Root/mm/balloon_compaction.c

1/*
2 * mm/balloon_compaction.c
3 *
4 * Common interface for making balloon pages movable by compaction.
5 *
6 * Copyright (C) 2012, Red Hat, Inc. Rafael Aquini <aquini@redhat.com>
7 */
8#include <linux/mm.h>
9#include <linux/slab.h>
10#include <linux/export.h>
11#include <linux/balloon_compaction.h>
12
13/*
14 * balloon_devinfo_alloc - allocates a balloon device information descriptor.
15 * @balloon_dev_descriptor: pointer to reference the balloon device which
16 * this struct balloon_dev_info will be servicing.
17 *
18 * Driver must call it to properly allocate and initialize an instance of
19 * struct balloon_dev_info which will be used to reference a balloon device
20 * as well as to keep track of the balloon device page list.
21 */
22struct balloon_dev_info *balloon_devinfo_alloc(void *balloon_dev_descriptor)
23{
24    struct balloon_dev_info *b_dev_info;
25    b_dev_info = kmalloc(sizeof(*b_dev_info), GFP_KERNEL);
26    if (!b_dev_info)
27        return ERR_PTR(-ENOMEM);
28
29    b_dev_info->balloon_device = balloon_dev_descriptor;
30    b_dev_info->mapping = NULL;
31    b_dev_info->isolated_pages = 0;
32    spin_lock_init(&b_dev_info->pages_lock);
33    INIT_LIST_HEAD(&b_dev_info->pages);
34
35    return b_dev_info;
36}
37EXPORT_SYMBOL_GPL(balloon_devinfo_alloc);
38
39/*
40 * balloon_page_enqueue - allocates a new page and inserts it into the balloon
41 * page list.
42 * @b_dev_info: balloon device decriptor where we will insert a new page to
43 *
44 * Driver must call it to properly allocate a new enlisted balloon page
45 * before definetively removing it from the guest system.
46 * This function returns the page address for the recently enqueued page or
47 * NULL in the case we fail to allocate a new page this turn.
48 */
49struct page *balloon_page_enqueue(struct balloon_dev_info *b_dev_info)
50{
51    unsigned long flags;
52    struct page *page = alloc_page(balloon_mapping_gfp_mask() |
53                    __GFP_NOMEMALLOC | __GFP_NORETRY);
54    if (!page)
55        return NULL;
56
57    /*
58     * Block others from accessing the 'page' when we get around to
59     * establishing additional references. We should be the only one
60     * holding a reference to the 'page' at this point.
61     */
62    BUG_ON(!trylock_page(page));
63    spin_lock_irqsave(&b_dev_info->pages_lock, flags);
64    balloon_page_insert(page, b_dev_info->mapping, &b_dev_info->pages);
65    spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
66    unlock_page(page);
67    return page;
68}
69EXPORT_SYMBOL_GPL(balloon_page_enqueue);
70
71/*
72 * balloon_page_dequeue - removes a page from balloon's page list and returns
73 * the its address to allow the driver release the page.
74 * @b_dev_info: balloon device decriptor where we will grab a page from.
75 *
76 * Driver must call it to properly de-allocate a previous enlisted balloon page
77 * before definetively releasing it back to the guest system.
78 * This function returns the page address for the recently dequeued page or
79 * NULL in the case we find balloon's page list temporarily empty due to
80 * compaction isolated pages.
81 */
82struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info)
83{
84    struct page *page, *tmp;
85    unsigned long flags;
86    bool dequeued_page;
87
88    dequeued_page = false;
89    list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
90        /*
91         * Block others from accessing the 'page' while we get around
92         * establishing additional references and preparing the 'page'
93         * to be released by the balloon driver.
94         */
95        if (trylock_page(page)) {
96            spin_lock_irqsave(&b_dev_info->pages_lock, flags);
97            /*
98             * Raise the page refcount here to prevent any wrong
99             * attempt to isolate this page, in case of coliding
100             * with balloon_page_isolate() just after we release
101             * the page lock.
102             *
103             * balloon_page_free() will take care of dropping
104             * this extra refcount later.
105             */
106            get_page(page);
107            balloon_page_delete(page);
108            spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
109            unlock_page(page);
110            dequeued_page = true;
111            break;
112        }
113    }
114
115    if (!dequeued_page) {
116        /*
117         * If we are unable to dequeue a balloon page because the page
118         * list is empty and there is no isolated pages, then something
119         * went out of track and some balloon pages are lost.
120         * BUG() here, otherwise the balloon driver may get stuck into
121         * an infinite loop while attempting to release all its pages.
122         */
123        spin_lock_irqsave(&b_dev_info->pages_lock, flags);
124        if (unlikely(list_empty(&b_dev_info->pages) &&
125                 !b_dev_info->isolated_pages))
126            BUG();
127        spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
128        page = NULL;
129    }
130    return page;
131}
132EXPORT_SYMBOL_GPL(balloon_page_dequeue);
133
134#ifdef CONFIG_BALLOON_COMPACTION
135/*
136 * balloon_mapping_alloc - allocates a special ->mapping for ballooned pages.
137 * @b_dev_info: holds the balloon device information descriptor.
138 * @a_ops: balloon_mapping address_space_operations descriptor.
139 *
140 * Driver must call it to properly allocate and initialize an instance of
141 * struct address_space which will be used as the special page->mapping for
142 * balloon device enlisted page instances.
