,
			sbi->s_frag_size,
			sbi->s_groups_count,
			EXT2_BLOCKS_PER_GROUP(sb),
			EXT2_INODES_PER_GROUP(sb),
			sbi->s_mount_opt);
	return res;
}
/*ext2檔案系統檢查組描述符*/
static int ext2_check_descriptors (struct super_block * sb)
{
	int i;
	int desc_block = 0;//目前是第幾塊組描述符
	struct ext2_sb_info *sbi = EXT2_SB(sb);
	/*第一個資料塊塊號*/
	unsigned long first_block = le32_to_cpu(sbi->s_es->s_first_data_block);
	unsigned long last_block;
	struct ext2_group_desc * gdp = NULL;//組描述符的緩衝塊

	ext2_debug ("Checking group descriptors");
	/*遍歷每一個組*/
	for (i = 0; i < sbi->s_groups_count; i++)
	{	/*判斷當前遍歷是不是到了最後一個組*/
		if (i == sbi->s_groups_count - 1)
			/*如果到了最後一個組,last_block指向最後一個塊,因為最後一組可能由於邊界的原因不是和之前的組有一樣的塊數目*/
			last_block = le32_to_cpu(sbi->s_es->s_blocks_count) - 1;
		else
			/*last_block是本組最後一個塊的塊號*/
			last_block = first_block +
				(EXT2_BLOCKS_PER_GROUP(sb) - 1);
/*組描述符是放在連續的塊上的,一個塊有若干個描述符,如果當前組過了一個塊,就使得gdp新的描述符塊緩衝區,因為每一個塊是對應一個緩衝區*/
		if ((i % EXT2_DESC_PER_BLOCK(sb)) == 0)
			gdp = (struct ext2_group_desc *) sbi->s_group_desc[desc_block++]->b_data;
		/*判斷組描述符的塊點陣圖編號是不是合法(大於本組內第一個資料塊,小於本組內最後一個塊)*/
		if (le32_to_cpu(gdp->bg_block_bitmap) < first_block ||
		    le32_to_cpu(gdp->bg_block_bitmap) > last_block)
		{
			ext2_error (sb, "ext2_check_descriptors",
				    "Block bitmap for group %d"
				    " not in group (block %lu)!",
				    i, (unsigned long) le32_to_cpu(gdp->bg_block_bitmap));
			return 0;
		}
		/*inode點陣圖的塊編號是不是合法*/
		if (le32_to_cpu(gdp->bg_inode_bitmap) < first_block ||
		    le32_to_cpu(gdp->bg_inode_bitmap) > last_block)
		{
			ext2_error (sb, "ext2_check_descriptors",
				    "Inode bitmap for group %d"
				    " not in group (block %lu)!",
				    i, (unsigned long) le32_to_cpu(gdp->bg_inode_bitmap));
			return 0;
		}
			/*判斷inodetable的塊編號是不是合法*/
		if (le32_to_cpu(gdp->bg_inode_table) < first_block ||
		    le32_to_cpu(gdp->bg_inode_table) + sbi->s_itb_per_group - 1 >//s_itb_per_group為inode table佔的塊數
		    last_block)
		{
			ext2_error (sb, "ext2_check_descriptors",
				    "Inode table for group %d"
				    " not in group (block %lu)!",
				    i, (unsigned long) le32_to_cpu(gdp->bg_inode_table));
			return 0;
		}
		/*如果都合法,說明這個組描述符是正確的,我們去找下一個組,first_block指向下一個組的第一個塊的塊號*/
		first_block += EXT2_BLOCKS_PER_GROUP(sb);
			/*組描述符指標指向下一個*/
		gdp++;
	}
	return 1;
}

/*
 * Maximal file size.  There is a direct, and {,double-,triple-}indirect
 * block limit, and also a limit of (2^32 - 1) 512-byte sectors in i_blocks.
 * We need to be 1 filesystem block less than the 2^32 sector limit.
 *//* 返回ext2檔案系統的最大檔案大小,因為ext2支援三重間接連線塊 */
static loff_t ext2_max_size(int bits)
{	/*res先賦值為直接塊的數目*/
	loff_t res = EXT2_NDIR_BLOCKS;
	/* This constant is calculated to be the largest file size for a
	 * dense, 4k-blocksize file such that the total number of
	 * sectors in the file, including data and all indirect blocks,
	 * does not exceed 2^32. */
	const loff_t upper_limit = 0x1ff7fffd000LL;/*檔案大小上限,不超過32位大小限制*/
	/*一級間接塊大小*/
	res += 1LL << (bits-2);
	res += 1LL << (2*(bits-2));	/*二級間接塊大小*/
	res += 1LL << (3*(bits-2));/*三級間接塊大小*/
	res <<= bits;
	if (res > upper_limit)/*超過最大限制,修改下*/
		res = upper_limit;
	return res;
}
//返回第nr個組描述符的快號
static unsigned long descriptor_loc(struct super_block *sb,
				    unsigned long logic_sb_block,
				    int nr)
{
	struct ext2_sb_info *sbi = EXT2_SB(sb);
	unsigned long bg, first_data_block, first_meta_bg;
	int has_super = 0;
	/*檔案系統的第一個資料塊的塊號*/
	first_data_block = le32_to_cpu(sbi->s_es->s_first_data_block);
	first_meta_bg = le32_to_cpu(sbi->s_es->s_first_meta_bg);/*第一個處理塊組*/

