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SubjectRe: [PATCH 00/13] mm: Asynchronous + multithreaded memmap init for ZONE_DEVICE
On Wed 04-07-18 23:49:02, Dan Williams wrote:
> In order to keep pfn_to_page() a simple offset calculation the 'struct
> page' memmap needs to be mapped and initialized in advance of any usage
> of a page. This poses a problem for large memory systems as it delays
> full availability of memory resources for 10s to 100s of seconds.
>
> For typical 'System RAM' the problem is mitigated by the fact that large
> memory allocations tend to happen after the kernel has fully initialized
> and userspace services / applications are launched. A small amount, 2GB
> of memory, is initialized up front. The remainder is initialized in the
> background and freed to the page allocator over time.
>
> Unfortunately, that scheme is not directly reusable for persistent
> memory and dax because userspace has visibility to the entire resource
> pool and can choose to access any offset directly at its choosing. In
> other words there is no allocator indirection where the kernel can
> satisfy requests with arbitrary pages as they become initialized.
>
> That said, we can approximate the optimization by performing the
> initialization in the background, allow the kernel to fully boot the
> platform, start up pmem block devices, mount filesystems in dax mode,
> and only incur the delay at the first userspace dax fault.
>
> With this change an 8 socket system was observed to initialize pmem
> namespaces in ~4 seconds whereas it was previously taking ~4 minutes.
>
> These patches apply on top of the HMM + devm_memremap_pages() reworks
> [1]. Andrew, once the reviews come back, please consider this series for
> -mm as well.
>
> [1]: https://lkml.org/lkml/2018/6/19/108

One question: Why not (in addition to background initialization) have
->direct_access() initialize a block of struct pages around the pfn it
needs if it finds it's not initialized yet? That would make devices usable
immediately without waiting for init to complete...

Honza
>
> ---
>
> Dan Williams (9):
> mm: Plumb dev_pagemap instead of vmem_altmap to memmap_init_zone()
> mm: Enable asynchronous __add_pages() and vmemmap_populate_hugepages()
> mm: Teach memmap_init_zone() to initialize ZONE_DEVICE pages
> mm: Multithread ZONE_DEVICE initialization
> mm: Allow an external agent to wait for memmap initialization
> filesystem-dax: Make mount time pfn validation a debug check
> libnvdimm, pmem: Initialize the memmap in the background
> device-dax: Initialize the memmap in the background
> libnvdimm, namespace: Publish page structure init state / control
>
> Huaisheng Ye (4):
> nvdimm/pmem: check the validity of the pointer pfn
> nvdimm/pmem-dax: check the validity of the pointer pfn
> s390/block/dcssblk: check the validity of the pointer pfn
> fs/dax: Assign NULL to pfn of dax_direct_access if useless
>
>
> arch/ia64/mm/init.c | 5 +
> arch/powerpc/mm/mem.c | 5 +
> arch/s390/mm/init.c | 8 +
> arch/sh/mm/init.c | 5 +
> arch/x86/mm/init_32.c | 8 +
> arch/x86/mm/init_64.c | 27 +++--
> drivers/dax/Kconfig | 10 ++
> drivers/dax/dax-private.h | 2
> drivers/dax/device-dax.h | 2
> drivers/dax/device.c | 16 +++
> drivers/dax/pmem.c | 5 +
> drivers/dax/super.c | 64 +++++++-----
> drivers/nvdimm/nd.h | 2
> drivers/nvdimm/pfn_devs.c | 54 ++++++++--
> drivers/nvdimm/pmem.c | 17 ++-
> drivers/nvdimm/pmem.h | 1
> drivers/s390/block/dcssblk.c | 5 +
> fs/dax.c | 10 +-
> include/linux/memmap_async.h | 55 ++++++++++
> include/linux/memory_hotplug.h | 18 ++-
> include/linux/memremap.h | 31 ++++++
> include/linux/mm.h | 8 +
> kernel/memremap.c | 85 ++++++++-------
> mm/memory_hotplug.c | 73 ++++++++++---
> mm/page_alloc.c | 215 +++++++++++++++++++++++++++++++++------
> mm/sparse-vmemmap.c | 56 ++++++++--
> tools/testing/nvdimm/pmem-dax.c | 11 ++
> 27 files changed, 610 insertions(+), 188 deletions(-)
> create mode 100644 include/linux/memmap_async.h
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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