linux/include/linux/balloon_compaction.h
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   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 */
  56struct 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
  64extern struct page *balloon_page_enqueue(struct balloon_dev_info *b_dev_info);
  65extern struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info);
  66extern struct balloon_dev_info *balloon_devinfo_alloc(
  67                                                void *balloon_dev_descriptor);
  68
  69static 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 */
  82static 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
  97extern bool balloon_page_isolate(struct page *page);
  98extern void balloon_page_putback(struct page *page);
  99extern int balloon_page_migrate(struct page *newpage,
 100                                struct page *page, enum migrate_mode mode);
 101extern struct address_space
 102*balloon_mapping_alloc(struct balloon_dev_info *b_dev_info,
 103                        const struct address_space_operations *a_ops);
 104
 105static 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 */
 120static 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 */
 128static 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 */
 148static 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 */
 171static 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 */
 187static 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 */
 197static 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
 206static inline gfp_t balloon_mapping_gfp_mask(void)
 207{
 208        return GFP_HIGHUSER_MOVABLE;
 209}
 210
 211static inline bool balloon_compaction_check(void)
 212{
 213        return true;
 214}
 215
 216#else /* !CONFIG_BALLOON_COMPACTION */
 217
 218static inline void *balloon_mapping_alloc(void *balloon_device,
 219                                const struct address_space_operations *a_ops)
 220{
 221        return ERR_PTR(-EOPNOTSUPP);
 222}
 223
 224static inline void balloon_mapping_free(struct address_space *balloon_mapping)
 225{
 226        return;
 227}
 228
 229static 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
 236static inline void balloon_page_delete(struct page *page)
 237{
 238        list_del(&page->lru);
 239}
 240
 241static inline bool balloon_page_movable(struct page *page)
 242{
 243        return false;
 244}
 245
 246static inline bool balloon_page_isolate(struct page *page)
 247{
 248        return false;
 249}
 250
 251static inline void balloon_page_putback(struct page *page)
 252{
 253        return;
 254}
 255
 256static inline int balloon_page_migrate(struct page *newpage,
 257                                struct page *page, enum migrate_mode mode)
 258{
 259        return 0;
 260}
 261
 262static inline gfp_t balloon_mapping_gfp_mask(void)
 263{
 264        return GFP_HIGHUSER;
 265}
 266
 267static inline bool balloon_compaction_check(void)
 268{
 269        return false;
 270}
 271#endif /* CONFIG_BALLOON_COMPACTION */
 272#endif /* _LINUX_BALLOON_COMPACTION_H */
 273
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