	if (!EXT2_HAS_INCOMPAT_FEATURE(sb, EXT2_FEATURE_INCOMPAT_META_BG) ||
	    nr < first_meta_bg)
		return (logic_sb_block + nr + 1);
	bg = sbi->s_desc_per_block * nr;
	if (ext2_bg_has_super(sb, bg))/*如果這個塊組有超級塊*/
		has_super = 1;
	return (first_data_block + has_super + (bg * sbi->s_blocks_per_group));//first_meta_bg塊全是儲存的描述符
}
 /*ext2的填充超級塊的函式*/
static int ext2_fill_super(struct super_block *sb, void *data, int silent)
{
	struct buffer_head * bh;
	struct ext2_sb_info * sbi;
	struct ext2_super_block * es;
	struct inode *root;
	unsigned long block;
	/*之前講過的函式,從data的字串引數中獲得超級塊的塊號*/
	unsigned long sb_block = get_sb_block(&data);
	unsigned long logic_sb_block;
	unsigned long offset = 0;
	unsigned long def_mount_opts;
	int blocksize = BLOCK_SIZE;
	int db_count;
	int i, j;
	__le32 features;
	int err;
	/*分配超級塊在記憶體的資訊結構體ext2_sb_info*/
	sbi = kzalloc(sizeof(*sbi), GFP_KERNEL);
	if (!sbi)
		return -ENOMEM;
	sb->s_fs_info = sbi;
	sbi->s_sb_block = sb_block;

	/*
	 * See what the current blocksize for the device is, and
	 * use that as the blocksize.  Otherwise (or if the blocksize
	 * is smaller than the default) use the default.
	 * This is important for devices that have a hardware
	 * sectorsize that is larger than the default.
	 *//* 看一下當前的裝置的塊大小,使用作為檔案系統的塊大小 */
	blocksize = sb_min_blocksize(sb, BLOCK_SIZE);
	if (!blocksize) {
		printk ("EXT2-fs: unable to set blocksize\n");
		goto failed_sbi;
	}

	/*
	 * If the superblock doesn't start on a hardware sector boundary,
	 * calculate the offset.  
	 *//* 如果塊大小不一致,那麼剛才獲得的超級塊也不對了 */
	if (blocksize != BLOCK_SIZE) {
		logic_sb_block = (sb_block*BLOCK_SIZE) / blocksize;
		offset = (sb_block*BLOCK_SIZE) % blocksize;
	} else {
		logic_sb_block = sb_block;
	}
	/*得到了超級塊的塊號,就可以讀取超級塊了*/
	if (!(bh = sb_bread(sb, logic_sb_block))) {
		printk ("EXT2-fs: unable to read superblock\n");
		goto failed_sbi;
	}
	/*
	 * Note: s_es must be initialized as soon as possible because
	 *       some ext2 macro-instructions depend on its value
	 *//* 使用ext2_super_block指標es指向緩衝區中ext2的超級塊 */
	es = (struct ext2_super_block *) (((char *)bh->b_data) + offset);
	sbi->s_es = es;//指向緩衝區中超級快的指標
	/*根據讀取到的超級塊資訊填充vfs層的檔案系統超級塊結構體*/
	sb->s_magic = le16_to_cpu(es->s_magic);
	/*魔數如果不是ext2的,說明不是ext2檔案系統,直接退出吧*/
	if (sb->s_magic != EXT2_SUPER_MAGIC)
		goto cantfind_ext2;
	/* 設定sbi結構體,在我們轉化掛載選項的之前,先設定為預設值*/
	/* Set defaults before we parse the mount options */
	def_mount_opts = le32_to_cpu(es->s_default_mount_opts);
	/*除錯選項,使用者id和組id,*/
	if (def_mount_opts & EXT2_DEFM_DEBUG)
		set_opt(sbi->s_mount_opt, DEBUG);
	if (def_mount_opts & EXT2_DEFM_BSDGROUPS)
		set_opt(sbi->s_mount_opt, GRPID);
	if (def_mount_opts & EXT2_DEFM_UID16)
		set_opt(sbi->s_mount_opt, NO_UID32);
	/*屬性選項*/
#ifdef CONFIG_EXT2_FS_XATTR
	if (def_mount_opts & EXT2_DEFM_XATTR_USER)
		set_opt(sbi->s_mount_opt, XATTR_USER);
#endif
/*是不是支援acl*/

