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Date:   Thu, 16 Jun 2022 09:21:35 +0200
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Subject: Re: [PATCH v2 2/2] mm: memory_hotplug: introduce
 SECTION_CANNOT_OPTIMIZE_VMEMMAP
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To:     Muchun Song <songmuchun@bytedance.com>
Cc:     corbet@lwn.net, akpm@linux-foundation.org, paulmck@kernel.org,
        mike.kravetz@oracle.com, osalvador@suse.de,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, duanxiongchun@bytedance.com, smuchun@gmail.com
References: <20220520025538.21144-1-songmuchun@bytedance.com>
 <20220520025538.21144-3-songmuchun@bytedance.com>
 <53024884-0182-df5f-9ca2-00652c64ce36@redhat.com>
 <YqqZOj+zby1fLGv/@FVFYT0MHHV2J.usts.net>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 16.06.22 04:45, Muchun Song wrote:
> On Wed, Jun 15, 2022 at 11:51:49AM +0200, David Hildenbrand wrote:
>> On 20.05.22 04:55, Muchun Song wrote:
>>> For now, the feature of hugetlb_free_vmemmap is not compatible with the
>>> feature of memory_hotplug.memmap_on_memory, and hugetlb_free_vmemmap
>>> takes precedence over memory_hotplug.memmap_on_memory. However, someone
>>> wants to make memory_hotplug.memmap_on_memory takes precedence over
>>> hugetlb_free_vmemmap since memmap_on_memory makes it more likely to
>>> succeed memory hotplug in close-to-OOM situations.  So the decision
>>> of making hugetlb_free_vmemmap take precedence is not wise and elegant.
>>> The proper approach is to have hugetlb_vmemmap.c do the check whether
>>> the section which the HugeTLB pages belong to can be optimized.  If
>>> the section's vmemmap pages are allocated from the added memory block
>>> itself, hugetlb_free_vmemmap should refuse to optimize the vmemmap,
>>> otherwise, do the optimization.  Then both kernel parameters are
>>> compatible.  So this patch introduces SECTION_CANNOT_OPTIMIZE_VMEMMAP
>>> to indicate whether the section could be optimized.
>>>
>>
>> In theory, we have that information stored in the relevant memory block,
>> but I assume that lookup in the xarray + locking is impractical.
>>
>> I wonder if we can derive that information simply from the vmemmap pages
>> themselves, because *drumroll*
>>
>> For one vmemmap page (the first one), the vmemmap corresponds to itself
>> -- what?!
>>
>>
>> [	hotplugged memory	]
>> [ memmap ][      usable memory	]
>>       |    |                    |
>>   ^---     |                    |
>>    ^-------                     |
>>          ^----------------------
>>
>> The memmap of the first page of hotplugged memory falls onto itself.
>> We'd have to derive from actual "usable memory" that condition.
>>
>>
>> We currently support memmap_on_memory memory only within fixed-size
>> memory blocks. So "hotplugged memory" is guaranteed to be aligned to
>> memory_block_size_bytes() and the size is memory_block_size_bytes().
>>
>> If we'd have a page falling into usbale memory, we'd simply lookup the
>> first page and test if the vmemmap maps to itself.
>>
> 
> I think this can work. Should we use this approach in next version?
> 

Either that or more preferable, flagging the vmemmap pages eventually.
That's might be future proof.

>>
>> Of course, once we'd support variable-sized memory blocks, it would be
>> different.
>>
>>
>> An easier/future-proof approach might simply be flagging the vmemmap
>> pages as being special. We reuse page flags for that, which don't have
>> semantics yet (i.e., PG_reserved indicates a boot-time allocation via
>> memblock).
>>
> 
> I think you mean flag vmemmap pages' struct page as PG_reserved if it
> can be optimized, right? When the vmemmap pages are allocated in
> hugetlb_vmemmap_alloc(), is it valid to flag them as PG_reserved (they
> are allocated from buddy allocator not memblock)?
> 

Sorry I wasn't clear. I'd flag them with some other
not-yet-used-for-vmemmap-pages flag. Reusing PG_reserved could result in
trouble.


-- 
Thanks,

David / dhildenb