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Subject[RFC PATCH 00/31] Generating physically contiguous memory after page allocation
Date
From: Zi Yan <ziy@nvidia.com>

Hi all,

This patchset produces physically contiguous memory by moving in-use pages
without allocating any new pages. It targets two scenarios that complements
khugepaged use cases: 1) avoiding page reclaim and memory compaction when the
system is under memory pressure because this patchset does not allocate any new
pages, 2) generating pages larger than 2^MAX_ORDER without changing the buddy
allocator.

To demonstrate its use, I add very basic 1GB THP support and enable promoting
512 2MB THPs to a 1GB THP in my patchset. Promoting 512 4KB pages to a 2MB
THP is also implemented.

The patches are on top of v5.0-rc5. They are posted as part of my upcoming
LSF/MM proposal.

Motivation 
---- 

The goal of this patchset is to provide alternative way of generating physically
contiguous memory and making it available as arbitrary sized large pages. This
patchset generates physically contiguous memory/arbitrary size pages after pages
are allocated by moving virtually-contiguous pages to become physically
contiguous at any size, thus it does not require changes to memory allocators.
On the other hand, it works only for moveable pages, so it also faces the same
fragmentation issues as memory compaction, i.e., if non-moveable pages spread
across the entire memory, this patchset can only generate contiguity between
any two non-moveable pages. 

Large pages and physically contiguous memory are important to devices, such as
GPUs, FPGAs, NICs and RDMA controllers, because they can often achieve better
performance when operating on large pages. The same can be said of CPU
performance, of course, but there is an important difference: GPUs and
high-throughput devices often take a more severe performance hit, in the event
of a TLB miss and subsequent page table walks, as compared to a CPU. The effect
is sufficiently large that such devices *really* want a highly reliable way to
allocate large pages to minimize the number of potential TLB misses and the time
spent on the induced page table walks. 

Vendors (like Oracle, Mellanox, IBM, NVIDIA) are interested in generating
physically contiguous memory beyond THP sizes and looking for solutions [1],[2],[3].
This patchset provides an alternative approach, compared to allocating
physically contiguous memory at page allocation time, to generating physically
contiguous memory after pages are allocated. This approach can avoid page
reclaim and memory compaction, which happen during the process of page
allocation, but still produces comparable physically contiguous memory. 

In terms of THPs, it helps, but we are interested in even larger contiguous
ranges (or page size support) to further reduce the address translation overheads.
With this patchset, we can generate pages larger than PMD-level THPs without
requiring MAX_ORDER changes in the buddy allocators. 


Patch structure 
---- 

The patchset I developed to generate physically contiguous memory/arbitrary
sized pages merely moves pages around. There are three components in this
patchset:

1) a new page migration mechanism, called exchange pages, that exchanges the
content of two in-use pages instead of performing two back-to-back page
migration. It saves on overheads and avoids page reclaim and memory compaction
in the page allocation path, although it is not strictly required if enough
free memory is available in the system.

2) a new mechanism that utilizes both page migration and exchange pages to
produce physically contiguous memory/arbitrary sized pages without allocating
any new pages, unlike what khugepaged does. It works on per-VMA basis, creating
physically contiguous memory out of each VMA, which is virtually contiguous.
A simple range tree is used to ensure no two VMAs are overlapping with each
other in the physical address space.

3) a use case of the new physically contiguous memory producing mechanism that
generates 1GB THPs by migrating and exchanging pages and promoting 512
contiguous 2MB THPs to a 1GB THP, although even larger physically contiguous
memory ranges can be generated. The 1GB THP implement is very basic, which can
handle 1GB THP faults when buddy allocator is modified to allocate 1GB pages,
support 1GB THP split to 2MB THP and in-place promotion from 2MB THP to 1GB THP,
and PMD/PTE-mapped 1GB THP. These are not fully tested.


