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From: Pasha Tatashin <pasha.tatashin@soleen.com>
Date: Wed, 12 Jun 2024 13:53:20 -0400
Message-ID: <CA+CK2bA75p3LW95i79uiEfkg9AS0cKVfhKZMatHHQfRB4PJFZw@mail.gmail.com>
Subject: Re: [PATCH v13] mm: report per-page metadata information
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Sourav Panda <souravpanda@google.com>, corbet@lwn.net, gregkh@linuxfoundation.org, 
	rafael@kernel.org, mike.kravetz@oracle.com, muchun.song@linux.dev, 
	rppt@kernel.org, david@redhat.com, rdunlap@infradead.org, 
	chenlinxuan@uniontech.com, yang.yang29@zte.com.cn, tomas.mudrunka@gmail.com, 
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On Tue, Jun 11, 2024 at 6:30=E2=80=AFPM Andrew Morton <akpm@linux-foundatio=
n.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 runtim=
e:

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