#ifdef CONFIG_EXT2_FS_POSIX_ACL
	if (def_mount_opts & EXT2_DEFM_ACL)
		set_opt(sbi->s_mount_opt, POSIX_ACL);
#endif
	/*記錄發生的錯誤*/
	if (le16_to_cpu(sbi->s_es->s_errors) == EXT2_ERRORS_PANIC)
		set_opt(sbi->s_mount_opt, ERRORS_PANIC);
	else if (le16_to_cpu(sbi->s_es->s_errors) == EXT2_ERRORS_RO)
		set_opt(sbi->s_mount_opt, ERRORS_RO);
	else
		set_opt(sbi->s_mount_opt, ERRORS_CONT);
	/*保留uid和gid*/
	sbi->s_resuid = le16_to_cpu(es->s_def_resuid);
	sbi->s_resgid = le16_to_cpu(es->s_def_resgid);
	
	set_opt(sbi->s_mount_opt, RESERVATION);
 /*轉化選項並設定*/
	if (!parse_options ((char *) data, sbi))
		goto failed_mount;
 /*acl轉化*/
	sb->s_flags = (sb->s_flags & ~MS_POSIXACL) |
		((EXT2_SB(sb)->s_mount_opt & EXT2_MOUNT_POSIX_ACL) ?
		 MS_POSIXACL : 0);
	/*看塊裝置是不是支援片內操作*/
	ext2_xip_verify_sb(sb); /* see if bdev supports xip, unset
				    EXT2_MOUNT_XIP if not */
	/*修改級別和feature是不是適合*/
	if (le32_to_cpu(es->s_rev_level) == EXT2_GOOD_OLD_REV &&
	    (EXT2_HAS_COMPAT_FEATURE(sb, ~0U) ||
	     EXT2_HAS_RO_COMPAT_FEATURE(sb, ~0U) ||
	     EXT2_HAS_INCOMPAT_FEATURE(sb, ~0U)))
		printk("EXT2-fs warning: feature flags set on rev 0 fs, "
		       "running e2fsck is recommended\n");
	/*
	 * Check feature flags regardless of the revision level, since we
	 * previously didn't change the revision level when setting the flags,
	 * so there is a chance incompat flags are set on a rev 0 filesystem.
	 *//*選項檢查*/
	features = EXT2_HAS_INCOMPAT_FEATURE(sb, ~EXT2_FEATURE_INCOMPAT_SUPP);
	if (features) {
		printk("EXT2-fs: %s: couldn't mount because of "
		       "unsupported optional features (%x).\n",
		       sb->s_id, le32_to_cpu(features));
		goto failed_mount;
	}
	/*如果檔案系統不是隻讀掛載,且feature欄位不支援,返回掛載失敗*/
	if (!(sb->s_flags & MS_RDONLY) &&
	    (features = EXT2_HAS_RO_COMPAT_FEATURE(sb, ~EXT2_FEATURE_RO_COMPAT_SUPP))){
		printk("EXT2-fs: %s: couldn't mount RDWR because of "
		       "unsupported optional features (%x).\n",
		       sb->s_id, le32_to_cpu(features));
		goto failed_mount;
	}
	/*取到ext2_super_block中儲存的blocksize */
	blocksize = BLOCK_SIZE << le32_to_cpu(sbi->s_es->s_log_block_size);
	/*片內執行的裝置的話,實際的塊大小和我們猜想的不一致,就不支援*/
	if ((ext2_use_xip(sb)) && ((blocksize != PAGE_SIZE) ||
				  (sb->s_blocksize != blocksize))) {
		if (!silent)
			printk("XIP: Unsupported blocksize\n");
		goto failed_mount;
	}
	/* 如果讀取到的真實的塊大小不匹配的話 */
	/* If the blocksize doesn't match, re-read the thing.. */
	if (sb->s_blocksize != blocksize) {
		brelse(bh);
		/*先設定塊大小,如果失敗了,說明設定的快大小太小*/
		if (!sb_set_blocksize(sb, blocksize)) {
			printk(KERN_ERR "EXT2-fs: blocksize too small for device.\n");
			goto failed_sbi;
		}
		/*根據新的塊大小重新計算超級塊號和偏移*/
		logic_sb_block = (sb_block*BLOCK_SIZE) / blocksize;
		offset = (sb_block*BLOCK_SIZE) % blocksize;
		bh = sb_bread(sb, logic_sb_block);
		if(!bh) {
			printk("EXT2-fs: Couldn't read superblock on "
			       "2nd try.\n");
			goto failed_sbi;
		}
		es = (struct ext2_super_block *) (((char *)bh->b_data) + offset);
		sbi->s_es = es;
		if (es->s_magic != cpu_to_le16(EXT2_SUPER_MAGIC)) {
			printk ("EXT2-fs: Magic mismatch, very weird !\n");
			goto failed_mount;
		}
	}
	/*繼續用ext2_super_block中的內容來填充ext2_sb_info*/
	/*檔案的最大位元組數*/
	sb->s_maxbytes = ext2_max_size(sb->s_blocksize_bits);
	/*根據修改級別的不同,inode的大小和開始編號不一樣*/
	if (le32_to_cpu(es->s_rev_level) == EXT2_GOOD_OLD_REV) {
		sbi->s_inode_size = EXT2_GOOD_OLD_INODE_SIZE;
		sbi->s_first_ino = EXT2_GOOD_OLD_FIRST_INO;
	} else {
		sbi->s_inode_size = le16_to_cpu(es->s_inode_size);
		sbi->s_first_ino = le32_to_cpu(es->s_first_ino);
		if ((sbi->s_inode_size < EXT2_GOOD_OLD_INODE_SIZE) ||
		    !is_power_of_2(sbi->s_inode_size) ||
		    (sbi->s_inode_size > blocksize)) {
			printk ("EXT2-fs: unsupported inode size: %d\n",
				sbi->s_inode_size);
			goto failed_mount;
		}
	}
	/*碎片的資訊填充*/
	sbi->s_frag_size = EXT2_MIN_FRAG_SIZE <<
				   le32_to_cpu(es->s_log_frag_size);
	if (sbi->s_frag_size == 0)
		goto cantfind_ext2;
	sbi->s_frags_per_block = sb->s_blocksize / sbi->s_frag_size;
	/*組的資料塊數目,碎片數目,inode數目*/
	sbi->s_blocks_per_group = le32_to_cpu(es->s_blocks_per_group);
	sbi->s_frags_per_group = le32_to_cpu(es->s_frags_per_group);
	sbi->s_inodes_per_group = le32_to_cpu(es->s_inodes_per_group);

