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Rafael Aquini18468d92012-12-11 16:02:38 -08001/*
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 */