Re: [PATCH] Add /proc/pid_generation

On Wed 2018-11-21 18:06:33, Matthew Wilcox wrote:

On Wed, Nov 21, 2018 at 12:38:20PM -0800, Daniel Colascione wrote:

quoted

On Wed, Nov 21, 2018 at 12:31 PM Matthew Wilcox [off-list ref] wrote:

quoted

On Wed, Nov 21, 2018 at 12:14:44PM -0800, Daniel Colascione wrote:

quoted

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