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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  */
 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 */
298 

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