Re: [HMM 03/15] mm/unaddressable-memory: new type of ZONE_DEVICE for unaddressable memory

[Dan Williams]

On Fri, Apr 21, 2017 at 8:30 PM, JÃrÃme Glisse <jglisse@xxxxxxxxxx> 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.
>
> Signed-off-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> ---
>  fs/proc/task_mmu.c       |  7 +++++
>  include/linux/ioport.h   |  1 +
>  include/linux/memremap.h | 82 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/swap.h     | 24 ++++++++++++--
>  include/linux/swapops.h  | 68 +++++++++++++++++++++++++++++++++++++++
>  kernel/memremap.c        | 43 ++++++++++++++++++++++++-
>  mm/Kconfig               | 13 ++++++++
>  mm/memory.c              | 61 +++++++++++++++++++++++++++++++++++
>  mm/memory_hotplug.c      | 10 ++++--
>  mm/mprotect.c            | 14 +++++++++
>  10 files changed, 317 insertions(+), 6 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index f0c8b33..a12ba94 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -542,6 +542,8 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
>                         }
>                 } else if (is_migration_entry(swpent))
>                         page = migration_entry_to_page(swpent);
> +               else if (is_device_entry(swpent))
> +                       page = device_entry_to_page(swpent);
>         } else if (unlikely(IS_ENABLED(CONFIG_SHMEM) && mss->check_shmem_swap
>                                                         && pte_none(*pte))) {
>                 page = find_get_entry(vma->vm_file->f_mapping,
> @@ -704,6 +706,8 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
>
>                 if (is_migration_entry(swpent))
>                         page = migration_entry_to_page(swpent);
> +               else if (is_device_entry(swpent))
> +                       page = device_entry_to_page(swpent);
>         }
>         if (page) {
>                 int mapcount = page_mapcount(page);
> @@ -1196,6 +1200,9 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
>                 flags |= PM_SWAP;
>                 if (is_migration_entry(entry))
>                         page = migration_entry_to_page(entry);
> +
> +               if (is_device_entry(entry))
> +                       page = device_entry_to_page(entry);
>         }
>
>         if (page && !PageAnon(page))
> diff --git a/include/linux/ioport.h b/include/linux/ioport.h
> index 6230064..ec619dc 100644
> --- a/include/linux/ioport.h
> +++ b/include/linux/ioport.h
> @@ -130,6 +130,7 @@ enum {
>         IORES_DESC_ACPI_NV_STORAGE              = 3,
>         IORES_DESC_PERSISTENT_MEMORY            = 4,
>         IORES_DESC_PERSISTENT_MEMORY_LEGACY     = 5,
> +       IORES_DESC_DEVICE_MEMORY_UNADDRESSABLE  = 6,
>  };
>
>  /Fput* helpers to define resources */
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 9341619..365fb4e 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -35,24 +35,101 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
>  }
>  #endif
>
> +/*
> + * Specialize ZONE_DEVICE memory into multiple types each having differents
> + * usage.
> + *
> + * MEMORY_DEVICE_PERSISTENT:
> + * Persistent device memory (pmem): struct page might be allocated in different
> + * memory and architecture might want to perform special actions. It is similar
> + * to regular memory, in that the CPU can access it transparently. However,
> + * it is likely to have different bandwidth and latency than regular memory.
> + * See Documentation/nvdimm/nvdimm.txt for more information.
> + *
> + * MEMORY_DEVICE_UNADDRESSABLE:
> + * Device memory that is not directly addressable by the CPU: CPU can neither
> + * read nor write _UNADDRESSABLE memory. In this case, we do still have struct
> + * pages backing the device memory. Doing so simplifies the implementation, but
> + * it is important to remember that there are certain points at which the struct
> + * page must be treated as an opaque object, rather than a "normal" struct page.
> + * A more complete discussion of unaddressable memory may be found in
> + * include/linux/hmm.h and Documentation/vm/hmm.txt.
> + */
> +enum memory_type {
> +       MEMORY_DEVICE_PERSISTENT = 0,
> +       MEMORY_DEVICE_UNADDRESSABLE,
> +};

Ok, this is a bikeshed, but I think it is important. I think these
should be called MEMORY_DEVICE_PUBLIC and MEMORY_DEVICE_PRIVATE. The
reason is that persistence has nothing to do with the code paths that
deal with the pmem use case of ZONE_DEVICE. The only property the mm
cares about is that the address range behaves the same as host memory
for dma and cpu accesses. The "unaddressable" designation always
confuses me because a memory range isn't memory if it's
"unaddressable". It is addressable, it's just "private" to the device.

