include/linux/balloon_compaction.h

   1 /*
   2  * include/linux/balloon_compaction.h
   3  *
   4  * Common interface definitions for making balloon pages movable by compaction.
   5  *
   6  * Despite being perfectly possible to perform ballooned pages migration, they
   7  * make a special corner case to compaction scans because balloon pages are not
   8  * enlisted at any LRU list like the other pages we do compact / migrate.
   9  *
  10  * As the page isolation scanning step a compaction thread does is a lockless
  11  * procedure (from a page standpoint), it might bring some racy situations while
  12  * performing balloon page compaction. In order to sort out these racy scenarios
  13  * and safely perform balloon's page compaction and migration we must, always,
  14  * ensure following these three simple rules:
  15  *
  16  *   i. when updating a balloon's page ->mapping element, strictly do it under
  17  *      the following lock order, independently of the far superior
  18  *      locking scheme (lru_lock, balloon_lock):
  19  *          +-page_lock(page);
  20  *            +--spin_lock_irq(&b_dev_info->pages_lock);
  21  *                  ... page->mapping updates here ...
  22  *
  23  *  ii. before isolating or dequeueing a balloon page from the balloon device
  24  *      pages list, the page reference counter must be raised by one and the
  25  *      extra refcount must be dropped when the page is enqueued back into
  26  *      the balloon device page list, thus a balloon page keeps its reference
  27  *      counter raised only while it is under our special handling;
  28  *
  29  * iii. after the lockless scan step have selected a potential balloon page for
  30  *      isolation, re-test the page->mapping flags and the page ref counter
  31  *      under the proper page lock, to ensure isolating a valid balloon page
  32  *      (not yet isolated, nor under release procedure)
  33  *
  34  * The functions provided by this interface are placed to help on coping with
  35  * the aforementioned balloon page corner case, as well as to ensure the simple
  36  * set of exposed rules are satisfied while we are dealing with balloon pages
  37  * compaction / migration.
  38  *
  39  * Copyright (C) 2012, Red Hat, Inc.  Rafael Aquini <aquini@redhat.com>
  40  */
  41 #ifndef _LINUX_BALLOON_COMPACTION_H
  42 #define _LINUX_BALLOON_COMPACTION_H
  43 #include <linux/pagemap.h>
  44 #include <linux/page-flags.h>
  45 #include <linux/migrate.h>
  46 #include <linux/gfp.h>
  47 #include <linux/err.h>
  48
  49 /*
  50  * Balloon device information descriptor.
  51  * This struct is used to allow the common balloon compaction interface
  52  * procedures to find the proper balloon device holding memory pages they'll
  53  * have to cope for page compaction / migration, as well as it serves the
  54  * balloon driver as a page book-keeper for its registered balloon devices.
  55  */
  56 struct balloon_dev_info {
  57         void *balloon_device;           /* balloon device descriptor */
  58         struct address_space *mapping;  /* balloon special page->mapping */
  59         unsigned long isolated_pages;   /* # of isolated pages for migration */
  60         spinlock_t pages_lock;          /* Protection to pages list */
  61         struct list_head pages;         /* Pages enqueued & handled to Host */
  62 };
  63
  64 extern struct page *balloon_page_enqueue(struct balloon_dev_info *b_dev_info);
  65 extern struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info);
  66 extern struct balloon_dev_info *balloon_devinfo_alloc(
  67                                                 void *balloon_dev_descriptor);
  68
  69 static inline void balloon_devinfo_free(struct balloon_dev_info *b_dev_info)
  70 {
  71         kfree(b_dev_info);
  72 }
  73
  74 /*
  75  * balloon_page_free - release a balloon page back to the page free lists
  76  * @page: ballooned page to be set free
  77  *
  78  * This function must be used to properly set free an isolated/dequeued balloon
  79  * page at the end of a sucessful page migration, or at the balloon driver's
  80  * page release procedure.
  81  */
  82 static inline void balloon_page_free(struct page *page)
  83 {
  84         /*
  85          * Balloon pages always get an extra refcount before being isolated
  86          * and before being dequeued to help on sorting out fortuite colisions
  87          * between a thread attempting to isolate and another thread attempting
  88          * to release the very same balloon page.
  89          *
  90          * Before we handle the page back to Buddy, lets drop its extra refcnt.
  91          */
  92         put_page(page);
  93         __free_page(page);
  94 }
  95
  96 #ifdef CONFIG_BALLOON_COMPACTION
  97 extern bool balloon_page_isolate(struct page *page);
  98 extern void balloon_page_putback(struct page *page);
  99 extern int balloon_page_migrate(struct page *newpage,
 100                                 struct page *page, enum migrate_mode mode);
 101 extern struct address_space
 102 *balloon_mapping_alloc(struct balloon_dev_info *b_dev_info,
 103                         const struct address_space_operations *a_ops);
 104
 105 static inline void balloon_mapping_free(struct address_space *balloon_mapping)
 106 {
 107         kfree(balloon_mapping);
 108 }
 109
 110 /*
 111  * page_flags_cleared - helper to perform balloon @page ->flags tests.
