mm, dax, pmem: introduce {get|put}_dev_pagemap() for dax-gup

get_dev_page() enables paths like get_user_pages() to pin a dynamically
mapped pfn-range (devm_memremap_pages()) while the resulting struct page
objects are in use.  Unlike get_page() it may fail if the device is, or
is in the process of being, disabled.  While the initial lookup of the
range may be an expensive list walk, the result is cached to speed up
subsequent lookups which are likely to be in the same mapped range.

devm_memremap_pages() now requires a reference counter to be specified
at init time.  For pmem this means moving request_queue allocation into
pmem_alloc() so the existing queue usage counter can track "device
pages".

ZONE_DEVICE pages always have an elevated count and will never be on an
lru reclaim list.  That space in 'struct page' can be redirected for
other uses, but for safety introduce a poison value that will always
trip __list_add() to assert.  This allows half of the struct list_head
storage to be reclaimed with some assurance to back up the assumption
that the page count never goes to zero and a list_add() is never
attempted.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Logan Gunthorpe <logang@deltatee.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Matthew Wilcox <willy@linux.intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 47 insertions, 2 deletions

diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index aa3e82a80d7b..bcaa634139a9 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -1,6 +1,8 @@
 #ifndef _LINUX_MEMREMAP_H_
 #define _LINUX_MEMREMAP_H_
 #include <linux/mm.h>
+#include <linux/ioport.h>
+#include <linux/percpu-refcount.h>
 
 struct resource;
 struct device;
@@ -36,21 +38,25 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
 /**
  * struct dev_pagemap - metadata for ZONE_DEVICE mappings
  * @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
  */
 struct dev_pagemap {
         struct vmem_altmap *altmap;
         const struct resource *res;
+        struct percpu_ref *ref;
         struct device *dev;
 };
 
 #ifdef CONFIG_ZONE_DEVICE
 void *devm_memremap_pages(struct device *dev, struct resource *res,
-                struct vmem_altmap *altmap);
+                struct percpu_ref *ref, struct vmem_altmap *altmap);
 struct dev_pagemap *find_dev_pagemap(resource_size_t phys);
 #else
 static inline void *devm_memremap_pages(struct device *dev,
-                struct resource *res, struct vmem_altmap *altmap)
+                struct resource *res, struct percpu_ref *ref,
+                struct vmem_altmap *altmap)
 {
         /*
          * Fail attempts to call devm_memremap_pages() without
@@ -66,4 +72,43 @@ static inline struct dev_pagemap *find_dev_pagemap(resource_size_t phys)
         return NULL;
 }
 #endif
+
+/**
+ * get_dev_pagemap() - take a new live reference on the dev_pagemap for @pfn
+ * @pfn: page frame number to lookup page_map
+ * @pgmap: optional known pgmap that already has a reference
+ *
+ * @pgmap allows the overhead of a lookup to be bypassed when @pfn lands in the
+ * same mapping.
+ */
+static inline struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
+                struct dev_pagemap *pgmap)
+{
+        const struct resource *res = pgmap ? pgmap->res : NULL;
+        resource_size_t phys = PFN_PHYS(pfn);
+
+        /*
+         * In the cached case we're already holding a live reference so
+         * we can simply do a blind increment
+         */
+        if (res && phys >= res->start && phys <= res->end) {
+                percpu_ref_get(pgmap->ref);
+                return pgmap;
+        }
+
+        /* fall back to slow path lookup */
+        rcu_read_lock();
+        pgmap = find_dev_pagemap(phys);
+        if (pgmap && !percpu_ref_tryget_live(pgmap->ref))
+                pgmap = NULL;
+        rcu_read_unlock();
+
+        return pgmap;
+}
+
+static inline void put_dev_pagemap(struct dev_pagemap *pgmap)
+{
+        if (pgmap)
+                percpu_ref_put(pgmap->ref);
+}
 #endif /* _LINUX_MEMREMAP_H_ */