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Date:   Thu, 2 Nov 2023 17:09:27 +0100
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Subject: Re: [PATCH v5 1/1] mm: report per-page metadata information
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To:     Pasha Tatashin <pasha.tatashin@soleen.com>
Cc:     Wei Xu <weixugc@google.com>, Sourav Panda <souravpanda@google.com>,
        corbet@lwn.net, gregkh@linuxfoundation.org, rafael@kernel.org,
        akpm@linux-foundation.org, mike.kravetz@oracle.com,
        muchun.song@linux.dev, rppt@kernel.org, rdunlap@infradead.org,
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        yosryahmed@google.com, hannes@cmpxchg.org, shakeelb@google.com,
        kirill.shutemov@linux.intel.com, wangkefeng.wang@huawei.com,
        adobriyan@gmail.com, vbabka@suse.cz, Liam.Howlett@oracle.com,
        surenb@google.com, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-mm@kvack.org, willy@infradead.org,
        Greg Thelen <gthelen@google.com>
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From:   David Hildenbrand <david@redhat.com>
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On 02.11.23 17:02, Pasha Tatashin wrote:
> On Thu, Nov 2, 2023 at 11:53 AM David Hildenbrand <david@redhat.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 does on memory hotplug. But for things like crashkernel size 
detection doesn't really care about that.

> 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

Probably; but ideally the metadata overhead will be smaller with 
memdesc. And we'll talk about that once it gets real ;)

> memory should really be included into MemTotal, otherwise we will have
> this struggle in the future when we try to optimize struct page
> memory.
How far do we want to go, do we want to include crashkernel reserved 
memory in MemTotal because it is system memory? Only metadata? what else 
allocated using memblock?

Again, right now it's simple: MemTotal is memory managed by the buddy.

The spirit of this patch set is good, modifying existing counters needs 
good justification.

-- 
Cheers,

David / dhildenb