	if (EXT2_INODE_SIZE(sb) == 0)
		goto cantfind_ext2;
	/*每塊的inode數*/
	sbi->s_inodes_per_block = sb->s_blocksize / EXT2_INODE_SIZE(sb);
	if (sbi->s_inodes_per_block == 0 || sbi->s_inodes_per_group == 0)
		goto cantfind_ext2;
	/*每組的inodetable塊數目*/
	sbi->s_itb_per_group = sbi->s_inodes_per_group /
					sbi->s_inodes_per_block;
	/*每塊的描述符數目*/
	sbi->s_desc_per_block = sb->s_blocksize /
					sizeof (struct ext2_group_desc);
	sbi->s_sbh = bh;
	sbi->s_mount_state = le16_to_cpu(es->s_state);
	sbi->s_addr_per_block_bits =
		ilog2 (EXT2_ADDR_PER_BLOCK(sb));
	sbi->s_desc_per_block_bits =
		ilog2 (EXT2_DESC_PER_BLOCK(sb));

	if (sb->s_magic != EXT2_SUPER_MAGIC)
		goto cantfind_ext2;
	/*裝置的塊大小是不是支援*/
	if (sb->s_blocksize != bh->b_size) {
		if (!silent)
			printk ("VFS: Unsupported blocksize on dev "
				"%s.\n", sb->s_id);
		goto failed_mount;
	}
	/*碎片大小是不是不等於塊大小*/
	if (sb->s_blocksize != sbi->s_frag_size) {
		printk ("EXT2-fs: fragsize %lu != blocksize %lu (not supported yet)\n",
			sbi->s_frag_size, sb->s_blocksize);
		goto failed_mount;
	}
	/*每組的塊數目必須大於8*/
	if (sbi->s_blocks_per_group > sb->s_blocksize * 8) {
		printk ("EXT2-fs: #blocks per group too big: %lu\n",
			sbi->s_blocks_per_group);
		goto failed_mount;
	}/*每組的碎片必須多於8塊*/
	if (sbi->s_frags_per_group > sb->s_blocksize * 8) {
		printk ("EXT2-fs: #fragments per group too big: %lu\n",
			sbi->s_frags_per_group);
		goto failed_mount;
	}/*inode必須多於8塊*/
	if (sbi->s_inodes_per_group > sb->s_blocksize * 8) {
		printk ("EXT2-fs: #inodes per group too big: %lu\n",
			sbi->s_inodes_per_group);
		goto failed_mount;
	}

	if (EXT2_BLOCKS_PER_GROUP(sb) == 0)
		goto cantfind_ext2;
	/*記錄組的數目*/
 	sbi->s_groups_count = ((le32_to_cpu(es->s_blocks_count) -
 				le32_to_cpu(es->s_first_data_block) - 1)
 					/ EXT2_BLOCKS_PER_GROUP(sb)) + 1;
	db_count = (sbi->s_groups_count + EXT2_DESC_PER_BLOCK(sb) - 1) /
		   EXT2_DESC_PER_BLOCK(sb);
	/*為組描述符分配空間*/
	sbi->s_group_desc = kmalloc (db_count * sizeof (struct buffer_head *), GFP_KERNEL);
	if (sbi->s_group_desc == NULL) {
		printk ("EXT2-fs: not enough memory\n");
		goto failed_mount;
	}
	bgl_lock_init(&sbi->s_blockgroup_lock);
	/*檔案系統的負載分配空間*/
	sbi->s_debts = kcalloc(sbi->s_groups_count, sizeof(*sbi->s_debts), GFP_KERNEL);
	if (!sbi->s_debts) {
		printk ("EXT2-fs: not enough memory\n");
		goto failed_mount_group_desc;
	}
	/*初始化每一組的負載的值,並讀入組描述符*/
	for (i = 0; i < db_count; i++) {
		block = descriptor_loc(sb, logic_sb_block, i);//定位第i個組描述符的塊號
		sbi->s_group_desc[i] = sb_bread(sb, block);//讀入快
		if (!sbi->s_group_desc[i]) {//如果讀入失敗
			for (j = 0; j < i; j++)
				brelse (sbi->s_group_desc[j]);
			printk ("EXT2-fs: unable to read group descriptors\n");
			goto failed_mount_group_desc;
		}
	}
	if (!ext2_check_descriptors (sb)) {/*ext2檔案系統檢查組描述符一致性*/
		printk ("EXT2-fs: group descriptors corrupted!\n");
		goto failed_mount2;
	}
	sbi->s_gdb_count = db_count;
	get_random_bytes(&sbi->s_next_generation, sizeof(u32));
	spin_lock_init(&sbi->s_next_gen_lock);