> +
> +/*
> + * For MEMORY_DEVICE_UNADDRESSABLE we use ZONE_DEVICE and extend it with two
> + * callbacks:
> + *   page_fault()
> + *   page_free()
> + *
> + * Additional notes about MEMORY_DEVICE_UNADDRESSABLE may be found in
> + * include/linux/hmm.h and Documentation/vm/hmm.txt. There is also a brief
> + * explanation in include/linux/memory_hotplug.h.
> + *
> + * The page_fault() callback must migrate page back, from device memory to
> + * system memory, so that the CPU can access it. This might fail for various
> + * reasons (device issues,  device have been unplugged, ...). When such error
> + * conditions happen, the page_fault() callback must return VM_FAULT_SIGBUS and
> + * set the CPU page table entry to "poisoned".
> + *
> + * Note that because memory cgroup charges are transferred to the device memory,
> + * this should never fail due to memory restrictions. However, allocation
> + * of a regular system page might still fail because we are out of memory. If
> + * that happens, the page_fault() callback must return VM_FAULT_OOM.
> + *
> + * The page_fault() callback can also try to migrate back multiple pages in one
> + * chunk, as an optimization. It must, however, prioritize the faulting address
> + * over all the others.
> + *
> + *
> + * The page_free() callback is called once the page refcount reaches 1
> + * (ZONE_DEVICE pages never reach 0 refcount unless there is a refcount bug.
> + * This allows the device driver to implement its own memory management.)
> + */
> +typedef int (*dev_page_fault_t)(struct vm_area_struct *vma,
> +                               unsigned long addr,
> +                               struct page *page,
> +                               unsigned int flags,
> +                               pmd_t *pmdp);
> +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;
> +       dev_page_free_t page_free;
>         struct vmem_altmap *altmap;
>         const struct resource *res;
>         struct percpu_ref *ref;
>         struct device *dev;
> +       void *data;
> +       enum memory_type type;
>  };

I think the only attribute that belongs here is @type, the rest are
specific to the device_private case.

struct dev_private_pagemap {
    dev_page_fault_t page_fault;
    dev_page_free_t page_free;
    void *data;
    struct dev_pagemap pgmap;
};

As Logan pointed out we can kill the internal struct page_map in
kernel/memremap.c and let devm_memremap_pages() take a "struct
dev_pagemap *" pointer. That way when future dev_pagemap users come
along they can wrap the core structure with whatever specific data
they need for their use case.

