On Tue, 2014-04-01 at 14:10 -0400, KOSAKI Motohiro wrote: > On Tue, Apr 1, 2014 at 1:01 PM, Davidlohr Bueso <davidlohr@xxxxxx> wrote: > > On Mon, 2014-03-31 at 17:05 -0700, Andrew Morton wrote: > >> On Mon, 31 Mar 2014 16:25:32 -0700 Davidlohr Bueso <davidlohr@xxxxxx> wrote: > >> > >> > On Mon, 2014-03-31 at 16:13 -0700, Andrew Morton wrote: > >> > > On Mon, 31 Mar 2014 15:59:33 -0700 Davidlohr Bueso <davidlohr@xxxxxx> wrote: > >> > > > >> > > > > > >> > > > > - Shouldn't there be a way to alter this namespace's shm_ctlmax? > >> > > > > >> > > > Unfortunately this would also add the complexity I previously mentioned. > >> > > > >> > > But if the current namespace's shm_ctlmax is too small, you're screwed. > >> > > Have to shut down the namespace all the way back to init_ns and start > >> > > again. > >> > > > >> > > > > - What happens if we just nuke the limit altogether and fall back to > >> > > > > the next check, which presumably is the rlimit bounds? > >> > > > > >> > > > afaik we only have rlimit for msgqueues. But in any case, while I like > >> > > > that simplicity, it's too late. Too many workloads (specially DBs) rely > >> > > > heavily on shmmax. Removing it and relying on something else would thus > >> > > > cause a lot of things to break. > >> > > > >> > > It would permit larger shm segments - how could that break things? It > >> > > would make most or all of these issues go away? > >> > > > >> > > >> > So sysadmins wouldn't be very happy, per man shmget(2): > >> > > >> > EINVAL A new segment was to be created and size < SHMMIN or size > > >> > SHMMAX, or no new segment was to be created, a segment with given key > >> > existed, but size is greater than the size of that segment. > >> > >> So their system will act as if they had set SHMMAX=enormous. What > >> problems could that cause? > > > > So, just like any sysctl configurable, only privileged users can change > > this value. If we remove this option, users can theoretically create > > huge segments, thus ignoring any custom limit previously set. This is > > what I fear. Think of it kind of like mlock's rlimit. And for that > > matter, why does sysctl exist at all, the same would go for the rest of > > the limits. > > Hmm. It's hard to agree. AFAIK 32MB is just borrowed from other Unix > and it doesn't respect any Linux internals. Agreed, it's stupid, but it's what Linux chose to use since forever. > Look, non privileged user > can user unlimited memory, at least on linux. So I don't find out any > difference between regular anon and shmem. Fine, let's try it, if users complain we can revert. > > So, I personally like 0 byte per default. If by this you mean 0 bytes == unlimited, then I agree. It's less harsh then removing it entirely. So instead of removing the limit we can just set it by default to 0, and in newseg() if shm_ctlmax == 0 then we don't return EINVAL if the passed size is great (obviously), otherwise, if the user _explicitly_ set it via sysctl then we respect that. Andrew, do you agree with this? If so I'll send a patch. 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>