	/* per fileystem reservation list head & lock */
	spin_lock_init(&sbi->s_rsv_window_lock);
	sbi->s_rsv_window_root = RB_ROOT;
	/*
	 * Add a single, static dummy reservation to the start of the
	 * reservation window list --- it gives us a placeholder for
	 * append-at-start-of-list which makes the allocation logic
	 * _much_ simpler.
	 */
	sbi->s_rsv_window_head.rsv_start = EXT2_RESERVE_WINDOW_NOT_ALLOCATED;
	sbi->s_rsv_window_head.rsv_end = EXT2_RESERVE_WINDOW_NOT_ALLOCATED;
	sbi->s_rsv_window_head.rsv_alloc_hit = 0;
	sbi->s_rsv_window_head.rsv_goal_size = 0;
	ext2_rsv_window_add(sb, &sbi->s_rsv_window_head);
 	/*初始化percpu相關的幾個變數*/
	err = percpu_counter_init(&sbi->s_freeblocks_counter,
				ext2_count_free_blocks(sb));
	if (!err) {
		err = percpu_counter_init(&sbi->s_freeinodes_counter,
				ext2_count_free_inodes(sb));
	}
	if (!err) {
		err = percpu_counter_init(&sbi->s_dirs_counter,
				ext2_count_dirs(sb));
	}
	if (err) {
		printk(KERN_ERR "EXT2-fs: insufficient memory\n");
		goto failed_mount3;
	}
	/*/*初始化ok
	 * set up enough so that it can read an inode
	 */
	sb->s_op = &ext2_sops;
	sb->s_export_op = &ext2_export_ops;
	sb->s_xattr = ext2_xattr_handlers;/*開始讀入根節點的inode*/
	root = iget(sb, EXT2_ROOT_INO);
	sb->s_root = d_alloc_root(root);
	/*檢查讀入的inode是不是有問題*/
	if (!sb->s_root) {
		iput(root);
		printk(KERN_ERR "EXT2-fs: get root inode failed\n");
		goto failed_mount3;
	}
	if (!S_ISDIR(root->i_mode) || !root->i_blocks || !root->i_size) {
		dput(sb->s_root);
		sb->s_root = NULL;
		printk(KERN_ERR "EXT2-fs: corrupt root inode, run e2fsck\n");
		goto failed_mount3;
	}
	if (EXT2_HAS_COMPAT_FEATURE(sb, EXT3_FEATURE_COMPAT_HAS_JOURNAL))
		ext2_warning(sb, __FUNCTION__,
			"mounting ext3 filesystem as ext2");
	/*檢查超級塊,檢查完寫入裝置(裝載操作會修改超級快的某些值,裝載計數,上一次裝載日期)*/
	ext2_setup_super (sb, es, sb->s_flags & MS_RDONLY);
	return 0;

cantfind_ext2:
	if (!silent)
		printk("VFS: Can't find an ext2 filesystem on dev %s.\n",
		       sb->s_id);
	goto failed_mount;
failed_mount3:
	percpu_counter_destroy(&sbi->s_freeblocks_counter);
	percpu_counter_destroy(&sbi->s_freeinodes_counter);
	percpu_counter_destroy(&sbi->s_dirs_counter);
failed_mount2:
	for (i = 0; i < db_count; i++)
		brelse(sbi->s_group_desc[i]);
failed_mount_group_desc:
	kfree(sbi->s_group_desc);
	kfree(sbi->s_debts);
failed_mount:
	brelse(bh);
failed_sbi:
	sb->s_fs_info = NULL;
	kfree(sbi);
	return -EINVAL;
}
/*提交超級塊的修改*/
static void ext2_commit_super (struct super_block * sb,
			       struct ext2_super_block * es)
{/*記錄修改時間,標記超級塊髒*/
	es->s_wtime = cpu_to_le32(get_seconds());
	mark_buffer_dirty(EXT2_SB(sb)->s_sbh);
	sb->s_dirt = 0;
}
/*同步超級塊和硬碟上的*/
static void ext2_sync_super(struct super_block *sb, struct ext2_super_block *es)                                                                                     
{	/*更新空閒塊和inode的資訊,以及修改時間*/
	es->s_free_blocks_count = cpu_to_le32(ext2_count_free_blocks(sb));
	es->s_free_inodes_count = cpu_to_le32(ext2_count_free_inodes(sb));
	es->s_wtime = cpu_to_le32(get_seconds());
	/*標記髒,然後同步*/
	mark_buffer_dirty(EXT2_SB(sb)->s_sbh);
	sync_dirty_buffer(EXT2_SB(sb)->s_sbh);
	/*同步ok,標記不髒*/                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
	sb->s_dirt = 0;
}

