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Date:   Thu, 12 May 2022 18:38:44 +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,
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        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, smuchun@gmail.com
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 12.05.22 15:59, Muchun Song wrote:
> On Thu, May 12, 2022 at 03:04:57PM +0200, David Hildenbrand wrote:
>> 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.
>>
> 
> I thought the users who enable hugetlb_free_vmemmap value memory
> savings more, so I made a decision in commit 4bab4964a59f.  Seems
> I made a bad decision from your description.

Depends on the perspective I guess. :)

>  
>> 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.
>>
> 
> I mean I want to have hugetlb_vmemmap.c do the check whether the section
> which the HugeTLB pages belong to can be optimized instead of making
> hugetlb_free_vmemmap take precedence.  E.g. If the section's vmemmap pages
> are allocated from the added memory block itself, hugetlb_free_vmemmap will
> refuse to optimize the vmemmap, otherwise, do the optimization.  Then
> both kernel parameters are compatible.  I have done those patches, but
> haven't send them out.

Yeah, that's exactly what I thought. How complicated are they? If they
are easy, can we just avoid this patch here and do it "properly"? :)

-- 
Thanks,

David / dhildenb