Re: [PATCH V3 1/4] mm: Define coherent device memory (CDM) node

[Bob Liu]

Hi Anshuman,

I have a few questions about coherent device memory.

On Wed, Feb 15, 2017 at 8:07 PM, Anshuman Khandual
<khandual@xxxxxxxxxxxxxxxxxx> wrote:
> There are certain devices like specialized accelerator, GPU cards, network
> cards, FPGA cards etc which might contain onboard memory which is coherent
> along with the existing system RAM while being accessed either from the CPU
> or from the device. They share some similar properties with that of normal

What's the general size of this kind of memory?

> system RAM but at the same time can also be different with respect to
> system RAM.
>
> User applications might be interested in using this kind of coherent device

What kind of applications?

> memory explicitly or implicitly along side the system RAM utilizing all
> possible core memory functions like anon mapping (LRU), file mapping (LRU),
> page cache (LRU), driver managed (non LRU), HW poisoning, NUMA migrations

I didn't see the benefit to manage the onboard memory same way as system RAM.
Why not just map this kind of onborad memory to userspace directly?
And only those specific applications can manage/access/use it.

It sounds not very good to complicate the core memory framework a lot
because of some not widely used devices and uncertain applications.

-
Regards,
Bob

> etc. To achieve this kind of tight integration with core memory subsystem,
> the device onboard coherent memory must be represented as a memory only
> NUMA node. At the same time arch must export some kind of a function to
> identify of this node as a coherent device memory not any other regular
> cpu less memory only NUMA node.
>
> After achieving the integration with core memory subsystem coherent device
> memory might still need some special consideration inside the kernel. There
> can be a variety of coherent memory nodes with different expectations from
> the core kernel memory. But right now only one kind of special treatment is
> considered which requires certain isolation.
>
> Now consider the case of a coherent device memory node type which requires
> isolation. This kind of coherent memory is onboard an external device
> attached to the system through a link where there is always a chance of a
> link failure taking down the entire memory node with it. More over the
> memory might also have higher chance of ECC failure as compared to the
> system RAM. Hence allocation into this kind of coherent memory node should
> be regulated. Kernel allocations must not come here. Normal user space
> allocations too should not come here implicitly (without user application
> knowing about it). This summarizes isolation requirement of certain kind of
> coherent device memory node as an example. There can be different kinds of
> isolation requirement also.
>
> Some coherent memory devices might not require isolation altogether after
> all. Then there might be other coherent memory devices which might require
> some other special treatment after being part of core memory representation
> . For now, will look into isolation seeking coherent device memory node not
> the other ones.
>
> To implement the integration as well as isolation, the coherent memory node
> must be present in N_MEMORY and a new N_COHERENT_DEVICE node mask inside
> the node_states[] array. During memory hotplug operations, the new nodemask
> N_COHERENT_DEVICE is updated along with N_MEMORY for these coherent device
> memory nodes. This also creates the following new sysfs based interface to
> list down all the coherent memory nodes of the system.
>
>         /sys/devices/system/node/is_coherent_node
>
> Architectures must export function arch_check_node_cdm() which identifies
> any coherent device memory node in case they enable CONFIG_COHERENT_DEVICE.
>
> Signed-off-by: Anshuman Khandual <khandual@xxxxxxxxxxxxxxxxxx>
> ---
>  Documentation/ABI/stable/sysfs-devices-node |  7 ++++
>  arch/powerpc/Kconfig                        |  1 +
>  arch/powerpc/mm/numa.c                      |  7 ++++
>  drivers/base/node.c                         |  6 +++
>  include/linux/nodemask.h                    | 58 ++++++++++++++++++++++++++++-
>  mm/Kconfig                                  |  4 ++
>  mm/memory_hotplug.c                         |  3 ++
>  mm/page_alloc.c                             |  8 +++-
>  8 files changed, 91 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/ABI/stable/sysfs-devices-node b/Documentation/ABI/stable/sysfs-devices-node
> index 5b2d0f0..5df18f7 100644
> --- a/Documentation/ABI/stable/sysfs-devices-node
> +++ b/Documentation/ABI/stable/sysfs-devices-node
> @@ -29,6 +29,13 @@ Description:
>                 Nodes that have regular or high memory.
>                 Depends on CONFIG_HIGHMEM.
>
> +What:          /sys/devices/system/node/is_cdm_node
> +Date:          January 2017
> +Contact:       Linux Memory Management list <linux-mm@xxxxxxxxx>
> +Description:
> +               Lists the nodemask of nodes that have coherent device memory.
> +               Depends on CONFIG_COHERENT_DEVICE.
> +
>  What:          /sys/devices/system/node/nodeX
>  Date:          October 2002
>  Contact:       Linux Memory Management list <linux-mm@xxxxxxxxx>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 281f4f1..1cff239 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -164,6 +164,7 @@ config PPC
>         select ARCH_HAS_SCALED_CPUTIME if VIRT_CPU_ACCOUNTING_NATIVE
>         select HAVE_ARCH_HARDENED_USERCOPY
>         select HAVE_KERNEL_GZIP
