Thanks everyone for participating in this discussion and looking into this. On Fri, May 13, 2022 at 9:12 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Fri, May 13, 2022, Johannes Weiner wrote: > > On Thu, May 12, 2022 at 11:29:38PM +0000, Sean Christopherson wrote: > > > On Thu, May 12, 2022, Johannes Weiner wrote: > > > > On Mon, May 02, 2022 at 11:46:26AM -0700, Yosry Ahmed wrote: > > > > > On Mon, May 2, 2022 at 3:01 AM Marc Zyngier <maz@xxxxxxxxxx> wrote: > > > > > > What do you plan to do for IOMMU page tables? After all, they serve > > > > > > the exact same purpose, and I'd expect these to be handled the same > > > > > > way (i.e. why is this KVM specific?). > > > > > > > > > > The reason this was named NR_SECONDARY_PAGTABLE instead of > > > > > NR_KVM_PAGETABLE is exactly that. To leave room to incrementally > > > > > account other types of secondary page tables to this stat. It is just > > > > > that we are currently interested in the KVM MMU usage. > > > > > > > > Do you actually care at the supervisor level that this memory is used > > > > for guest page tables? > > > > > > Hmm, yes? KVM does have a decent number of large-ish allocations that aren't > > > for page tables, but except for page tables, the number/size of those allocations > > > scales linearly with either the number of vCPUs or the amount of memory assigned > > > to the VM (with no room for improvement barring KVM changes). > > > > > > Off the top of my head, KVM's secondary page tables are the only allocations that > > > don't scale linearly, especially when nested virtualization is in use. > > > > Thanks, that's useful information. > > > > Are these other allocations accounted somewhere? If not, are they > > potential containment holes that will need fixing eventually? > > All allocations that are tied to specific VM/vCPU are tagged GFP_KERNEL_ACCOUNT, > so we should be good on that front. > > > > > It seems to me you primarily care that it is reported *somewhere* > > > > (hence the piggybacking off of NR_PAGETABLE at first). And whether > > > > it's page tables or iommu tables or whatever else allocated for the > > > > purpose of virtualization, it doesn't make much of a difference to the > > > > host/cgroup that is tracking it, right? > > > > > > > > (The proximity to nr_pagetable could also be confusing. A high page > > > > table count can be a hint to userspace to enable THP. It seems > > > > actionable in a different way than a high number of kvm page tables or > > > > iommu page tables.) > > > > > > I don't know about iommu page tables, but on the KVM side a high count can also > > > be a good signal that enabling THP would be beneficial. > > > > Well, maybe. > > > > It might help, but ultimately it's the process that's in control in > > all cases: it's unmovable kernel memory allocated to manage virtual > > address space inside the task. > > > > So I'm still a bit at a loss whether these things should all be lumped > > in together or kept separately. meminfo and memory.stat are permanent > > ABI, so we should try to establish in advance whether the new itme is > > really a first-class consumer or part of something bigger. > > > > The patch initially piggybacked on NR_PAGETABLE. I found an email of > > you asking why it couldn't be a separate item, but it didn't provide a > > reasoning for that decision. Could you share your thoughts on that? > > It was mostly an honest question, I too am trying to understand what userspace > wants to do with this information. I was/am also trying to understand the benefits > of doing the tracking through page_state and not a dedicated KVM stat. E.g. KVM > already has specific stats for the number of leaf pages mapped into a VM, why not > do the same for non-leaf pages? Let me cast some light on this. The reason this started being piggybacked on NR_PAGETABLE is that we had a remnant of an old internal implementation of NR_PAGETABLE before it was introduced upstream, that accounted KVM secondary page tables as normal page tables. This made me think this behavior was preferable. Personally, I wanted to make it a separate thing since the beginning. When I found opinions here that also suggested a separate stat I went ahead for that. As for where to put this information, it does not have to be NR_SECONDARY_PAGETABLE. Ultimately, people working on KVM are the ones that will interpret and act upon this data, so if you have somewhere else in mind please let me know, Sean.
