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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 : 0 : put_page(page);
93 : 0 : __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 : 0 : 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 */
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