Re: [PATCH v13] mm: report per-page metadata information

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On Fri, Aug 2, 2024 at 3:02 PM Alison Schofield
<alison.schofield@intel.com> wrote:
>
> ++ nvdimm, linux-cxl, Yu Zhang
>
> On Wed, Jun 05, 2024 at 10:27:51PM +0000, Sourav Panda wrote:
> > Today, we do not have any observability of per-page metadata
> > and how much it takes away from the machine capacity. Thus,
> > we want to describe the amount of memory that is going towards
> > per-page metadata, which can vary depending on build
> > configuration, machine architecture, and system use.
> >
> > This patch adds 2 fields to /proc/vmstat that can used as shown
> > below:
> >
> > Accounting per-page metadata allocated by boot-allocator:
> >       /proc/vmstat:nr_memmap_boot * PAGE_SIZE
> >
> > Accounting per-page metadata allocated by buddy-allocator:
> >       /proc/vmstat:nr_memmap * PAGE_SIZE
> >
> > Accounting total Perpage metadata allocated on the machine:
> >       (/proc/vmstat:nr_memmap_boot +
> >        /proc/vmstat:nr_memmap) * PAGE_SIZE
> >
> > Utility for userspace:
> >
> > Observability: Describe the amount of memory overhead that is
> > going to per-page metadata on the system at any given time since
> > this overhead is not currently observable.
> >
> > Debugging: Tracking the changes or absolute value in struct pages
> > can help detect anomalies as they can be correlated with other
> > metrics in the machine (e.g., memtotal, number of huge pages,
> > etc).
> >
> > page_ext overheads: Some kernel features such as page_owner
> > page_table_check that use page_ext can be optionally enabled via
> > kernel parameters. Having the total per-page metadata information
> > helps users precisely measure impact. Furthermore, page-metadata
> > metrics will reflect the amount of struct pages reliquished
> > (or overhead reduced) when hugetlbfs pages are reserved which
> > will vary depending on whether hugetlb vmemmap optimization is
> > enabled or not.
> >
> > For background and results see:
> > lore.kernel.org/all/20240220214558.3377482-1-souravpanda@google.com
> >
> > Acked-by: David Rientjes <rientjes@google.com>
> > Signed-off-by: Sourav Panda <souravpanda@google.com>
> > Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
>
> This patch is leading to Oops in 6.11-rc1 when CONFIG_MEMORY_HOTPLUG
> is enabled. Folks hitting it have had success with reverting this patch.
> Disabling CONFIG_MEMORY_HOTPLUG is not a long term solution.
>
> Reported here:
> https://lore.kernel.org/linux-cxl/CAHj4cs9Ax1=CoJkgBGP_+sNu6-6=6v=_L-ZBZY0bVLD3wUWZQg@mail.gmail.com/

Thank you for the heads up. Can you please attach a full config file,
also was anyone able to reproduce this problem in qemu with emulated
nvdimm?

Pasha