static int ext4_has_free_clusters(struct ext4_sb_info *sbi,
				  s64 nclusters, unsigned int flags)
{
	s64 free_clusters, dirty_clusters, rsv, resv_clusters;
	struct percpu_counter *fcc = &sbi->s_freeclusters_counter;
	struct percpu_counter *dcc = &sbi->s_dirtyclusters_counter;

	free_clusters  = percpu_counter_read_positive(fcc);
	dirty_clusters = percpu_counter_read_positive(dcc);
	resv_clusters = atomic64_read(&sbi->s_resv_clusters);

	/*
	 * r_blocks_count should always be multiple of the cluster ratio so
	 * we are safe to do a plane bit shift only.
	 */
	rsv = (ext4_r_blocks_count(sbi->s_es) >> sbi->s_cluster_bits) +
	      resv_clusters;
…
	/* Check whether we have space after accounting for current
	 * dirty clusters & root reserved clusters.
	 */
	if (free_clusters >= (rsv + nclusters + dirty_clusters))
		return 1;

	/* Hm, nope.  Are (enough) root reserved clusters available? */
	if (uid_eq(sbi->s_resuid, current_fsuid()) ||
	    (!gid_eq(sbi->s_resgid, GLOBAL_ROOT_GID) && in_group_p(sbi->s_resgid)) ||
	    capable(CAP_SYS_RESOURCE) ||
	    (flags & EXT4_MB_USE_ROOT_BLOCKS)) {

		if (free_clusters >= (nclusters + dirty_clusters +
				      resv_clusters))
			return 1;
	}
	/* No free blocks. Let's see if we can dip into reserved pool */
	if (flags & EXT4_MB_USE_RESERVED) {
		if (free_clusters >= (nclusters + dirty_clusters))
			return 1;
	}

	return 0;
}
…
int ext4_claim_free_clusters(struct ext4_sb_info *sbi,
			     s64 nclusters, unsigned int flags)
{
	if (ext4_has_free_clusters(sbi, nclusters, flags)) {
		percpu_counter_add(&sbi->s_dirtyclusters_counter, nclusters);
		return 0;
	} else
		return -ENOSPC;
}

Warm-up