/*
 * In the second extended file system, it is not necessary to
 * write the super block since we use a mapping of the
 * disk super block in a buffer.
 *
 * However, this function is still used to set the fs valid
 * flags to 0.  We need to set this flag to 0 since the fs
 * may have been checked while mounted and e2fsck may have
 * set s_state to EXT2_VALID_FS after some corrections.
 */	
	/*ext2寫入超級塊結構體到硬碟上 */
void ext2_write_super (struct super_block * sb)
{
	struct ext2_super_block * es;
	lock_kernel();
	/*只讀掛載的是不可以寫的*/
	if (!(sb->s_flags & MS_RDONLY)) {
		es = EXT2_SB(sb)->s_es;/*得到ext2_super_block結構體*/
	/*如果狀態是合法的檔案系統的話*/
		if (le16_to_cpu(es->s_state) & EXT2_VALID_FS) {
			ext2_debug ("setting valid to 0\n");
			/*記錄一些資訊*/
			es->s_state = cpu_to_le16(le16_to_cpu(es->s_state) &
						  ~EXT2_VALID_FS);
			es->s_free_blocks_count = cpu_to_le32(ext2_count_free_blocks(sb));
			es->s_free_inodes_count = cpu_to_le32(ext2_count_free_inodes(sb));
			es->s_mtime = cpu_to_le32(get_seconds());
			/*然後同步*/
			ext2_sync_super(sb, es);
		} else
			/*不可寫,就不進行實際上的寫操作*/
			ext2_commit_super (sb, es);
	}
	sb->s_dirt = 0;
	unlock_kernel();
}
/*ext2的重新掛載操作*/
static int ext2_remount (struct super_block * sb, int * flags, char * data)
{
	struct ext2_sb_info * sbi = EXT2_SB(sb);
	struct ext2_super_block * es;
	unsigned long old_mount_opt = sbi->s_mount_opt;
	struct ext2_mount_options old_opts;
	unsigned long old_sb_flags;
	int err;
	/* 儲存原先掛載的選項 */
	/* Store the old options */
	old_sb_flags = sb->s_flags;
	old_opts.s_mount_opt = sbi->s_mount_opt;
	old_opts.s_resuid = sbi->s_resuid;
	old_opts.s_resgid = sbi->s_resgid;

	/*
	 * Allow the "check" option to be passed as a remount option.
	 *//* 轉化選項 ,將data中相應選項儲存入sbi*/
	if (!parse_options (data, sbi)) {
		err = -EINVAL;
		goto restore_opts;
	}
	/*原來的acl的flag轉化為sb指標結構體上的flag*/
	sb->s_flags = (sb->s_flags & ~MS_POSIXACL) |
		((sbi->s_mount_opt & EXT2_MOUNT_POSIX_ACL) ? MS_POSIXACL : 0);
	/*看看塊裝置是不是支援xip,如果不支援,EXT2_MOUNT_XIP位就取消掉*/
	ext2_xip_verify_sb(sb); /* see if bdev supports xip, unset
				    EXT2_MOUNT_XIP if not */
	/*如果使用了xip塊大小就必須等於頁大小*/
	if ((ext2_use_xip(sb)) && (sb->s_blocksize != PAGE_SIZE)) {
		printk("XIP: Unsupported blocksize\n");
		err = -EINVAL;
		goto restore_opts;
	}

