linux-kernel - Re: [PATCH v3 1/5] mm,memory_hotplug: Allocate memmap from the added memory range

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Message-ID: <51662b87-35bd-ff18-17dd-b2a99d1095d7@redhat.com>
Date:   Mon, 8 Mar 2021 15:06:49 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>, Zi Yan <ziy@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pasha.tatashin@...een.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH v3 1/5] mm,memory_hotplug: Allocate memmap from the added
 memory range

On 08.03.21 15:04, Oscar Salvador wrote:
> On Sun, Mar 07, 2021 at 10:16:36PM -0500, Zi Yan wrote:
>> +Mike for hugetlb discussion.
>>
>> Just thinking about how it might impact gigantic page allocation like hugetlb.
>> When MHP_MEMMAP_ON_MEMORY is on, memmap pages are placed at the beginning
>> of each hot added memory block, so available PFNs from two consecutive
>> hot added memory blocks are not all contiguous, separated by memmap pages.
>> If the memory block size is <= 1GB, there is no way of reserving gigantic
>> pages for hugetlb during runtime using alloc_contig_pages from any hot
>> added memory. Am I getting this right?
> 
> Yes, that is why it is stated both in boot parameter documentation and
> patch changelog that this feature does not play well in those setups
> where your workload is in need of large contiguous chunks of memory,
> that being gigantic hugetlb or just normal memory.
> 
>> I see this implication is documented at the high level in patch 3. Just
>> wonder if we want to be more specific. Or hugetlb is rarely used along
>> with hot-add memory.
> 
> I think it is quite normal to see hugetlb and hotplug operations in the
> same environment.
> One thing excludes the other, just need to be careful when it comes to
> potential pitfalls during offline operations.
> 
> I guess we could mention hugetlb pages in the documentation, if it feels
> it is necesary.

Runtime allocation of gigantic pages without CMA is absolutely 
unreliable either way. IMHO, a tunable for the admin is good enough.

-- 
Thanks,

David / dhildenb