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Message-ID: <284eec3f-a79d-c5f0-3cd6-53b8e64100cd@redhat.com>
Date:   Thu, 12 May 2022 15:04:57 +0200
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Subject: Re: [PATCH v10 2/4] mm: memory_hotplug: override memmap_on_memory
 when hugetlb_free_vmemmap=on
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To:     Muchun Song <songmuchun@bytedance.com>
Cc:     corbet@lwn.net, mike.kravetz@oracle.com, akpm@linux-foundation.org,
        mcgrof@kernel.org, keescook@chromium.org, yzaikin@google.com,
        osalvador@suse.de, masahiroy@kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, smuchun@gmail.com
References: <20220509062703.64249-1-songmuchun@bytedance.com>
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 <ebffd794-697b-9bf1-f41b-4b2d52c100fc@redhat.com>
 <Yn0Ck5isxx4ghQi6@FVFYT0MHHV2J.usts.net>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 12.05.22 14:50, Muchun Song wrote:
> On Thu, May 12, 2022 at 09:36:15AM +0200, David Hildenbrand wrote:
>> On 09.05.22 08:27, Muchun Song wrote:
>>> Optimizing HugeTLB vmemmap pages is not compatible with allocating memmap on
>>> hot added memory. If "hugetlb_free_vmemmap=on" and
>>> memory_hotplug.memmap_on_memory" are both passed on the kernel command line,
>>> optimizing hugetlb pages takes precedence. 
>>
>> Why?
>>
> 
> Because both two features are not compatible since hugetlb_free_vmemmap cannot
> optimize the vmemmap pages allocated from alternative allocator (when
> memory_hotplug.memmap_on_memory=1). So when the feature of hugetlb_free_vmemmap
> is introduced, I made hugetlb_free_vmemmap take precedence.  BTW, I have a plan
> to remove this restriction, I'll post it out ASAP.

I was asking why vmemmap optimization should take precedence.
memmap_on_memory makes it more likely to succeed memory hotplug in
close-to-OOM situations -- which is IMHO more important than a vmemmap
optimization.

But anyhow, the proper approach should most probably be to simply not
mess with the vmemmap if we stumble over a vmemmap that's special due to
memmap_on_memory. I assume that's what you're talking about sending out.

-- 
Thanks,

David / dhildenb