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

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Date:   Thu, 2 Nov 2023 12:02:34 -0400
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Wei Xu <weixugc@...gle.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

On Thu, Nov 2, 2023 at 11:53 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 02.11.23 16:50, Pasha Tatashin wrote:
> >>> Adding reserved memory to MemTotal is a cleaner approach IMO as well.
> >>> But it changes the semantics of MemTotal, which may have compatibility
> >>> issues.
> >>
> >> I object.
> >
> > Could you please elaborate what you object (and why): you object that
> > it will have compatibility issues, or  you object to include memblock
> > reserves into MemTotal?
>
> Sorry, I object to changing the semantics of MemTotal. MemTotal is
> traditionally the memory managed by the buddy, not all memory in the
> system. I know people/scripts that are relying on that [although it's
> been source of confusion a couple of times].

What if one day we change so that struct pages are allocated from
buddy allocator (i.e. allocate deferred struct pages from buddy) will
it break those MemTotal scripts? What if the size of struct pages
changes significantly, but the overhead will come from other metadata
(i.e. memdesc) will that break those scripts? I feel like struct page
memory should really be included into MemTotal, otherwise we will have
this struggle in the future when we try to optimize struct page
memory.