struct dentry_stat定义在linux/include/linux/dcache.h，包含了dentry的一些信息。
struct dentry_stat_t dentry_stat {
int nr_dentry;
int nr_unused;
int age_limit; /* age in seconds */
int want_pages; /* pages requested by system */
int nr_negative; /* # of unused negative dentries */
int dummy; /* Reserved for future use */
};
nr_dentry shows the total number of dentries allocated (active + unused).
nr_unused shows the number of dentries that are not actively used, but are saved in the LRU list for future reuse.
age_limit is the age in seconds after which dcache entries can be reclaimed when memory is short and
want_pages is nonzero when shrink_dcache_pages() has been called and the dcache isn't pruned yet.
nr_negative shows the number of unused dentries that are also negative dentries which do not map to any files.
Instead, they help speeding up rejection of non-existing files provided by the users.
# cat /proc/sys/fs/dentry-state
8379 7523 45 0 1 0