On Thu, Mar 28, 2019 at 06:28:36PM -0700, Shakeel Butt wrote: > A VCPU of a VM can allocate upto three pages which can be mmap'ed by the > user space application. At the moment this memory is not charged. On a > large machine running large number of VMs (or small number of VMs having > large number of VCPUs), this unaccounted memory can be very significant. > So, this memory should be charged to a kmemcg. However that is not > possible as these pages are mmapped to the userspace and PageKmemcg() > was designed with the assumption that such pages will never be mmapped > to the userspace. > > One way to solve this problem is by introducing an additional memcg > charging API similar to mem_cgroup_[un]charge_skmem(). However skmem > charging API usage is contained and shared and no new users are > expected but the pages which can be mmapped and should be charged to > kmemcg can and will increase. So, requiring the usage for such API will > increase the maintenance burden. The simplest solution is to remove the > assumption of no mmapping PageKmemcg() pages to user space. The usual response under these circumstances is "No, you can't have a page flag bit". I don't understand why we need a PageKmemcg anyway. We already have an entire pointer in struct page; can we not just check whether page->mem_cgroup is NULL or not?