 112  *
 113  * As balloon pages are obtained from buddy and we do not play with page->flags
 114  * at driver level (exception made when we get the page lock for compaction),
 115  * we can safely identify a ballooned page by checking if the
 116  * PAGE_FLAGS_CHECK_AT_PREP page->flags are all cleared.  This approach also
 117  * helps us skip ballooned pages that are locked for compaction or release, thus
 118  * mitigating their racy check at balloon_page_movable()
 119  */
 120 static inline bool page_flags_cleared(struct page *page)
 121 {
 122         return !(page->flags & PAGE_FLAGS_CHECK_AT_PREP);
 123 }
 124
 125 /*
 126  * __is_movable_balloon_page - helper to perform @page mapping->flags tests
 127  */
 128 static inline bool __is_movable_balloon_page(struct page *page)
 129 {
 130         struct address_space *mapping = page->mapping;
 131         return mapping_balloon(mapping);
 132 }
 133
 134 /*
 135  * balloon_page_movable - test page->mapping->flags to identify balloon pages
 136  *                        that can be moved by compaction/migration.
 137  *
 138  * This function is used at core compaction's page isolation scheme, therefore
 139  * most pages exposed to it are not enlisted as balloon pages and so, to avoid
 140  * undesired side effects like racing against __free_pages(), we cannot afford
 141  * holding the page locked while testing page->mapping->flags here.
 142  *
 143  * As we might return false positives in the case of a balloon page being just
 144  * released under us, the page->mapping->flags need to be re-tested later,
 145  * under the proper page lock, at the functions that will be coping with the
 146  * balloon page case.
 147  */
 148 static inline bool balloon_page_movable(struct page *page)
 149 {
 150         /*
 151          * Before dereferencing and testing mapping->flags, let's make sure
 152          * this is not a page that uses ->mapping in a different way
 153          */
 154         if (page_flags_cleared(page) && !page_mapped(page) &&
 155             page_count(page) == 1)
 156                 return __is_movable_balloon_page(page);
 157
 158         return false;
 159 }
 160
 161 /*
 162  * balloon_page_insert - insert a page into the balloon's page list and make
 163  *                       the page->mapping assignment accordingly.
 164  * @page    : page to be assigned as a 'balloon page'
 165  * @mapping : allocated special 'balloon_mapping'
 166  * @head    : balloon's device page list head
 167  *
 168  * Caller must ensure the page is locked and the spin_lock protecting balloon
 169  * pages list is held before inserting a page into the balloon device.
 170  */
 171 static inline void balloon_page_insert(struct page *page,
 172                                        struct address_space *mapping,
 173                                        struct list_head *head)
 174 {
 175         page->mapping = mapping;
 176         list_add(&page->lru, head);
 177 }
 178
 179 /*
 180  * balloon_page_delete - delete a page from balloon's page list and clear
 181  *                       the page->mapping assignement accordingly.
 182  * @page    : page to be released from balloon's page list
 183  *
 184  * Caller must ensure the page is locked and the spin_lock protecting balloon
 185  * pages list is held before deleting a page from the balloon device.
 186  */
 187 static inline void balloon_page_delete(struct page *page)
 188 {
 189         page->mapping = NULL;
 190         list_del(&page->lru);
 191 }
 192
 193 /*
 194  * balloon_page_device - get the b_dev_info descriptor for the balloon device
 195  *                       that enqueues the given page.
 196  */
 197 static inline struct balloon_dev_info *balloon_page_device(struct page *page)
 198 {
 199         struct address_space *mapping = page->mapping;
 200         if (likely(mapping))
 201                 return mapping->private_data;
 202
 203         return NULL;
 204 }
 205
 206 static inline gfp_t balloon_mapping_gfp_mask(void)
 207 {
 208         return GFP_HIGHUSER_MOVABLE;
 209 }
 210
 211 static inline bool balloon_compaction_check(void)
 212 {
 213         return true;
 214 }
 215
 216 #else /* !CONFIG_BALLOON_COMPACTION */
 217
 218 static inline void *balloon_mapping_alloc(void *balloon_device,
 219                                 const struct address_space_operations *a_ops)
 220 {
 221         return ERR_PTR(-EOPNOTSUPP);
 222 }
 223
 224 static inline void balloon_mapping_free(struct address_space *balloon_mapping)
 225 {
 226         return;
 227 }
 228
 229 static inline void balloon_page_insert(struct page *page,
 230                                        struct address_space *mapping,
 231                                        struct list_head *head)
 232 {
 233         list_add(&page->lru, head);
 234 }
 235
 236 static inline void balloon_page_delete(struct page *page)
 237 {
 238         list_del(&page->lru);
 239 }
 240
 241 static inline bool balloon_page_movable(struct page *page)
 242 {
 243         return false;
 244 }
 245
 246 static inline bool balloon_page_isolate(struct page *page)
 247 {
 248         return false;
 249 }
 250
 251 static inline void balloon_page_putback(struct page *page)
 252 {
 253         return;
 254 }
 255
 256 static inline int balloon_page_migrate(struct page *newpage,
 257                                 struct page *page, enum migrate_mode mode)
 258 {
 259         return 0;
 260 }
 261
 262 static inline gfp_t balloon_mapping_gfp_mask(void)
 263 {
 264         return GFP_HIGHUSER;
 265 }
 266
 267 static inline bool balloon_compaction_check(void)
 268 {
 269         return false;
 270 }
 271 #endif /* CONFIG_BALLOON_COMPACTION */
 272 #endif /* _LINUX_BALLOON_COMPACTION_H */