On Wed, Jul 29, 2020 at 11:35:20AM +0200, David Hildenbrand wrote:

On 29.07.20 11:31, Mike Rapoport wrote:

quoted

Hi Justin,

On Wed, Jul 29, 2020 at 08:27:58AM +0000, Justin He wrote:

quoted

Hi David

quoted

quoted

Without this series, if qemu creates a 4G bytes nvdimm device, we can

only

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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

quoted

limitation. It wastes too much memory space.

Decreasing the SECTION_SIZE_BITS on arm64 might be an alternative, but

there

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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

quoted

with memory_block_size_bytes().

Tested on arm64 guest and x86 guest, qemu creates a 4G pmem device. dax

pmem

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can be used as ram with smaller gap. Also the kmem hotplug add/remove

are both

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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

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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.

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