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

[Mike Rapoport]

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 (thatâs 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.
 
> 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"

> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.