> three transitions to kernel mode, It's not the number of "transitions to kernel mode" (which is actually really fast), but the cost of performing the UID map

> three transitions to kernel mode, It's not the number of "transitions to kernel mode" (which is actually really fast), but the cost of performing the UID mapping, especially considering that this effort is frequently wasted when programs don't bother inspecting st_uid and friends. That's why I wish the Linux xstat syscall, the one that let programs ask for just what they need, had gone through: it'd have made other systems more efficient too.