ext4 buddy块分配算法源码剖析

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已于 2023-07-23 23:07:56 修改 · 502 阅读

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#linux #linux操作系统 #内核 #文件系统

于 2023-07-23 23:07:38 首次发布

文章详细介绍了ext4文件系统中用于块分配的ext4_mb_regular_allocator函数，该函数首先尝试从目标块分配，然后按照不同严格程度（cr值）搜索空闲块，考虑了2的幂次方分配和RAID条带化的优化。在找不到满足条件的连续空闲块时，会尝试找到最佳的空闲块进行分配，以避免长时间搜索导致的性能损失。

概述

ext4 buddy块分配算法的函数是ext4_mb_regular_allocator，阅读本文之前需要先看下ext4 mballoc之buddy算法_nginux的博客-优快云博客

ext4_mb_regular_allocator源码


static noinline_for_stack int
ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
{
	ext4_group_t prefetch_grp = 0, ngroups, group, i;
	int cr = -1;
	int err = 0, first_err = 0;
	unsigned int nr = 0, prefetch_ios = 0;
	struct ext4_sb_info *sbi;
	struct super_block *sb;
	struct ext4_buddy e4b;
	int lost;

	sb = ac->ac_sb;
	sbi = EXT4_SB(sb);
	ngroups = ext4_get_groups_count(sb);
	/* non-extent files are limited to low blocks/groups */
	if (!(ext4_test_inode_flag(ac->ac_inode, EXT4_INODE_EXTENTS)))
		ngroups = sbi->s_blockfile_groups;

	BUG_ON(ac->ac_status == AC_STATUS_FOUND);

	/* first, try the goal */
    //首先尝试从goal分配物理块，如果分配到goto out返回
	err = ext4_mb_find_by_goal(ac, &e4b);
	if (err || ac->ac_status == AC_STATUS_FOUND)
		goto out;

	if (unlikely(ac->ac_flags & EXT4_MB_HINT_GOAL_ONLY))
		goto out;

	/*
	 * ac->ac_2order is set only if the fe_len is a power of 2
	 * if ac->ac_2order is set we also set criteria to 0 so that we
	 * try exact allocation using buddy.
	 */
	i = fls(ac->ac_g_ex.fe_len);
	ac->ac_2order = 0;
	/*
	 * We search using buddy data only if the order of the request
	 * is greater than equal to the sbi_s_mb_order2_reqs
	 * You can tune it via /sys/fs/ext4/<partition>/mb_order2_req
	 * We also support searching for power-of-two requests only for
	 * requests upto maximum buddy size we have constructed.
	 */
	if (i >= sbi->s_mb_order2_reqs && i <= sb->s_blocksize_bits + 2) {
		/*
		 * This should tell if fe_len is exactly power of 2
		 */
        //申请分配的数量刚好是2的N次方,比如申请1024个block，那么ac->ac_2order = 10
		if ((ac->ac_g_ex.fe_len & (~(1 << (i - 1)))) == 0)
			ac->ac_2order = array_index_nospec(i - 1,
							   sb->s_blocksize_bits + 2);
	}

	/* if stream allocation is enabled, use global goal */
    //由于goal目标物理块无法申请到空间，那么从文件系统上次分配的地方开始分配
	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
		/* TBD: may be hot point */
		spin_lock(&sbi->s_md_lock);
		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
		spin_unlock(&sbi->s_md_lock);
	}

	/* Let's just scan groups to find more-less suitable blocks */
	cr = ac->ac_2order ? 0 : 1;
    //申请有个严苛程度的概念，如果是2^N方申请量，那么就用cr=0代表精确分配请求数量的物理物理块
	/*
	 * cr == 0 try to get exact allocation,
	 * cr == 3  try to get anything
	 */
repeat:
	for (; cr < 4 && ac->ac_status == AC_STATUS_CONTINUE; cr++) {
		ac->ac_criteria = cr;
		/*
		 * searching for the right group start
		 * from the goal value specified
		 */
		group = ac->ac_g_ex.fe_group;
		prefetch_grp = group;

		for (i = 0; i < ngroups; group++, i++) {
			int ret = 0;
			cond_resched();
			/*
			 * Arti