>
>  #ifdef CONFIG_ZONE_DEVICE
>  void *devm_memremap_pages(struct device *dev, struct resource *res,
>                 struct percpu_ref *ref, struct vmem_altmap *altmap);
>  struct dev_pagemap *find_dev_pagemap(resource_size_t phys);
> +
> +static inline bool is_device_unaddressable_page(const struct page *page)
> +{
> +       /* See MEMORY_DEVICE_UNADDRESSABLE in include/linux/memory_hotplug.h */
> +       return ((page_zonenum(page) == ZONE_DEVICE) &&
> +               (page->pgmap->type == MEMORY_DEVICE_UNADDRESSABLE));
> +}
>  #else
>  static inline void *devm_memremap_pages(struct device *dev,
>                 struct resource *res, struct percpu_ref *ref,
> @@ -71,6 +148,11 @@ static inline struct dev_pagemap *find_dev_pagemap(resource_size_t phys)
>  {
>         return NULL;
>  }
> +
> +static inline bool is_device_unaddressable_page(const struct page *page)
> +{
> +       return false;
> +}
>  #endif
>
>  /**
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index ba58824..52a137d 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -51,6 +51,23 @@ static inline int current_is_kswapd(void)
>   */
>
>  /*
> + * Unaddressable device memory support. See include/linux/hmm.h and
> + * Documentation/vm/hmm.txt. Short description is we need struct pages for
> + * device memory that is unaddressable (inaccessible) by CPU, so that we can
> + * migrate part of a process memory to device memory.
> + *
> + * When a page is migrated from CPU to device, we set the CPU page table entry
> + * to a special SWP_DEVICE_* entry.
> + */
> +#ifdef CONFIG_DEVICE_UNADDRESSABLE
> +#define SWP_DEVICE_NUM 2
> +#define SWP_DEVICE_WRITE (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM)
> +#define SWP_DEVICE_READ (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM+1)
> +#else
> +#define SWP_DEVICE_NUM 0
> +#endif
> +
> +/*
>   * NUMA node memory migration support
>   */
>  #ifdef CONFIG_MIGRATION
> @@ -72,7 +89,8 @@ static inline int current_is_kswapd(void)
>  #endif
>
>  #define MAX_SWAPFILES \
> -       ((1 << MAX_SWAPFILES_SHIFT) - SWP_MIGRATION_NUM - SWP_HWPOISON_NUM)
> +       ((1 << MAX_SWAPFILES_SHIFT) - SWP_DEVICE_NUM - \
> +       SWP_MIGRATION_NUM - SWP_HWPOISON_NUM)
>
>  /*
>   * Magic header for a swap area. The first part of the union is
> @@ -432,8 +450,8 @@ static inline void show_swap_cache_info(void)
>  {
>  }
>
> -#define free_swap_and_cache(swp)       is_migration_entry(swp)
> -#define swapcache_prepare(swp)         is_migration_entry(swp)
> +#define free_swap_and_cache(e) (is_migration_entry(e) || is_device_entry(e))
> +#define swapcache_prepare(e) (is_migration_entry(e) || is_device_entry(e))
>
>  static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask)
>  {
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 5c3a5f3..960eb6b 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -100,6 +100,74 @@ static inline void *swp_to_radix_entry(swp_entry_t entry)
>         return (void *)(value | RADIX_TREE_EXCEPTIONAL_ENTRY);
>  }
>
> +#if IS_ENABLED(CONFIG_DEVICE_UNADDRESSABLE)
> +static inline swp_entry_t make_device_entry(struct page *page, bool write)
> +{
> +       return swp_entry(write ? SWP_DEVICE_WRITE : SWP_DEVICE_READ,
> +                        page_to_pfn(page));
> +}
> +
> +static inline bool is_device_entry(swp_entry_t entry)
> +{
> +       int type = swp_type(entry);
> +       return type == SWP_DEVICE_READ || type == SWP_DEVICE_WRITE;
> +}
> +
> +static inline void make_device_entry_read(swp_entry_t *entry)
> +{
> +       *entry = swp_entry(SWP_DEVICE_READ, swp_offset(*entry));
> +}
> +
> +static inline bool is_write_device_entry(swp_entry_t entry)
> +{
> +       return unlikely(swp_type(entry) == SWP_DEVICE_WRITE);
> +}
> +
> +static inline struct page *device_entry_to_page(swp_entry_t entry)
> +{
> +       return pfn_to_page(swp_offset(entry));
> +}
> +
> +int device_entry_fault(struct vm_area_struct *vma,
> +                      unsigned long addr,
> +                      swp_entry_t entry,
> +                      unsigned int flags,
> +                      pmd_t *pmdp);


The use of the "device" term is ambiguous. Since these changes are
specific to the MEMORY_DEVICE_PRIVATE can we reflect "private" in the
name? Change "device" to "devpriv" or "device_private"?


> +#else /* CONFIG_DEVICE_UNADDRESSABLE */
> +static inline swp_entry_t make_device_entry(struct page *page, bool write)
> +{
> +       return swp_entry(0, 0);
> +}
> +
> +static inline void make_device_entry_read(swp_entry_t *entry)
> +{
> +}
> +
> +static inline bool is_device_entry(swp_entry_t entry)
> +{
> +       return false;
> +}
> +
> +static inline bool is_write_device_entry(swp_entry_t entry)
> +{
> +       return false;
> +}
> +
> +static inline struct page *device_entry_to_page(swp_entry_t entry)
> +{
> +       return NULL;
> +}
> +
> +static inline int device_entry_fault(struct vm_area_struct *vma,
> +                                    unsigned long addr,
> +                                    swp_entry_t entry,
> +                                    unsigned int flags,
> +                                    pmd_t *pmdp)
> +{
> +       return VM_FAULT_SIGBUS;
> +}
> +#endif /* CONFIG_DEVICE_UNADDRESSABLE */
> +
>  #ifdef CONFIG_MIGRATION
>  static inline swp_entry_t make_migration_entry(struct page *page, int write)
>  {
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index 97ef676..dce3b29 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -18,6 +18,8 @@
>  #include <linux/io.h>
>  #include <linux/mm.h>
>  #include <linux/memory_hotplug.h>
> +#include <linux/swap.h>
> +#include <linux/swapops.h>
>
>  #ifndef ioremap_cache
>  /* temporary while we convert existing ioremap_cache users to memremap */
> @@ -194,12 +196,47 @@ void put_zone_device_page(struct page *page)
>          * ZONE_DEVICE page refcount should never reach 0 and never be freed
>          * to kernel memory allocator.
>          */
> -       page_ref_dec(page);
> +       int count = page_ref_dec_return(page);
> +
> +       /*
> +        * If refcount is 1 then page is freed and refcount is stable as nobody
> +        * holds a reference on the page.
> +        */
> +       if (page->pgmap->page_free && count == 1)
> +               page->pgmap->page_free(page, page->pgmap->data);


