On Mon 03-11-14 17:36:26, Johannes Weiner wrote: [...] > Also, nobody is using that space currently, and I can save memory by > moving the pointer in there. Should we later add another pointer to > struct page we are only back to the status quo - with the difference > that booting with cgroup_disable=memory will no longer save the extra > pointer per page, but again, if you care that much, you can disable > memory cgroups at compile-time. There would be a slight inconvenience for 32b machines with distribution kernels which cannot simply drop CONFIG_MEMCG from the config. Especially those 32b machines with a lot of memory. I have checked configuration used for OpenSUSE PAE kernel. Both the struct page and the code size grow. There are additional 4B with SLAB and SLUB gets 8 because of the alignment in the struct page. So the overhead is 4B per page with SLUB. This doesn't sound too bad to me considering that 64b actually even saves some space with SLUB and it is at the same level with SLAB and more importantly gets rid of the lookup in hot paths. The code size grows (~1.5k) most probably due to struct page pointer arithmetic (but I haven't checked that) but the data section shrinks for SLAB. So we have additional 1.6k for SLUB. I guess this is acceptable. text data bss dec hex filename 8427489 887684 3186688 12501861 bec365 mmotm/vmlinux.slab 8429060 883588 3186688 12499336 beb988 page_cgroup/vmlinux.slab 8438894 883428 3186688 12509010 bedf52 mmotm/vmlinux.slub 8440529 883428 3186688 12510645 bee5b5 page_cgroup/vmlinux.slub So to me it sounds like the savings for 64b are worth minor inconvenience for 32b which is clearly on decline and I would definitely not encourage people to use PAE kernels with a lot of memory where the difference might matter. For the most x86 32b deployments (laptops with 4G) the difference shouldn't be noticeable. I am not familiar with other archs so the situation might be different there. If this would be a problem for some reason, though, we can reintroduce the external page descriptor and translation layer conditionally depending on the arch. It seems there will be some users of the external descriptors anyway so a struct page_external can hold memcg pointer as well. This should probably go into the changelog, I guess. -- Michal Hocko SUSE Labs -- 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>