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

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Date: Wed, 12 Jun 2024 13:53:20 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Sourav Panda <souravpanda@...gle.com>, corbet@....net, gregkh@...uxfoundation.org, 
	rafael@...nel.org, mike.kravetz@...cle.com, muchun.song@...ux.dev, 
	rppt@...nel.org, david@...hat.com, rdunlap@...radead.org, 
	chenlinxuan@...ontech.com, yang.yang29@....com.cn, tomas.mudrunka@...il.com, 
	bhelgaas@...gle.com, ivan@...udflare.com, yosryahmed@...gle.com, 
	hannes@...xchg.org, shakeelb@...gle.com, kirill.shutemov@...ux.intel.com, 
	wangkefeng.wang@...wei.com, adobriyan@...il.com, vbabka@...e.cz, 
	Liam.Howlett@...cle.com, surenb@...gle.com, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org, linux-mm@...ck.org, 
	willy@...radead.org, weixugc@...gle.com, David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v13] mm: report per-page metadata information

On Tue, Jun 11, 2024 at 6:30 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Wed,  5 Jun 2024 22:27:51 +0000 Sourav Panda <souravpanda@...gle.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