Reading this it isn't clear whether we are dealing with the public or
private case. I think this should be:

if (count == 1)
        zone_device_page_free(page);

...where zone_device_page_free() is a helper that is explicit about
the expectations of which ZONE_DEVICE types need to take action on a
'free' event.

static void zone_device_page_free(struct page *page)
{
        struct dev_private_pagemap *priv_pgmap;

        if (page->pgmap->type == MEMORY_DEVICE_PUBLIC)
                return;

        priv_pgmap = container_of(page->pgmap, typeof(*priv_pgmap), pgmap);
        priv_pgmap->page_free(page);
}

>
>         put_dev_pagemap(page->pgmap);
>  }
>  EXPORT_SYMBOL(put_zone_device_page);
>
> +#if IS_ENABLED(CONFIG_DEVICE_UNADDRESSABLE)
> +int device_entry_fault(struct vm_area_struct *vma,
> +                      unsigned long addr,
> +                      swp_entry_t entry,
> +                      unsigned int flags,
> +                      pmd_t *pmdp)
> +{
> +       struct page *page = device_entry_to_page(entry);
> +
> +       /*
> +        * The page_fault() callback must migrate page back to system memory
> +        * so that CPU can access it. This might fail for various reasons
> +        * (device issue, device was unsafely unplugged, ...). When such
> +        * error conditions happen, the callback must return VM_FAULT_SIGBUS.
> +        *
> +        * Note that because memory cgroup charges are accounted to the device
> +        * memory, this should never fail because of memory restrictions (but
> +        * allocation of regular system page might still fail because we are
> +        * out of memory).
> +        *
> +        * There is a more in-depth description of what that callback can and
> +        * cannot do, in include/linux/memremap.h
> +        */
> +       return page->pgmap->page_fault(vma, addr, page, flags, pmdp);
> +}
> +EXPORT_SYMBOL(device_entry_fault);
> +#endif /* CONFIG_DEVICE_UNADDRESSABLE */
> +
>  static void pgmap_radix_release(struct resource *res)
>  {
>         resource_size_t key, align_start, align_size, align_end;
> @@ -332,6 +369,10 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
>         }
>         pgmap->ref = ref;
>         pgmap->res = &page_map->res;
> +       pgmap->type = MEMORY_DEVICE_PERSISTENT;
> +       pgmap->page_fault = NULL;
> +       pgmap->page_free = NULL;
> +       pgmap->data = NULL;


This goes away if we convert to passing in the dev_pagemap, and I
think that is a useful cleanup.


