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.

[[email protected]: fix warning]
  Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jérôme Glisse <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>
Acked-by: Dan Williams <[email protected]>
Cc: Ross Zwisler <[email protected]>
Cc: Aneesh Kumar <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: David Nellans <[email protected]>
Cc: Evgeny Baskakov <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Mark Hairgrove <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Sherry Cheung <[email protected]>
Cc: Subhash Gutti <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Bob Liu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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 */
@@ -219,6 +221,34 @@ static unsigned long order_at(struct resource *res, unsigned long pgoff)
 	for (pgoff = 0, order = order_at((res), pgoff); order < ULONG_MAX; \
 			pgoff += 1UL << order, order = order_at((res), pgoff))
 
+#if IS_ENABLED(CONFIG_DEVICE_PRIVATE)
+int device_private_entry_fault(struct vm_area_struct *vma,
+		       unsigned long addr,
+		       swp_entry_t entry,
+		       unsigned int flags,
+		       pmd_t *pmdp)
+{
+	struct page *page = device_private_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_private_entry_fault);
+#endif /* CONFIG_DEVICE_PRIVATE */
+
 static void pgmap_radix_release(struct resource *res)
 {
 	unsigned long pgoff, order;
@@ -356,6 +386,10 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
 	}
 	pgmap->ref = ref;
 	pgmap->res = &page_map->res;
+	pgmap->type = MEMORY_DEVICE_HOST;
+	pgmap->page_fault = NULL;
+	pgmap->page_free = NULL;
+	pgmap->data = NULL;
 
 	mutex_lock(&pgmap_lock);
 	error = 0;