On Thu, Mar 28, 2019 at 7:36 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > 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 would say for systems having CONFIG_MEMCG_KMEM, a page flag bit is not that expensive. > 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? PageKmemcg is for kmem while page->mem_cgroup is used for anon, file and kmem memory. So, page->mem_cgroup can not be used for NULL check unless we unify them. Not sure how complicated would that be. Shakeel