> +       select COHERENT_DEVICE if PPC_BOOK3S_64 && NEED_MULTIPLE_NODES
>
>  config GENERIC_CSUM
>         def_bool CPU_LITTLE_ENDIAN
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index b1099cb..14f0b98 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -41,6 +41,13 @@
>  #include <asm/setup.h>
>  #include <asm/vdso.h>
>
> +#ifdef CONFIG_COHERENT_DEVICE
> +inline int arch_check_node_cdm(int nid)
> +{
> +       return 0;
> +}
> +#endif
> +
>  static int numa_enabled = 1;
>
>  static char *cmdline __initdata;
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 5548f96..f39e615 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -661,6 +661,9 @@ static struct node_attr node_state_attr[] = {
>         [N_MEMORY] = _NODE_ATTR(has_memory, N_MEMORY),
>  #endif
>         [N_CPU] = _NODE_ATTR(has_cpu, N_CPU),
> +#ifdef CONFIG_COHERENT_DEVICE
> +       [N_COHERENT_DEVICE] = _NODE_ATTR(is_cdm_node, N_COHERENT_DEVICE),
> +#endif
>  };
>
>  static struct attribute *node_state_attrs[] = {
> @@ -674,6 +677,9 @@ static struct attribute *node_state_attrs[] = {
>         &node_state_attr[N_MEMORY].attr.attr,
>  #endif
>         &node_state_attr[N_CPU].attr.attr,
> +#ifdef CONFIG_COHERENT_DEVICE
> +       &node_state_attr[N_COHERENT_DEVICE].attr.attr,
> +#endif
>         NULL
>  };
>
> diff --git a/include/linux/nodemask.h b/include/linux/nodemask.h
> index f746e44..175c2d6 100644
> --- a/include/linux/nodemask.h
> +++ b/include/linux/nodemask.h
> @@ -388,11 +388,14 @@ enum node_states {
>         N_HIGH_MEMORY = N_NORMAL_MEMORY,
>  #endif
>  #ifdef CONFIG_MOVABLE_NODE
> -       N_MEMORY,               /* The node has memory(regular, high, movable) */
> +       N_MEMORY,       /* The node has memory(regular, high, movable, cdm) */
>  #else
>         N_MEMORY = N_HIGH_MEMORY,
>  #endif
>         N_CPU,          /* The node has one or more cpus */
> +#ifdef CONFIG_COHERENT_DEVICE
> +       N_COHERENT_DEVICE,      /* The node has CDM memory */
> +#endif
>         NR_NODE_STATES
>  };
>
> @@ -496,6 +499,59 @@ static inline int node_random(const nodemask_t *mask)
>  }
>  #endif
>
> +#ifdef CONFIG_COHERENT_DEVICE
> +extern int arch_check_node_cdm(int nid);
> +
> +static inline nodemask_t system_mem_nodemask(void)
> +{
> +       nodemask_t system_mem;
> +
> +       nodes_clear(system_mem);
> +       nodes_andnot(system_mem, node_states[N_MEMORY],
> +                       node_states[N_COHERENT_DEVICE]);
> +       return system_mem;
> +}
> +
> +static inline bool is_cdm_node(int node)
> +{
> +       return node_isset(node, node_states[N_COHERENT_DEVICE]);
> +}
> +
> +static inline void node_set_state_cdm(int node)
> +{
> +       if (arch_check_node_cdm(node))
> +               node_set_state(node, N_COHERENT_DEVICE);
> +}
> +
> +static inline void node_clear_state_cdm(int node)
> +{
> +       if (arch_check_node_cdm(node))
> +               node_clear_state(node, N_COHERENT_DEVICE);
> +}
> +
> +#else
> +
> +static inline int arch_check_node_cdm(int nid) { return 0; }
> +
> +static inline nodemask_t system_mem_nodemask(void)
> +{
> +       return node_states[N_MEMORY];
> +}
> +
> +static inline bool is_cdm_node(int node)
> +{
> +       return false;
> +}
> +
> +static inline void node_set_state_cdm(int node)
> +{
> +}
> +
> +static inline void node_clear_state_cdm(int node)
> +{
> +}
> +#endif /* CONFIG_COHERENT_DEVICE */
> +
>  #define node_online_map        node_states[N_ONLINE]
>  #define node_possible_map      node_states[N_POSSIBLE]
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 9b8fccb..6263a65 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -143,6 +143,10 @@ config HAVE_GENERIC_RCU_GUP
>  config ARCH_DISCARD_MEMBLOCK
>         bool
>
> +config COHERENT_DEVICE
> +       bool
> +       default n
> +
>  config NO_BOOTMEM
>         bool
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index b8c11e0..6bce093 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1030,6 +1030,7 @@ static void node_states_set_node(int node, struct memory_notify *arg)
>         if (arg->status_change_nid_high >= 0)
>                 node_set_state(node, N_HIGH_MEMORY);
>
> +       node_set_state_cdm(node);
>         node_set_state(node, N_MEMORY);
>  }
>
> @@ -1843,6 +1844,8 @@ static void node_states_clear_node(int node, struct memory_notify *arg)
>         if ((N_MEMORY != N_HIGH_MEMORY) &&
>             (arg->status_change_nid >= 0))
>                 node_clear_state(node, N_MEMORY);
> +
> +       node_clear_state_cdm(node);
>  }
>
>  static int __ref __offline_pages(unsigned long start_pfn,
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f3e0c69..84d61bb 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6080,8 +6080,10 @@ static unsigned long __init early_calculate_totalpages(void)
>                 unsigned long pages = end_pfn - start_pfn;
>
>                 totalpages += pages;
> -               if (pages)
> +               if (pages) {
> +                       node_set_state_cdm(nid);
>                         node_set_state(nid, N_MEMORY);
> +               }
>         }
>         return totalpages;
>  }
> @@ -6392,8 +6394,10 @@ void __init free_area_init_nodes(unsigned long *max_zone_pfn)
>                                 find_min_pfn_for_node(nid), NULL);
>
>                 /* Any memory on that node */
> -               if (pgdat->node_present_pages)
> +               if (pgdat->node_present_pages) {
> +                       node_set_state_cdm(nid);
>                         node_set_state(nid, N_MEMORY);
> +               }
>                 check_for_memory(pgdat, nid);
>         }
>  }
> --
> 2.9.3