On Wed 2018-11-21 18:06:33, Matthew Wilcox wrote: > 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? Well, the cost of 64-bit vs. 32-bit is really small here... I'd go with 64bits. If you have 1000 CPUs, rollovers may be faster.. Best regards, Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
Attachment:
signature.asc
Description: Digital signature
