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Date:   Fri, 17 Jun 2022 09:39:27 +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>,
        Oscar Salvador <osalvador@suse.de>
Cc:     corbet@lwn.net, akpm@linux-foundation.org, paulmck@kernel.org,
        mike.kravetz@oracle.com, linux-doc@vger.kernel.org,
        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 17.06.22 09:28, Muchun Song wrote:
> On Fri, Jun 17, 2022 at 07:46:53AM +0200, Oscar Salvador wrote:
>> On Thu, Jun 16, 2022 at 09:30:33AM +0200, David Hildenbrand wrote:
>>> IIRC, that was used to skip these patches on the offlining path before
>>> we provided the ranges to offline_pages().
>>
>> Yeah, it was designed for that purpose back then.
>>
>>> I'd not mess with PG_reserved, and give them a clearer name, to not
>>> confuse them with other, ordinary, vmemmap pages that are not
>>> self-hosted (maybe in the future we might want to flag all vmemmap pages
>>> with a new type?).
>>
>> Not sure whether a new type is really needed, or to put it another way, I
>> cannot see the benefit.
>>
>>>
>>> I'd just try reusing the flag PG_owner_priv_1. And eventually, flag all
>>> (v)memmap pages with a type PG_memmap. However, the latter would be
>>> optional and might not be strictly required
>>>
>>>
>>> So what think could make sense is
>>>
>>> /* vmemmap pages that are self-hosted and cannot be optimized/freed. */
>>> PG_vmemmap_self_hosted = PG_owner_priv_1,
>>
>> Sure, I just lightly tested the below, and seems to work, but not sure
>> whether that is what you are referring to.
>> @Munchun: thoughts?
>>
> 
> I think it works and fits my requirement.
> 
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index e66f7aa3191d..a4556afd7bda 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -193,6 +193,11 @@ enum pageflags {
>>  
>>  	/* Only valid for buddy pages. Used to track pages that are reported */
>>  	PG_reported = PG_uptodate,
>> +
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> +	/* For self-hosted memmap pages */
>> +	PG_vmemmap_self_hosted = PG_owner_priv_1,
>> +#endif
>>  };
>>  
>>  #define PAGEFLAGS_MASK		((1UL << NR_PAGEFLAGS) - 1)
>> @@ -628,6 +633,10 @@ PAGEFLAG_FALSE(SkipKASanPoison, skip_kasan_poison)
>>   */
>>  __PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
>>  
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> +PAGEFLAG(Vmemmap_self_hosted, vmemmap_self_hosted, PF_ANY)
>> +#endif
>> +
>>  /*
>>   * On an anonymous page mapped into a user virtual memory area,
>>   * page->mapping points to its anon_vma, not to a struct address_space;
>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>> index 1089ea8a9c98..e2de7ed27e9e 100644
>> --- a/mm/hugetlb_vmemmap.c
>> +++ b/mm/hugetlb_vmemmap.c
>> @@ -101,6 +101,14 @@ void hugetlb_vmemmap_free(struct hstate *h, struct page *head)
>>  {
>>  	unsigned long vmemmap_addr = (unsigned long)head;
>>  	unsigned long vmemmap_end, vmemmap_reuse, vmemmap_pages;
>> +	struct mem_section *ms = __pfn_to_section(page_to_pfn(head));
>> +	struct page *memmap;
>> +
>> +	memmap = sparse_decode_mem_map(ms->section_mem_map,
>> +				       pfn_to_section_nr(page_to_pfn(head)));
>> +
>> +	if (PageVmemmap_self_hosted(memmap))
>> +		return;
> 
> I think here needs a loop if it is a 1GB page (spans multiple sections).
> Right?  Here is an implementation based on another approach. But I think
> your implementation is more simpler and efficient.  Would you mind me
> squash your diff into my patch and with your "Co-developed-by"?

Due to hugtlb alignment requirements, and the vmemmap pages being at the
start of the hotplugged memory region, I think that cannot currently
happen. Checking the first vmemmap page might be good enough for now,
and probably for the future.

-- 
Thanks,

David / dhildenb