mm/ZONE_DEVICE: new type of ZONE_DEVICE for unaddressable memory

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.

[arnd@arndb.de: fix warning]
  Link: http://lkml.kernel.org/r/20170823133213.712917-1-arnd@arndb.de
Link: http://lkml.kernel.org/r/20170817000548.32038-8-jglisse@redhat.com
Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Nellans <dnellans@nvidia.com>
Cc: Evgeny Baskakov <ebaskakov@nvidia.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Mark Hairgrove <mhairgrove@nvidia.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Sherry Cheung <SCheung@nvidia.com>
Cc: Subhash Gutti <sgutti@nvidia.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Bob Liu <liubo95@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 73 insertions, 0 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>
+
 struct resource;
 struct device;
 
@@ -35,18 +37,89 @@ 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_HOST:
+ * 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 private 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_HOST = 0,
+	MEMORY_DEVICE_PRIVATE,
+};
+
+/*
+ * For MEMORY_DEVICE_PRIVATE we use ZONE_DEVICE and extend it with two
+ * callbacks:
+ *   page_fault()
+ *   page_free()
+ *
+ * Additional notes about MEMORY_DEVICE_PRIVATE 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,
+				const 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;
 };
 
 #ifdef CONFIG_ZONE_DEVICE