Re: [RFC PATCH 0/6] decrease unnecessary gap due to pmem kmem alignment

[David Hildenbrand <david@redhat.com>]

On 29.07.20 15:00, Mike Rapoport wrote:
> On Wed, Jul 29, 2020 at 11:35:20AM +0200, David Hildenbrand wrote:
>> On 29.07.20 11:31, Mike Rapoport wrote:
>>> Hi Justin,
>>>
>>> On Wed, Jul 29, 2020 at 08:27:58AM +0000, Justin He wrote:
>>>> Hi David
>>>>>>
>>>>>> Without this series, if qemu creates a 4G bytes nvdimm device, we can
>>>>> only
>>>>>> use 2G bytes for dax pmem(kmem) in the worst case.
>>>>>> e.g.
>>>>>> 240000000-33fdfffff : Persistent Memory
>>>>>> We can only use the memblock between [240000000, 2ffffffff] due to the
>>>>> hard
>>>>>> limitation. It wastes too much memory space.
>>>>>>
>>>>>> Decreasing the SECTION_SIZE_BITS on arm64 might be an alternative, but
>>>>> there
>>>>>> are too many concerns from other constraints, e.g. PAGE_SIZE, hugetlb,
>>>>>> SPARSEMEM_VMEMMAP, page bits in struct page ...
>>>>>>
>>>>>> Beside decreasing the SECTION_SIZE_BITS, we can also relax the kmem
>>>>> alignment
>>>>>> with memory_block_size_bytes().
>>>>>>
>>>>>> Tested on arm64 guest and x86 guest, qemu creates a 4G pmem device. dax
>>>>> pmem
>>>>>> can be used as ram with smaller gap. Also the kmem hotplug add/remove
>>>>> are both
>>>>>> tested on arm64/x86 guest.
>>>>>>
>>>>>
>>>>> Hi,
>>>>>
>>>>> I am not convinced this use case is worth such hacks (that’s what it is)
>>>>> for now. On real machines pmem is big - your example (losing 50% is
>>>>> extreme).
>>>>>
>>>>> I would much rather want to see the section size on arm64 reduced. I
>>>>> remember there were patches and that at least with a base page size of 4k
>>>>> it can be reduced drastically (64k base pages are more problematic due to
>>>>> the ridiculous THP size of 512M). But could be a section size of 512 is
>>>>> possible on all configs right now.
>>>>
>>>> Yes, I once investigated how to reduce section size on arm64 thoughtfully:
>>>> There are many constraints for reducing SECTION_SIZE_BITS
>>>> 1. Given page->flags bits is limited, SECTION_SIZE_BITS can't be reduced too
>>>>    much.
>>>> 2. Once CONFIG_SPARSEMEM_VMEMMAP is enabled, section id will not be counted
>>>>    into page->flags.
>>>> 3. MAX_ORDER depends on SECTION_SIZE_BITS 
>>>>  - 3.1 mmzone.h
>>>> #if (MAX_ORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS
>>>> #error Allocator MAX_ORDER exceeds SECTION_SIZE
>>>> #endif
>>>>  - 3.2 hugepage_init()
>>>> MAYBE_BUILD_BUG_ON(HPAGE_PMD_ORDER >= MAX_ORDER);
>>>>
>>>> Hence when ARM64_4K_PAGES && CONFIG_SPARSEMEM_VMEMMAP are enabled,
>>>> SECTION_SIZE_BITS can be reduced to 27.
>>>> But when ARM64_64K_PAGES, given 3.2, MAX_ORDER > 29-16 = 13.
>>>> Given 3.1 SECTION_SIZE_BITS >= MAX_ORDER+15 > 28. So SECTION_SIZE_BITS can not
>>>> be reduced to 27.
>>>>
>>>> In one word, if we considered to reduce SECTION_SIZE_BITS on arm64, the Kconfig
>>>> might be very complicated,e.g. we still need to consider the case for
>>>> ARM64_16K_PAGES.
>>>
>>> It is not necessary to pollute Kconfig with that.
>>> arch/arm64/include/asm/sparesemem.h can have something like
>>>
>>> #ifdef CONFIG_ARM64_64K_PAGES
>>> #define SPARSE_SECTION_SIZE 29
>>> #elif defined(CONFIG_ARM16K_PAGES)
>>> #define SPARSE_SECTION_SIZE 28
>>> #elif defined(CONFIG_ARM4K_PAGES)
>>> #define SPARSE_SECTION_SIZE 27
>>> #else
>>> #error
>>> #endif
>>
>> ack
>>
>>>  
>>> There is still large gap with ARM64_64K_PAGES, though.
>>>
>>> As for SPARSEMEM without VMEMMAP, are there actual benefits to use it?
>>
>> I was asking myself the same question a while ago and didn't really find
>> a compelling one.
> 
> Memory overhead for VMEMMAP is larger, especially for arm64 that knows
> how to free empty parts of the memory map with "classic" SPARSEMEM.

You mean the hole punching within section memmap? (which is why their
pfn_valid() implementation is special)

(I do wonder why that shouldn't work with VMEMMAP, or is it simply not
implemented?)

>  
>> I think it's always enabled as default (SPARSEMEM_VMEMMAP_ENABLE) and
>> would require config tweaks to even disable it.
> 
> Nope, it's right there in menuconfig,
> 
> "Memory Management options" -> "Sparse Memory virtual memmap"

Ah, good to know.


-- 
Thanks,

David / dhildenb
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