On Wed, Nov 21, 2018 at 12:31 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Wed, Nov 21, 2018 at 12:14:44PM -0800, Daniel Colascione wrote: > > This change adds a per-pid-namespace 64-bit generation number, > > incremented on PID rollover, and exposes it via a new proc file > > /proc/pid_generation. By examining this file before and after /proc > > enumeration, user code can detect the potential reuse of a PID and > > restart the task enumeration process, repeating until it gets a > > coherent snapshot. > > > > PID rollover ought to be rare, so in practice, scan repetitions will > > be rare. > > Then why does it need to be 64-bit? [Resending because of accidental HTML. I really need to switch to a better email client.] Because 64 bits is enough for anyone. :-) A u64 is big enough that we'll never observe an overflow on a running system, and PID namespaces are rare enough that we won't miss the four extra bytes we use by upgrading from a u32. And after reading about some security problems caused by too-clever handling of 32-bit rollover, I'd rather the code be obviously correct than save a trivial amount of space.
