Hi Justin,

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

Hi David

quoted

quoted

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

only

quoted

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

quoted

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

quoted

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

are both

quoted

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
 
There is still large gap with ARM64_64K_PAGES, though.

As for SPARSEMEM without VMEMMAP, are there actual benefits to use it?

quoted

In the long term we might want to rework the memory block device model
(eventually supporting old/new as discussed with Michal some time ago
using a kernel parameter), dropping the fixed sizes

Has this been posted to Linux mm maillist? Sorry, searched and didn't find it.


--
Cheers,
Justin (Jia He)

quoted

- allowing sizes / addresses aligned with subsection size
- drastically reducing the number of devices for boot memory to only a
hand full (e.g., one per resource / DIMM we can actually unplug again.

Long story short, I don’t like this hack.

quoted

This patch series (mainly patch6/6) is based on the fixing patch, ~v5.8-

rc5 [2].

quoted

[1] https://lkml.org/lkml/2019/6/19/67
[2] https://lkml.org/lkml/2020/7/8/1546
Jia He (6):
 mm/memory_hotplug: remove redundant memory block size alignment check
 resource: export find_next_iomem_res() helper
 mm/memory_hotplug: allow pmem kmem not to align with memory_block_size
 mm/page_alloc: adjust the start,end in dax pmem kmem case
 device-dax: relax the memblock size alignment for kmem_start
 arm64: fall back to vmemmap_populate_basepages if not aligned  with
   PMD_SIZE

arch/arm64/mm/mmu.c    |  4 ++++
drivers/base/memory.c  | 24 ++++++++++++++++--------
drivers/dax/kmem.c     | 22 +++++++++++++---------
include/linux/ioport.h |  3 +++
kernel/resource.c      |  3 ++-
mm/memory_hotplug.c    | 39 ++++++++++++++++++++++++++++++++++++++-
mm/page_alloc.c        | 14 ++++++++++++++
7 files changed, 90 insertions(+), 19 deletions(-)

--
2.17.1

-- 
Sincerely yours,
Mike.

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