Date:   Thu, 20 Dec 2018 00:33:47 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jérôme Glisse <jglisse@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        Balbir Singh <bsingharora@...il.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [HMM-v25 07/19] mm/ZONE_DEVICE: new type of ZONE_DEVICE for
 unaddressable memory v5

On Wed, Aug 16, 2017 at 5:06 PM Jérôme Glisse <jglisse@...hat.com> wrote:
>
> HMM (heterogeneous memory management) need struct page to support migration
> from system main memory to device memory.  Reasons for HMM and migration to
> device memory is explained with HMM core patch.
>
> This patch deals with device memory that is un-addressable memory (ie CPU
> can not access it). Hence we do not want those struct page to be manage
> like regular memory. That is why we extend ZONE_DEVICE to support different
> types of memory.
>
> A persistent memory type is define for existing user of ZONE_DEVICE and a
> new device un-addressable type is added for the un-addressable memory type.
> There is a clear separation between what is expected from each memory type
> and existing user of ZONE_DEVICE are un-affected by new requirement and new
> use of the un-addressable type. All specific code path are protect with
> test against the memory type.
>
> Because memory is un-addressable we use a new special swap type for when
> a page is migrated to device memory (this reduces the number of maximum
> swap file).
>
> The main two additions beside memory type to ZONE_DEVICE is two callbacks.
> First one, page_free() is call whenever page refcount reach 1 (which means
> the page is free as ZONE_DEVICE page never reach a refcount of 0). This
> allow device driver to manage its memory and associated struct page.
>
> The second callback page_fault() happens when there is a CPU access to
> an address that is back by a device page (which are un-addressable by the
> CPU). This callback is responsible to migrate the page back to system
> main memory. Device driver can not block migration back to system memory,
> HMM make sure that such page can not be pin into device memory.
>
> If device is in some error condition and can not migrate memory back then
> a CPU page fault to device memory should end with SIGBUS.
>
> Changed since v4:
>   - s/DEVICE_PUBLIC/DEVICE_HOST (to free DEVICE_PUBLIC for HMM-CDM)
> Changed since v3:
>   - fix comments that was still using UNADDRESSABLE as keyword
>   - kernel configuration simplification
> Changed since v2:
>   - s/DEVICE_UNADDRESSABLE/DEVICE_PRIVATE
> Changed since v1:
>   - rename to device private memory (from device unaddressable)
>
> Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
> Acked-by: Dan Williams <dan.j.williams@...el.com>
> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
[..]
>  fs/proc/task_mmu.c       |  7 +++++
>  include/linux/ioport.h   |  1 +
>  include/linux/memremap.h | 73 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/mm.h       | 12 ++++++++
>  include/linux/swap.h     | 24 ++++++++++++++--
>  include/linux/swapops.h  | 68 ++++++++++++++++++++++++++++++++++++++++++++
>  kernel/memremap.c        | 34 ++++++++++++++++++++++
>  mm/Kconfig               | 11 +++++++-
>  mm/memory.c              | 61 ++++++++++++++++++++++++++++++++++++++++
>  mm/memory_hotplug.c      | 10 +++++--
>  mm/mprotect.c            | 14 ++++++++++
>  11 files changed, 309 insertions(+), 6 deletions(-)
>
[..]
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 93416196ba64..8e164ec9eed0 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -4,6 +4,8 @@
>  #include <linux/ioport.h>
>  #include <linux/percpu-refcount.h>
>
> +#include <asm/pgtable.h>
> +

So it turns out, over a year later, that this include was a mistake
and makes the build fragile.

>  struct resource;
>  struct device;
>
[..]
> +typedef int (*dev_page_fault_t)(struct vm_area_struct *vma,
> +                               unsigned long addr,
> +                               const struct page *page,
> +                               unsigned int flags,
> +                               pmd_t *pmdp);

I recently included this file somewhere that did not have a pile of
other mm headers included and 0day reports:

  In file included from arch/m68k/include/asm/pgtable_mm.h:148:0,
                    from arch/m68k/include/asm/pgtable.h:5,
                    from include/linux/memremap.h:7,
                    from drivers//dax/bus.c:3:
   arch/m68k/include/asm/motorola_pgtable.h: In function 'pgd_offset':
>> arch/m68k/include/asm/motorola_pgtable.h:199:11: error: dereferencing pointer to incomplete type 'const struct mm_struct'
     return mm->pgd + pgd_index(address);
              ^~
I assume this pulls in the entirety of pgtable.h just to get the pmd_t
definition?

> +typedef void (*dev_page_free_t)(struct page *page, void *data);
> +
>  /**
>   * struct dev_pagemap - metadata for ZONE_DEVICE mappings
> + * @page_fault: callback when CPU fault on an unaddressable device page
> + * @page_free: free page callback when page refcount reaches 1
>   * @altmap: pre-allocated/reserved memory for vmemmap allocations
>   * @res: physical address range covered by @ref
>   * @ref: reference count that pins the devm_memremap_pages() mapping
>   * @dev: host device of the mapping for debug
> + * @data: private data pointer for page_free()
> + * @type: memory type: see MEMORY_* in memory_hotplug.h
>   */
>  struct dev_pagemap {
> +       dev_page_fault_t page_fault;

Rather than try to figure out how to forward declare pmd_t, how about
just move dev_page_fault_t out of the generic dev_pagemap and into the
HMM specific container structure? This should be straightfoward on top
of the recent refactor.

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