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

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

On Tue, Jun 11, 2024 at 6:30 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Wed,  5 Jun 2024 22:27:51 +0000 Sourav Panda <souravpanda@google.com> 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
>
> Under what circumstances do these change?  Only hotplug?

Currently, there are several reasons these numbers can change during runtime:

1. Memory hotplug/hotremove
2. Adding/Removing hugetlb pages with vmemmap optimization
3. Adding/Removing Device DAX with vmemmap optimization.

>
> It's nasty, but would it be sufficient to simply emit these numbers
> into dmesg when they change?

These numbers should really be part of /proc/vmstat in order to
provide an interface for determining the system memory overhead.

Pasha