>         mutex_lock(&pgmap_lock);
>         error = 0;
> diff --git a/mm/Kconfig b/mm/Kconfig
> index c89f472..ad4bd50 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -700,6 +700,19 @@ config ZONE_DEVICE
>
>           If FS_DAX is enabled, then say Y.
>
> +config DEVICE_UNADDRESSABLE
> +       bool "Unaddressable device memory (GPU memory, ...)"
> +       depends on X86_64
> +       depends on ZONE_DEVICE
> +       depends on MEMORY_HOTPLUG
> +       depends on MEMORY_HOTREMOVE
> +       depends on SPARSEMEM_VMEMMAP
> +
> +       help
> +         Allows creation of struct pages to represent unaddressable device
> +         memory; i.e., memory that is only accessible from the device (or
> +         group of devices).
> +
>  config FRAME_VECTOR
>         bool
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 6ff5d72..4116ab2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -49,6 +49,7 @@
>  #include <linux/swap.h>
>  #include <linux/highmem.h>
>  #include <linux/pagemap.h>
> +#include <linux/memremap.h>
>  #include <linux/ksm.h>
>  #include <linux/rmap.h>
>  #include <linux/export.h>
> @@ -927,6 +928,35 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>                                         pte = pte_swp_mksoft_dirty(pte);
>                                 set_pte_at(src_mm, addr, src_pte, pte);
>                         }
> +               } else if (is_device_entry(entry)) {
> +                       page = device_entry_to_page(entry);
> +
> +                       /*
> +                        * Update rss count even for unaddressable pages, as
> +                        * they should treated just like normal pages in this
> +                        * respect.
> +                        *
> +                        * We will likely want to have some new rss counters
> +                        * for unaddressable pages, at some point. But for now
> +                        * keep things as they are.
> +                        */
> +                       get_page(page);
> +                       rss[mm_counter(page)]++;
> +                       page_dup_rmap(page, false);
> +
> +                       /*
> +                        * We do not preserve soft-dirty information, because so
> +                        * far, checkpoint/restore is the only feature that
> +                        * requires that. And checkpoint/restore does not work
> +                        * when a device driver is involved (you cannot easily
> +                        * save and restore device driver state).
> +                        */
> +                       if (is_write_device_entry(entry) &&
> +                           is_cow_mapping(vm_flags)) {
> +                               make_device_entry_read(&entry);
> +                               pte = swp_entry_to_pte(entry);
> +                               set_pte_at(src_mm, addr, src_pte, pte);
> +                       }
>                 }
>                 goto out_set_pte;
>         }
> @@ -1243,6 +1273,29 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>                         }
>                         continue;
>                 }
> +
> +               entry = pte_to_swp_entry(ptent);
> +               if (non_swap_entry(entry) && is_device_entry(entry)) {
> +                       struct page *page = device_entry_to_page(entry);
> +
> +                       if (unlikely(details && details->check_mapping)) {
> +                               /*
> +                                * unmap_shared_mapping_pages() wants to
> +                                * invalidate cache without truncating:
> +                                * unmap shared but keep private pages.
> +                                */
> +                               if (details->check_mapping !=
> +                                   page_rmapping(page))
> +                                       continue;
> +                       }
> +
> +                       pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> +                       rss[mm_counter(page)]--;
> +                       page_remove_rmap(page, false);
> +                       put_page(page);
> +                       continue;
> +               }
> +
>                 /* If details->check_mapping, we leave swap entries. */
>                 if (unlikely(details))
>                         continue;
> @@ -2690,6 +2743,14 @@ int do_swap_page(struct vm_fault *vmf)
>                 if (is_migration_entry(entry)) {
>                         migration_entry_wait(vma->vm_mm, vmf->pmd,
>                                              vmf->address);
> +               } else if (is_device_entry(entry)) {
> +                       /*
> +                        * For un-addressable device memory we call the pgmap
> +                        * fault handler callback. The callback must migrate
> +                        * the page back to some CPU accessible page.
> +                        */
> +                       ret = device_entry_fault(vma, vmf->address, entry,
> +                                                vmf->flags, vmf->pmd);
>                 } else if (is_hwpoison_entry(entry)) {
>                         ret = VM_FAULT_HWPOISON;
>                 } else {
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index de30822..d988fbe 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -156,7 +156,7 @@ void mem_hotplug_done(void)
>  /* add this memory to iomem resource */
>  static struct resource *register_memory_resource(u64 start, u64 size)
>  {
> -       struct resource *res;
> +       struct resource *res, *conflict;
>         res = kzalloc(sizeof(struct resource), GFP_KERNEL);
>         if (!res)
>                 return ERR_PTR(-ENOMEM);
> @@ -165,7 +165,13 @@ static struct resource *register_memory_resource(u64 start, u64 size)
>         res->start = start;
>         res->end = start + size - 1;
>         res->flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
> -       if (request_resource(&iomem_resource, res) < 0) {
> +       conflict =  request_resource_conflict(&iomem_resource, res);
> +       if (conflict) {
> +               if (conflict->desc == IORES_DESC_DEVICE_MEMORY_UNADDRESSABLE) {
> +                       pr_debug("Device unaddressable memory block "
> +                                "memory hotplug at %#010llx !\n",
> +                                (unsigned long long)start);
> +               }
>                 pr_debug("System RAM resource %pR cannot be added\n", res);
>                 kfree(res);
>                 return ERR_PTR(-EEXIST);
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 8edd0d5..dadb020 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -126,6 +126,20 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>
>                                 pages++;
>                         }
> +
> +                       if (is_write_device_entry(entry)) {
> +                               pte_t newpte;
> +
> +                               /*
> +                                * We do not preserve soft-dirtiness. See
> +                                * copy_one_pte() for explanation.
> +                                */
> +                               make_device_entry_read(&entry);
> +                               newpte = swp_entry_to_pte(entry);
> +                               set_pte_at(mm, addr, pte, newpte);
> +
> +                               pages++;
> +                       }
>                 }
>         } while (pte++, addr += PAGE_SIZE, addr != end);
>         arch_leave_lazy_mmu_mode();

I think this is a good example of how the usage of the generic
"device" term makes the new "private" code paths harder to spot /
discern from the current "public" requirements.