On Wed, Nov 21, 2018 at 12:38:20PM -0800, Daniel Colascione wrote: > 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. I don't think you understand how big 4 billion is. If it happens once a second, it will take 136 years for a 2^32 count to roll over. How often does a PID roll over happen?