143 */
144struct address_space *balloon_mapping_alloc(struct balloon_dev_info *b_dev_info,
145                const struct address_space_operations *a_ops)
146{
147    struct address_space *mapping;
148
149    mapping = kmalloc(sizeof(*mapping), GFP_KERNEL);
150    if (!mapping)
151        return ERR_PTR(-ENOMEM);
152
153    /*
154     * Give a clean 'zeroed' status to all elements of this special
155     * balloon page->mapping struct address_space instance.
156     */
157    address_space_init_once(mapping);
158
159    /*
160     * Set mapping->flags appropriately, to allow balloon pages
161     * ->mapping identification.
162     */
163    mapping_set_balloon(mapping);
164    mapping_set_gfp_mask(mapping, balloon_mapping_gfp_mask());
165
166    /* balloon's page->mapping->a_ops callback descriptor */
167    mapping->a_ops = a_ops;
168
169    /*
170     * Establish a pointer reference back to the balloon device descriptor
171     * this particular page->mapping will be servicing.
172     * This is used by compaction / migration procedures to identify and
173     * access the balloon device pageset while isolating / migrating pages.
174     *
175     * As some balloon drivers can register multiple balloon devices
176     * for a single guest, this also helps compaction / migration to
177     * properly deal with multiple balloon pagesets, when required.
178     */
179    mapping->private_data = b_dev_info;
180    b_dev_info->mapping = mapping;
181
182    return mapping;
183}
184EXPORT_SYMBOL_GPL(balloon_mapping_alloc);
185
186static inline void __isolate_balloon_page(struct page *page)
187{
188    struct balloon_dev_info *b_dev_info = page->mapping->private_data;
189    unsigned long flags;
190    spin_lock_irqsave(&b_dev_info->pages_lock, flags);
191    list_del(&page->lru);
192    b_dev_info->isolated_pages++;
193    spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
194}
195
196static inline void __putback_balloon_page(struct page *page)
197{
198    struct balloon_dev_info *b_dev_info = page->mapping->private_data;
199    unsigned long flags;
200    spin_lock_irqsave(&b_dev_info->pages_lock, flags);
201    list_add(&page->lru, &b_dev_info->pages);
202    b_dev_info->isolated_pages--;
203    spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
204}
205
206static inline int __migrate_balloon_page(struct address_space *mapping,
207        struct page *newpage, struct page *page, enum migrate_mode mode)
208{
209    return page->mapping->a_ops->migratepage(mapping, newpage, page, mode);
210}
211
212/* __isolate_lru_page() counterpart for a ballooned page */
213bool balloon_page_isolate(struct page *page)
214{
215    /*
216     * Avoid burning cycles with pages that are yet under __free_pages(),
217     * or just got freed under us.
218     *
219     * In case we 'win' a race for a balloon page being freed under us and
220     * raise its refcount preventing __free_pages() from doing its job
221     * the put_page() at the end of this block will take care of
222     * release this page, thus avoiding a nasty leakage.
223     */
224    if (likely(get_page_unless_zero(page))) {
225        /*
226         * As balloon pages are not isolated from LRU lists, concurrent
227         * compaction threads can race against page migration functions
228         * as well as race against the balloon driver releasing a page.
229         *
230         * In order to avoid having an already isolated balloon page
231         * being (wrongly) re-isolated while it is under migration,
232         * or to avoid attempting to isolate pages being released by
233         * the balloon driver, lets be sure we have the page lock
234         * before proceeding with the balloon page isolation steps.
235         */
236        if (likely(trylock_page(page))) {
237            /*
238             * A ballooned page, by default, has just one refcount.
239             * Prevent concurrent compaction threads from isolating
240             * an already isolated balloon page by refcount check.
241             */
242            if (__is_movable_balloon_page(page) &&
243                page_count(page) == 2) {
244                __isolate_balloon_page(page);
245                unlock_page(page);
246                return true;
247            }
248            unlock_page(page);
249        }
250        put_page(page);
251    }
252    return false;
253}
254
255/* putback_lru_page() counterpart for a ballooned page */
256void balloon_page_putback(struct page *page)
257{
258    /*
259     * 'lock_page()' stabilizes the page and prevents races against
260     * concurrent isolation threads attempting to re-isolate it.
261     */
262    lock_page(page);
263
264    if (__is_movable_balloon_page(page)) {
265        __putback_balloon_page(page);
266        /* drop the extra ref count taken for page isolation */
267        put_page(page);
268    } else {
269        WARN_ON(1);
270        dump_page(page);
271    }
272    unlock_page(page);
273}
274
275/* move_to_new_page() counterpart for a ballooned page */
276int balloon_page_migrate(struct page *newpage,
277             struct page *page, enum migrate_mode mode)
278{
279    struct address_space *mapping;
280    int rc = -EAGAIN;
281
282    /*
283     * Block others from accessing the 'newpage' when we get around to
284     * establishing additional references. We should be the only one
285     * holding a reference to the 'newpage' at this point.
286     */
287    BUG_ON(!trylock_page(newpage));
288
289    if (WARN_ON(!__is_movable_balloon_page(page))) {
290        dump_page(page);
291        unlock_page(newpage);
292        return rc;
293    }
294
295    mapping = page->mapping;
296    if (mapping)
297        rc = __migrate_balloon_page(mapping, newpage, page, mode);
298
299    unlock_page(newpage);
300    return rc;
301}
302#endif /* CONFIG_BALLOON_COMPACTION */
303

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