[1] https://lwn.net/Articles/736170/ 
[2] https://lwn.net/Articles/753167/ 
[3] https://blogs.nvidia.com/blog/2018/06/08/worlds-fastest-exascale-ai-supercomputer-summit/ 

Zi Yan (31):
mm: migrate: Add exchange_pages to exchange two lists of pages.
mm: migrate: Add THP exchange support.
mm: migrate: Add tmpfs exchange support.
mm: add mem_defrag functionality.
mem_defrag: split a THP if either src or dst is THP only.
mm: Make MAX_ORDER configurable in Kconfig for buddy allocator.
mm: deallocate pages with order > MAX_ORDER.
mm: add pagechain container for storing multiple pages.
mm: thp: 1GB anonymous page implementation.
mm: proc: add 1GB THP kpageflag.
mm: debug: print compound page order in dump_page().
mm: stats: Separate PMD THP and PUD THP stats.
mm: thp: 1GB THP copy on write implementation.
mm: thp: handling 1GB THP reference bit.
mm: thp: add 1GB THP split_huge_pud_page() function.
mm: thp: check compound_mapcount of PMD-mapped PUD THPs at free time.
mm: thp: split properly PMD-mapped PUD THP to PTE-mapped PUD THP.
mm: page_vma_walk: teach it about PMD-mapped PUD THP.
mm: thp: 1GB THP support in try_to_unmap().
mm: thp: split 1GB THPs at page reclaim.
mm: thp: 1GB zero page shrinker.
mm: thp: 1GB THP follow_p*d_page() support.
mm: support 1GB THP pagemap support.
sysctl: add an option to only print the head page virtual address.
mm: thp: add a knob to enable/disable 1GB THPs.
mm: thp: promote PTE-mapped THP to PMD-mapped THP.
mm: thp: promote PMD-mapped PUD pages to PUD-mapped PUD pages.
mm: vmstats: add page promotion stats.
mm: madvise: add madvise options to split PMD and PUD THPs.
mm: mem_defrag: thp: PMD THP and PUD THP in-place promotion support.
sysctl: toggle to promote PUD-mapped 1GB THP or not.

arch/x86/Kconfig | 15 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
arch/x86/include/asm/pgalloc.h | 69 +
arch/x86/include/asm/pgtable.h | 20 +
arch/x86/include/asm/sparsemem.h | 4 +-
arch/x86/mm/pgtable.c | 38 +
drivers/base/node.c | 3 +
fs/exec.c | 4 +
fs/proc/meminfo.c | 2 +
fs/proc/page.c | 2 +
fs/proc/task_mmu.c | 47 +-
include/asm-generic/pgtable.h | 110 +
include/linux/huge_mm.h | 78 +-
include/linux/khugepaged.h | 1 +
include/linux/ksm.h | 5 +
include/linux/mem_defrag.h | 60 +
include/linux/memcontrol.h | 5 +
include/linux/mm.h | 34 +
include/linux/mm_types.h | 5 +
include/linux/mmu_notifier.h | 13 +
include/linux/mmzone.h | 1 +
include/linux/page-flags.h | 79 +-
include/linux/pagechain.h | 73 +
include/linux/rmap.h | 10 +-
include/linux/sched/coredump.h | 4 +
include/linux/swap.h | 2 +
include/linux/syscalls.h | 3 +
include/linux/vm_event_item.h | 33 +
include/uapi/asm-generic/mman-common.h | 15 +
include/uapi/linux/kernel-page-flags.h | 2 +
kernel/events/uprobes.c | 4 +-
kernel/fork.c | 14 +
kernel/sysctl.c | 101 +-
mm/Makefile | 2 +
mm/compaction.c | 17 +-
mm/debug.c | 8 +-
mm/exchange.c | 878 +++++++
mm/filemap.c | 8 +
mm/gup.c | 60 +-
mm/huge_memory.c | 3360 ++++++++++++++++++++----
mm/hugetlb.c | 4 +-
mm/internal.h | 46 +
mm/khugepaged.c | 7 +-
mm/ksm.c | 39 +-
mm/madvise.c | 121 +
mm/mem_defrag.c | 1941 ++++++++++++++
mm/memcontrol.c | 13 +
mm/memory.c | 55 +-
mm/migrate.c | 14 +-
mm/mmap.c | 29 +
mm/page_alloc.c | 108 +-
mm/page_vma_mapped.c | 129 +-
mm/pgtable-generic.c | 78 +-
mm/rmap.c | 283 +-
mm/swap.c | 38 +
mm/swap_slots.c | 2 +
mm/swapfile.c | 4 +-
mm/userfaultfd.c | 2 +-
mm/util.c | 7 +
mm/vmscan.c | 55 +-
mm/vmstat.c | 32 +
61 files changed, 7452 insertions(+), 745 deletions(-)
create mode 100644 include/linux/mem_defrag.h
create mode 100644 include/linux/pagechain.h
create mode 100644 mm/exchange.c
create mode 100644 mm/mem_defrag.c

--
2.20.1

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