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

[David Hildenbrand]

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.

I think it's always enabled as default (SPARSEMEM_VMEMMAP_ENABLE) and
would require config tweaks to even disable it.

-- 
Thanks,

David / dhildenb