Re: [PATCH 00/16] IOMMU memory observability

[Yosry Ahmed]

On Tue, Nov 28, 2023 at 3:52 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Tue, Nov 28, 2023 at 03:03:30PM -0800, Yosry Ahmed wrote:
> > > Yes, another counter for KVM could be added. On the other hand KVM
> > > only can be computed by subtracting one from another as there are only
> > > two types of secondary page tables, KVM and IOMMU:
> > >
> > > /sys/devices/system/node/node0/meminfo
> > > Node 0 SecPageTables:    422204 kB
> > >
> > >  /sys/devices/system/node/nodeN/vmstat
> > > nr_iommu_pages 105555
> > >
> > > KVM only = SecPageTables - nr_iommu_pages * PAGE_SIZE / 1024
> > >
> >
> > Right, but as I mention above, if userspace starts depending on this
> > equation, we won't be able to add any more classes of "secondary" page
> > tables to SecPageTables. I'd like to avoid that if possible. We can do
> > the subtraction in the kernel.
>
> What Sean had suggested was that SecPageTables was always intended to
> account all the non-primary mmu memory used by page tables. If this is
> the case we shouldn't be trying to break it apart into finer
> counters. These are big picture counters, not detailed allocation by
> owner counters.

Right, I agree with that, but if SecPageTables includes page tables
from multiple sources, and it is observed to be suspiciously high, the
logical next step is to try to find the culprit, right?

>
> Jason