	es = sbi->s_es;
	if (((sbi->s_mount_opt & EXT2_MOUNT_XIP) !=
	    (old_mount_opt & EXT2_MOUNT_XIP)) &&
	    invalidate_inodes(sb))
		ext2_warning(sb, __FUNCTION__, "busy inodes while remounting "\
			     "xip remain in cache (no functional problem)");
	/*都是隻讀掛載的時候,就可以結束了*/
	if ((*flags & MS_RDONLY) == (sb->s_flags & MS_RDONLY))
		return 0;
	/*新的flag是隻讀的*/
	if (*flags & MS_RDONLY) {
		if (le16_to_cpu(es->s_state) & EXT2_VALID_FS ||
		    !(sbi->s_mount_state & EXT2_VALID_FS))
			return 0;
		/*
		 * OK, we are remounting a valid rw partition rdonly, so set
		 * the rdonly flag and then mark the partition as valid again.
		 *//* 把原來的讀寫掛載變成只讀掛載 */
		es->s_state = cpu_to_le16(sbi->s_mount_state);
		es->s_mtime = cpu_to_le32(get_seconds());
	} else {
		__le32 ret = EXT2_HAS_RO_COMPAT_FEATURE(sb,
					       ~EXT2_FEATURE_RO_COMPAT_SUPP);
		if (ret) {
			printk("EXT2-fs: %s: couldn't remount RDWR because of "
			       "unsupported optional features (%x).\n",
			       sb->s_id, le32_to_cpu(ret));
			err = -EROFS;
			goto restore_opts;
		}
		/*
		 * Mounting a RDONLY partition read-write, so reread and
		 * store the current valid flag.  (It may have been changed
		 * by e2fsck since we originally mounted the partition.)
		 *//* 把一個只讀掛載變成讀寫掛載 */
		sbi->s_mount_state = le16_to_cpu(es->s_state);
		if (!ext2_setup_super (sb, es, 0))
			sb->s_flags &= ~MS_RDONLY;
	}
	ext2_sync_super(sb, es);
	return 0;
restore_opts:
	sbi->s_mount_opt = old_opts.s_mount_opt;
	sbi->s_resuid = old_opts.s_resuid;
	sbi->s_resgid = old_opts.s_resgid;
	sb->s_flags = old_sb_flags;
	return err;
}
	/*把ext2的狀態存到kstatfs指標裡*/
static int ext2_statfs (struct dentry * dentry, struct kstatfs * buf)
{
	struct super_block *sb = dentry->d_sb;//目錄物件所在的超級塊
	struct ext2_sb_info *sbi = EXT2_SB(sb);//超級塊的記憶體資訊
	struct ext2_super_block *es = sbi->s_es;//超級塊的磁碟資訊
	u64 fsid;
	/*先計算第一個資料塊前邊的塊數目,如果有巨集,就是0*/
	if (test_opt (sb, MINIX_DF))
		sbi->s_overhead_last = 0;//上次計算管理資料的開銷
	else if (sbi->s_blocks_last != le32_to_cpu(es->s_blocks_count)) {//上次計算的快數!=磁碟上塊數
		unsigned long i, overhead = 0;
		smp_rmb();

		/*
		 * Compute the overhead (FS structures). This is constant
		 * for a given filesystem unless the number of block groups
		 * changes so we cache the previous value until it does.計算開銷(FS結構)。
		 除非塊組的數量發生更改,否則這對於給定的檔案系統而言是恆定的,因此我們將快取先前的值,
		 直到它改變為止。
		 */

		/*
		 * All of the blocks before first_data_block are
		 * overhead
		 */
		overhead = le32_to_cpu(es->s_first_data_block);

		/*
		 * Add the overhead attributed to the superblock and
		 * block group descriptors.  If the sparse superblocks
		 * feature is turned on, then not all groups have this.
		 *//* 增加前邊的每組的超級塊和描述符 為overhead*/
		for (i = 0; i < sbi->s_groups_count; i++)
			overhead += ext2_bg_has_super(sb, i) +
				ext2_bg_num_gdb(sb, i);

		/*
		 * Every block group has an inode bitmap, a block
		 * bitmap, and an inode table.
		 *//* inode點陣圖,塊點陣圖,inodetable */
		overhead += (sbi->s_groups_count *
			     (2 + sbi->s_itb_per_group));
		sbi->s_overhead_last = overhead;
		smp_wmb();
		sbi->s_blocks_last = le32_to_cpu(es->s_blocks_count);
	}
	/*填充buf,檔案系統魔數,塊大小,塊數,空閒資料塊數目*/
	buf->f_type = EXT2_SUPER_MAGIC;
	buf->f_bsize = sb->s_blocksize;
	buf->f_blocks = le32_to_cpu(es->s_blocks_count) - sbi->s_overhead_last;
	buf->f_bfree = ext2_count_free_blocks(sb);
	es->s_free_blocks_count = cpu_to_le32(buf->f_bfree);
	buf->f_bavail = buf->f_bfree - le32_to_cpu(es->s_r_blocks_count);/*除去保留塊的空閒塊數*/
	if (buf->f_bfree < le32_to_cpu(es->s_r_blocks_count))
		buf->f_bavail = 0;
	buf->f_files = le32_to_cpu(es->s_inodes_count);/*inode數目*/
	buf->f_ffree = ext2_count_free_inodes(sb);/*空閒inode*/
	es->s_free_inodes_count = cpu_to_le32(buf->f_ffree);
	buf->f_namelen = EXT2_NAME_LEN;/*最大名稱長度*/
	fsid = le64_to_cpup((void *)es->s_uuid) ^/*fsuid*/
	       le64_to_cpup((void *)es->s_uuid + sizeof(u64));
	buf->f_fsid.val[0] = fsid & 0xFFFFFFFFUL;
	buf->f_fsid.val[1] = (fsid >> 32) & 0xFFFFFFFFUL;
	return 0;
}
/*得到裝置的超級塊*/
static int ext2_get_sb(struct file_system_type *fs_type,
	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
{
	return get_sb_bdev(fs_type, flags, dev_name, data, ext2_fill_super, mnt);
}
	/*配額的函式*/
#ifdef CONFIG_QUOTA

