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  * isolated_balloon_page - identify an isolated balloon page on private
 163  *                         compaction/migration page lists.
 164  *
 165  * After a compaction thread isolates a balloon page for migration, it raises
 166  * the page refcount to prevent concurrent compaction threads from re-isolating
 167  * the same page. For that reason putback_movable_pages(), or other routines
 168  * that need to identify isolated balloon pages on private pagelists, cannot
 169  * rely on balloon_page_movable() to accomplish the task.
 170  */
 171 static inline bool isolated_balloon_page(struct page *page)
 172 {
 173         /* Already isolated balloon pages, by default, have a raised refcount */
 174         if (page_flags_cleared(page) && !page_mapped(page) &&
 175             page_count(page) >= 2)
 176                 return __is_movable_balloon_page(page);
 177
 178         return false;
 179 }
 180
 181 /*
 182  * balloon_page_insert - insert a page into the balloon's page list and make
 183  *                       the page->mapping assignment accordingly.
 184  * @page    : page to be assigned as a 'balloon page'
 185  * @mapping : allocated special 'balloon_mapping'
 186  * @head    : balloon's device page list head
 187  *
 188  * Caller must ensure the page is locked and the spin_lock protecting balloon
 189  * pages list is held before inserting a page into the balloon device.
 190  */
 191 static inline void balloon_page_insert(struct page *page,
 192                                        struct address_space *mapping,
 193                                        struct list_head *head)
 194 {
 195         page->mapping = mapping;
 196         list_add(&page->lru, head);
 197 }
 198
 199 /*
 200  * balloon_page_delete - delete a page from balloon's page list and clear
 201  *                       the page->mapping assignement accordingly.
 202  * @page    : page to be released from balloon's page list
 203  *
 204  * Caller must ensure the page is locked and the spin_lock protecting balloon
 205  * pages list is held before deleting a page from the balloon device.
 206  */
 207 static inline void balloon_page_delete(struct page *page)
 208 {
 209         page->mapping = NULL;
 210         list_del(&page->lru);
 211 }
 212
 213 /*
 214  * balloon_page_device - get the b_dev_info descriptor for the balloon device
 215  *                       that enqueues the given page.
 216  */
 217 static inline struct balloon_dev_info *balloon_page_device(struct page *page)
 218 {
 219         struct address_space *mapping = page->mapping;
 220         if (likely(mapping))
 221                 return mapping->private_data;
 222
 223         return NULL;
 224 }
 225
 226 static inline gfp_t balloon_mapping_gfp_mask(void)
 227 {
 228         return GFP_HIGHUSER_MOVABLE;
 229 }
 230
 231 static inline bool balloon_compaction_check(void)
 232 {
 233         return true;
 234 }
 235
 236 #else /* !CONFIG_BALLOON_COMPACTION */
 237
 238 static inline void *balloon_mapping_alloc(void *balloon_device,
 239                                 const struct address_space_operations *a_ops)
 240 {
 241         return ERR_PTR(-EOPNOTSUPP);
 242 }
 243
 244 static inline void balloon_mapping_free(struct address_space *balloon_mapping)
 245 {
 246         return;
 247 }
 248
 249 static inline void balloon_page_insert(struct page *page,
 250                                        struct address_space *mapping,
 251                                        struct list_head *head)
 252 {
 253         list_add(&page->lru, head);
 254 }
 255
 256 static inline void balloon_page_delete(struct page *page)
 257 {
 258         list_del(&page->lru);
 259 }
 260
 261 static inline bool balloon_page_movable(struct page *page)
 262 {
 263         return false;
 264 }
 265
 266 static inline bool isolated_balloon_page(struct page *page)
 267 {
 268         return false;
 269 }
 270
 271 static inline bool balloon_page_isolate(struct page *page)
 272 {
 273         return false;
 274 }
 275
 276 static inline void balloon_page_putback(struct page *page)
 277 {
 278         return;
 279 }
 280
 281 static inline int balloon_page_migrate(struct page *newpage,
 282                                 struct page *page, enum migrate_mode mode)
 283 {
 284         return 0;
 285 }
 286
 287 static inline gfp_t balloon_mapping_gfp_mask(void)
 288 {
 289         return GFP_HIGHUSER;
 290 }
 291
 292 static inline bool balloon_compaction_check(void)
 293 {
 294         return false;
 295 }
 296 #endif /* CONFIG_BALLOON_COMPACTION */
 297 #endif /* _LINUX_BALLOON_COMPACTION_H */