linux-kernel - Re: [PATCH v5 1/1] mm: report per-page metadata information

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Message-ID: <CA+CK2bAWbnapxXvOwHFXFJNqzKP-_=vroyLaeWBQ=d-ZJ4_R3w@mail.gmail.com>
Date:   Fri, 3 Nov 2023 11:18:53 -0400
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Wei Xu <weixugc@...gle.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Sourav Panda <souravpanda@...gle.com>, corbet@....net,
        gregkh@...uxfoundation.org, rafael@...nel.org,
        akpm@...ux-foundation.org, mike.kravetz@...cle.com,
        muchun.song@...ux.dev, rppt@...nel.org, 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,
        Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH v5 1/1] mm: report per-page metadata information

> > Since we are going to use two independent interfaces
> > /proc/meminfo/PageMetadata and nodeN/page_metadata (in a separate file
> > as requested by Greg) How about if in /proc/meminfo we provide only
> > the buddy allocator part, and in nodeN/page_metadata we provide the
> > total per-page overhead in the given node that include memblock
> > reserves, and buddy allocator memory?
>
> What we want is the system-wide breakdown of kernel memory usage. It
> works for this use case with the new PageMetadata counter in
> /proc/meminfo to report only buddy-allocated per-page metadata.

We want to report all PageMetadata, otherwise this effort is going to
be useless for the majority of users.

As you noted, /proc/meminfo allows us to report only the part of
per-page metadata that was allocated by the buddy allocator because of
an existing MemTotal bug that does not include memblock reserves.
However, we do not have this limitation when we create a new
nodeN/page_metadata interface, and we can document that in the sysfs
ABI documentation: sum(nodeN/page_metadata)  contains all per-page
metadata and is superset of /proc/meminfo.

The only question is how to name PageMetadata in the /proc/meminfo
appropriately, so users can understand that not all page metadata is
included? (of course we will also document that only the MemTotal part
of page metadata is reported in /proc/meminfo)

Pasha