linux-kernel - Re: [HMM 07/15] mm/ZONE_DEVICE: new type of ZONE

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Date:   Tue, 30 May 2017 10:43:55 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Jérôme Glisse <jglisse@...hat.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Dan Williams <dan.j.williams@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [HMM 07/15] mm/ZONE_DEVICE: new type of ZONE_DEVICE for
 unaddressable memory v3

On Wed, May 24, 2017 at 01:20:16PM -0400, Jérôme Glisse 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 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>
> ---
<>
> @@ -35,18 +37,88 @@ 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_PUBLIC:
> + * 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_PRIVATE:
> + * 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
		     _PRIVATE

Just noticed that one holdover from the DEVICE_UNADDRESSABLE naming.

> + * 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_PUBLIC = 0,
> +	MEMORY_DEVICE_PRIVATE,
> +};