file: /proc/sys/fs/dentrystate
variable: fs.dentrystate
Official reference

From linux/fs/dentry.c:

struct {
        int nr_dentry;
        int nr_unused;
        int age_limit;         /* age in seconds */
        int want_pages;        /* pages requested by system */
        int dummy[2];
} dentry_stat = {0, 0, 45, 0,};

Dentries are dynamically allocated and deallocated, and nr_dentry seems to be 0 all the time. Hence it’s safe to assume that only nr_unused, age_limit and want_pages are used. Nr_unused seems to be exactly what its name says. 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.