Re: [PATCH] arm64: mm: hugetlb: add support for free vmemmap pages of HugeTLB

From: David Hildenbrand <david@redhat.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>,
	Muchun Song <songmuchun@bytedance.com>,
	will@kernel.org, akpm@linux-foundation.org, bodeddub@amazon.com,
	osalvador@suse.de, mike.kravetz@oracle.com, rientjes@google.com
Cc: linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	duanxiongchun@bytedance.com, fam.zheng@bytedance.com,
	zhengqi.arch@bytedance.com
Subject: Re: [PATCH] arm64: mm: hugetlb: add support for free vmemmap pages of HugeTLB
Date: Wed, 19 May 2021 14:03:23 +0200	[thread overview]
Message-ID: <88114091-fbb2-340d-b69b-a572fa340265@redhat.com> (raw)
In-Reply-To: <1b9d008a-7544-cc85-5c2f-532b984eb5b5@arm.com>

On 19.05.21 13:45, Anshuman Khandual wrote:
> 
> 
> On 5/18/21 2:48 PM, Muchun Song wrote:
>> The preparation of supporting freeing vmemmap associated with each
>> HugeTLB page is ready, so we can support this feature for arm64.
>>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>>   arch/arm64/mm/mmu.c | 5 +++++
>>   fs/Kconfig          | 2 +-
>>   2 files changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 5d37e461c41f..967b01ce468d 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -23,6 +23,7 @@
>>   #include <linux/mm.h>
>>   #include <linux/vmalloc.h>
>>   #include <linux/set_memory.h>
>> +#include <linux/hugetlb.h>
>>   
>>   #include <asm/barrier.h>
>>   #include <asm/cputype.h>
>> @@ -1134,6 +1135,10 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>>   	pmd_t *pmdp;
>>   
>>   	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
>> +
>> +	if (is_hugetlb_free_vmemmap_enabled() && !altmap)
>> +		return vmemmap_populate_basepages(start, end, node, altmap);
> 
> Not considering the fact that this will force the kernel to have only
> base page size mapping for vmemmap (unless altmap is also requested)
> which might reduce the performance, it also enables vmemmap mapping to
> be teared down or build up at runtime which could potentially collide
> with other kernel page table walkers like ptdump or memory hotremove
> operation ! How those possible collisions are protected right now ?

Hi Anshuman,

Memory hotremove is not an issue IIRC. At the time memory is removed, 
all huge pages either have been migrated away or dissolved; the vmemmap 
is stable.

vmemmap access (accessing the memmap via a virtual address) itself is 
not an issue. Manually walking (vmemmap) page tables might behave 
differently, not sure if ptdump would require any synchronization.

> 
> Does not this vmemmap operation increase latency for HugeTLB usage ?
> Should not this runtime enablement also take into account some other
> qualifying information apart from potential memory save from struct
> page areas. Just wondering.

That's one of the reasons why it explicitly has to be enabled by an admin.

-- 
Thanks,

David / dhildenb

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