Description Alan Somers 2021-02-26 17:33:31 UTC

geom_stats_resync tries to mmap all of /dev/devstat.  But without knowing that device's size, it simply tries to mmap successively larger amounts of memory until it fails.  That means that the entire operation has O(n^2) syscalls, where n is related to the number of geom providers.  On a system with a few thousand providers, running "mdconfig -l" can take several minutes.

void
geom_stats_resync(void)
{
        void *p;

        if (statsfd == -1)
                return;
        for (;;) {
                p = mmap(statp, (npages + 1) * pagesize,
                    PROT_READ, MAP_SHARED, statsfd, 0);
                if (p == MAP_FAILED)
                        break;
                else
                        statp = p;
                npages++;
        }
}