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. Thoughts? Thanks, Davidlohr -- 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>