On Thu, 2014-04-03 at 19:39 -0400, KOSAKI Motohiro wrote: > On Thu, Apr 3, 2014 at 3:50 PM, Davidlohr Bueso <davidlohr@xxxxxx> wrote: > > On Thu, 2014-04-03 at 21:02 +0200, Manfred Spraul wrote: > >> Hi Davidlohr, > >> > >> On 04/03/2014 02:20 AM, Davidlohr Bueso wrote: > >> > The default size for shmmax is, and always has been, 32Mb. > >> > Today, in the XXI century, it seems that this value is rather small, > >> > making users have to increase it via sysctl, which can cause > >> > unnecessary work and userspace application workarounds[1]. > >> > > >> > Instead of choosing yet another arbitrary value, larger than 32Mb, > >> > this patch disables the use of both shmmax and shmall by default, > >> > allowing users to create segments of unlimited sizes. Users and > >> > applications that already explicitly set these values through sysctl > >> > are left untouched, and thus does not change any of the behavior. > >> > > >> > So a value of 0 bytes or pages, for shmmax and shmall, respectively, > >> > implies unlimited memory, as opposed to disabling sysv shared memory. > >> > This is safe as 0 cannot possibly be used previously as SHMMIN is > >> > hardcoded to 1 and cannot be modified. > > > >> Are we sure that no user space apps uses shmctl(IPC_INFO) and prints a > >> pretty error message if shmall is too small? > >> We would break these apps. > > > > Good point. 0 bytes/pages would definitely trigger an unexpected error > > message if users did this. But on the other hand I'm not sure this > > actually is a _real_ scenario, since upon overflow the value can still > > end up being 0, which is totally bogus and would cause the same > > breakage. > > > > So I see two possible workarounds: > > (i) Use ULONG_MAX for the shmmax default instead. This would make shmall > > default to 1152921504606846720 and 268435456, for 64 and 32bit systems, > > respectively. > > > > (ii) Keep the 0 bytes, but add a new a "transition" tunable that, if set > > (default off), would allow 0 bytes to be unlimited. With time, users > > could hopefully update their applications and we could eventually get > > rid of it. This _seems_ to be the less aggressive way to go. > > Do you mean > > set 0: IPC_INFO return shmmax = 0. > set 1: IPC_INFO return shmmax = ULONG_MAX. > > ? > > That makes sense. Well I was mostly referring to: set 0: leave things as there are now. set 1: this patch. I don't think it makes much sense to set unlimited for both 0 and ULONG_MAX, that would probably just create even more confusion. But then again, we shouldn't even care about breaking things with shmmax or shmall with 0 value, it just makes no sense from a user PoV. shmmax cannot be 0 unless there's an overflow, which voids any valid cases, and thus shmall cannot be 0 either as it would go against any values set for shmmax. I think it's safe to ignore this. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>