/* Read data from quotafile - avoid pagecache and such because we cannot afford
 * acquiring the locks... As quota files are never truncated and quota code
 * itself serializes the operations (and noone else should touch the files)
 * we don't have to be afraid of races *//* 從配額檔案裡讀取資料 */
static ssize_t ext2_quota_read(struct super_block *sb, int type, char *data,
			       size_t len, loff_t off)
{	/*得到硬碟上的配額檔案的inode*/
	struct inode *inode = sb_dqopt(sb)->files[type];
	sector_t blk = off >> EXT2_BLOCK_SIZE_BITS(sb);
	int err = 0;
	int offset = off & (sb->s_blocksize - 1);
	int tocopy;
	size_t toread;
	struct buffer_head tmp_bh;
	struct buffer_head *bh;
	/*得到檔案的大小*/
	loff_t i_size = i_size_read(inode);
	/*偏移超過檔案大小,出錯了*/
	if (off > i_size)
		return 0;
	/*讀的長度超出檔案尾部,也不可能*/
	if (off+len > i_size)
		len = i_size-off;
	toread = len;
	while (toread > 0) {
		/*將要讀的長度*/
		tocopy = sb->s_blocksize - offset < toread ?
				sb->s_blocksize - offset : toread;

		tmp_bh.b_state = 0;
		/*讀取檔案內容*/
		err = ext2_get_block(inode, blk, &tmp_bh, 0);
		if (err < 0)
			return err;
		/*如果沒有對映,賦值為0*/
		if (!buffer_mapped(&tmp_bh))	/* A hole? */
			memset(data, 0, tocopy);
		else {
			bh = sb_bread(sb, tmp_bh.b_blocknr);
			if (!bh)
				return -EIO;
			memcpy(data, bh->b_data+offset, tocopy);
			brelse(bh);
		}
		offset = 0;
		toread -= tocopy;
		data += tocopy;
		blk++;
	}
	return len;
}

/* Write to quotafile *//* 寫資料到配額檔案 */
static ssize_t ext2_quota_write(struct super_block *sb, int type,
				const char *data, size_t len, loff_t off)
{
	struct inode *inode = sb_dqopt(sb)->files[type];
	sector_t blk = off >> EXT2_BLOCK_SIZE_BITS(sb);
	int err = 0;
	int offset = off & (sb->s_blocksize - 1);
	int tocopy;
	size_t towrite = len;
	struct buffer_head tmp_bh;
	struct buffer_head *bh;

	mutex_lock_nested(&inode->i_mutex, I_MUTEX_QUOTA);
	while (towrite > 0) {/*迴圈寫入*/
		tocopy = sb->s_blocksize - offset < towrite ?/*先寫的資料長度*/
				sb->s_blocksize - offset : towrite;

		tmp_bh.b_state = 0;/*得到要寫入的檔案*/
		err = ext2_get_block(inode, blk, &tmp_bh, 1);
		if (err < 0)
			goto out;
		if (offset || tocopy != EXT2_BLOCK_SIZE(sb))/*得到緩衝區*/
			bh = sb_bread(sb, tmp_bh.b_blocknr);
		else
			bh = sb_getblk(sb, tmp_bh.b_blocknr);
		if (!bh) {
			err = -EIO;
			goto out;
		}
		lock_buffer(bh);
		memcpy(bh->b_data+offset, data, tocopy);/*複製資料到檔案緩衝區*/
		flush_dcache_page(bh->b_page);/*清空緩衝區,寫入到裝置上*/
		set_buffer_uptodate(bh);
		mark_buffer_dirty(bh);
		unlock_buffer(bh);
		brelse(bh);
		offset = 0;/*更新這些變數*/
		towrite -= tocopy;
		data += tocopy;
		blk++;
	}
out:
	if (len == towrite)
		return err;
	if (inode->i_size < off+len-towrite)
		i_size_write(inode, off+len-towrite);
	inode->i_version++;
	inode->i_mtime = inode->i_ctime = CURRENT_TIME;
	mark_inode_dirty(inode);
	mutex_unlock(&inode->i_mutex);
	return len - towrite;
}

#endif
/*ext2檔案系統型別結構體*/
static struct file_system_type ext2_fs_type = {
	.owner		= THIS_MODULE,
	.name		= "ext2",
	.get_sb		= ext2_get_sb,
	.kill_sb	= kill_block_super,
	.fs_flags	= FS_REQUIRES_DEV,
};	
	/*ext2的模組初始化*/
static int __init init_ext2_fs(void)
{/*初始化屬性*/
	int err = init_ext2_xattr();
	if (err)
		return err;
	err = init_inodecache();/*初始化inode快取*/
	if (err)
		goto out1;
        err = register_filesystem(&ext2_fs_type);/*登記ext2*/
	if (err)
		goto out;
	return 0;
out:
	destroy_inodecache();
out1:
	exit_ext2_xattr();
	return err;
}
/*ext2的模組清理*/
static void __exit exit_ext2_fs(void)
{	/*登出ext2*/
	unregister_filesystem(&ext2_fs_type);
	destroy_inodecache();
	exit_ext2_xattr();
}

module_init(init_ext2_fs)
module_exit(exit_ext2_fs)

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