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. -- 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>