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, 11 insertions, 0 deletions

diff --git a/include/linux/list.h b/include/linux/list.h
index 5356f4d661a7..30cf4200ab40 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -113,6 +113,17 @@ extern void __list_del_entry(struct list_head *entry);
 extern void list_del(struct list_head *entry);
 #endif
 
+#ifdef CONFIG_DEBUG_LIST
+/*
+ * See devm_memremap_pages() which wants DEBUG_LIST=y to assert if one
+ * of the pages it allocates is ever passed to list_add()
+ */
+extern void list_force_poison(struct list_head *entry);
+#else
+/* fallback to the less strict LIST_POISON* definitions */
+#define list_force_poison list_del
+#endif
+
 /**
  * list_replace - replace old entry by new one
  * @old : the element to be replaced