linux-kernel - Re: [PATCH v6 1/1] mm: report per-page metadata information

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Date: Fri, 5 Jan 2024 13:34:39 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Sourav Panda <souravpanda@...gle.com>
Cc: corbet@....net, gregkh@...uxfoundation.org, rafael@...nel.org, 
	akpm@...ux-foundation.org, mike.kravetz@...cle.com, muchun.song@...ux.dev, 
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	wangkefeng.wang@...wei.com, adobriyan@...il.com, vbabka@...e.cz, 
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	linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org, linux-mm@...ck.org, 
	willy@...radead.org, weixugc@...gle.com
Subject: Re: [PATCH v6 1/1] mm: report per-page metadata information

On Tue, Dec 5, 2023 at 5:31 PM Sourav Panda <souravpanda@...gle.com> wrote:
>
> Adds two new per-node fields, namely nr_page_metadata and
> nr_page_metadata_boot, to /sys/devices/system/node/nodeN/vmstat
> and a global PageMetadata field to /proc/meminfo. This information can
> be used by users to see how much memory is being used by per-page
> metadata, which can vary depending on build configuration, machine
> architecture, and system use.
>
> Per-page metadata is the amount of memory that Linux needs in order to
> manage memory at the page granularity. The majority of such memory is
> used by "struct page" and "page_ext" data structures. In contrast to
> most other memory consumption statistics, per-page metadata might not
> be included in MemTotal. For example, MemTotal does not include memblock
> allocations but includes buddy allocations. In this patch, exported
> field nr_page_metadata in /sys/devices/system/node/nodeN/vmstat would

It is OK to have nr_page_metadata field in nodeN/vmstat based on this
discussion:
https://lore.kernel.org/linux-mm/CA+CK2bB2=raEP8W5GDW_JY7TDvwtSCbkQjvn=SvbjUjPETXZow@mail.gmail.com

> exclusively track buddy allocations while nr_page_metadata_boot would
> exclusively track memblock allocations. Furthermore, PageMetadata in
> /proc/meminfo would exclusively track buddy allocations allowing it to
> be compared against MemTotal.
>
> This memory depends on build configurations, machine architectures, and
> the way system is used:
>
> Build configuration may include extra fields into "struct page",
> and enable / disable "page_ext"
> Machine architecture defines base page sizes. For example 4K x86,
> 8K SPARC, 64K ARM64 (optionally), etc. The per-page metadata
> overhead is smaller on machines with larger page sizes.
> System use can change per-page overhead by using vmemmap
> optimizations with hugetlb pages, and emulated pmem devdax pages.
> Also, boot parameters can determine whether page_ext is needed
> to be allocated. This memory can be part of MemTotal or be outside
> MemTotal depending on whether the memory was hot-plugged, booted with,
> or hugetlb memory was returned back to the system.
>
> Suggested-by: Pasha Tatashin <pasha.tatashin@...een.com>
> Signed-off-by: Sourav Panda <souravpanda@...gle.com>

Reviewed-by: Pasha Tatashin <pasha.tatashin@...een.com>

Pasha