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8174c430 NP |
1 | /* |
2 | * Lockless get_user_pages_fast for x86 | |
3 | * | |
4 | * Copyright (C) 2008 Nick Piggin | |
5 | * Copyright (C) 2008 Novell Inc. | |
6 | */ | |
7 | #include <linux/sched.h> | |
8 | #include <linux/mm.h> | |
9 | #include <linux/vmstat.h> | |
10 | #include <linux/highmem.h> | |
11 | ||
12 | #include <asm/pgtable.h> | |
13 | ||
14 | static inline pte_t gup_get_pte(pte_t *ptep) | |
15 | { | |
16 | #ifndef CONFIG_X86_PAE | |
17 | return *ptep; | |
18 | #else | |
19 | /* | |
20 | * With get_user_pages_fast, we walk down the pagetables without taking | |
21 | * any locks. For this we would like to load the pointers atoimcally, | |
22 | * but that is not possible (without expensive cmpxchg8b) on PAE. What | |
23 | * we do have is the guarantee that a pte will only either go from not | |
24 | * present to present, or present to not present or both -- it will not | |
25 | * switch to a completely different present page without a TLB flush in | |
26 | * between; something that we are blocking by holding interrupts off. | |
27 | * | |
28 | * Setting ptes from not present to present goes: | |
29 | * ptep->pte_high = h; | |
30 | * smp_wmb(); | |
31 | * ptep->pte_low = l; | |
32 | * | |
33 | * And present to not present goes: | |
34 | * ptep->pte_low = 0; | |
35 | * smp_wmb(); | |
36 | * ptep->pte_high = 0; | |
37 | * | |
38 | * We must ensure here that the load of pte_low sees l iff pte_high | |
39 | * sees h. We load pte_high *after* loading pte_low, which ensures we | |
40 | * don't see an older value of pte_high. *Then* we recheck pte_low, | |
41 | * which ensures that we haven't picked up a changed pte high. We might | |
42 | * have got rubbish values from pte_low and pte_high, but we are | |
43 | * guaranteed that pte_low will not have the present bit set *unless* | |
44 | * it is 'l'. And get_user_pages_fast only operates on present ptes, so | |
45 | * we're safe. | |
46 | * | |
47 | * gup_get_pte should not be used or copied outside gup.c without being | |
48 | * very careful -- it does not atomically load the pte or anything that | |
49 | * is likely to be useful for you. | |
50 | */ | |
51 | pte_t pte; | |
52 | ||
53 | retry: | |
54 | pte.pte_low = ptep->pte_low; | |
55 | smp_rmb(); | |
56 | pte.pte_high = ptep->pte_high; | |
57 | smp_rmb(); | |
58 | if (unlikely(pte.pte_low != ptep->pte_low)) | |
59 | goto retry; | |
60 | ||
61 | return pte; | |
62 | #endif | |
63 | } | |
64 | ||
65 | /* | |
66 | * The performance critical leaf functions are made noinline otherwise gcc | |
67 | * inlines everything into a single function which results in too much | |
68 | * register pressure. | |
69 | */ | |
70 | static noinline int gup_pte_range(pmd_t pmd, unsigned long addr, | |
71 | unsigned long end, int write, struct page **pages, int *nr) | |
72 | { | |
73 | unsigned long mask; | |
74 | pte_t *ptep; | |
75 | ||
76 | mask = _PAGE_PRESENT|_PAGE_USER; | |
77 | if (write) | |
78 | mask |= _PAGE_RW; | |
79 | ||
80 | ptep = pte_offset_map(&pmd, addr); | |
81 | do { | |
82 | pte_t pte = gup_get_pte(ptep); | |
83 | struct page *page; | |
84 | ||
85 | if ((pte_val(pte) & (mask | _PAGE_SPECIAL)) != mask) { | |
86 | pte_unmap(ptep); | |
87 | return 0; | |
88 | } | |
89 | VM_BUG_ON(!pfn_valid(pte_pfn(pte))); | |
90 | page = pte_page(pte); | |
91 | get_page(page); | |
92 | pages[*nr] = page; | |
93 | (*nr)++; | |
94 | ||
95 | } while (ptep++, addr += PAGE_SIZE, addr != end); | |
96 | pte_unmap(ptep - 1); | |
97 | ||
98 | return 1; | |
99 | } | |
100 | ||
101 | static inline void get_head_page_multiple(struct page *page, int nr) | |
102 | { | |
103 | VM_BUG_ON(page != compound_head(page)); | |
104 | VM_BUG_ON(page_count(page) == 0); | |
105 | atomic_add(nr, &page->_count); | |
106 | } | |
107 | ||
108 | static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr, | |
109 | unsigned long end, int write, struct page **pages, int *nr) | |
110 | { | |
111 | unsigned long mask; | |
112 | pte_t pte = *(pte_t *)&pmd; | |
113 | struct page *head, *page; | |
114 | int refs; | |
115 | ||
116 | mask = _PAGE_PRESENT|_PAGE_USER; | |
117 | if (write) | |
118 | mask |= _PAGE_RW; | |
119 | if ((pte_val(pte) & mask) != mask) | |
120 | return 0; | |
121 | /* hugepages are never "special" */ | |
122 | VM_BUG_ON(pte_val(pte) & _PAGE_SPECIAL); | |
123 | VM_BUG_ON(!pfn_valid(pte_pfn(pte))); | |
124 | ||
125 | refs = 0; | |
126 | head = pte_page(pte); | |
127 | page = head + ((addr & ~HPAGE_MASK) >> PAGE_SHIFT); | |
128 | do { | |
129 | VM_BUG_ON(compound_head(page) != head); | |
130 | pages[*nr] = page; | |
131 | (*nr)++; | |
132 | page++; | |
133 | refs++; | |
134 | } while (addr += PAGE_SIZE, addr != end); | |
135 | get_head_page_multiple(head, refs); | |
136 | ||
137 | return 1; | |
138 | } | |
139 | ||
140 | static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end, | |
141 | int write, struct page **pages, int *nr) | |
142 | { | |
143 | unsigned long next; | |
144 | pmd_t *pmdp; | |
145 | ||
146 | pmdp = pmd_offset(&pud, addr); | |
147 | do { | |
148 | pmd_t pmd = *pmdp; | |
149 | ||
150 | next = pmd_addr_end(addr, end); | |
151 | if (pmd_none(pmd)) | |
152 | return 0; | |
153 | if (unlikely(pmd_large(pmd))) { | |
154 | if (!gup_huge_pmd(pmd, addr, next, write, pages, nr)) | |
155 | return 0; | |
156 | } else { | |
157 | if (!gup_pte_range(pmd, addr, next, write, pages, nr)) | |
158 | return 0; | |
159 | } | |
160 | } while (pmdp++, addr = next, addr != end); | |
161 | ||
162 | return 1; | |
163 | } | |
164 | ||
165 | static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end, | |
166 | int write, struct page **pages, int *nr) | |
167 | { | |
168 | unsigned long next; | |
169 | pud_t *pudp; | |
170 | ||
171 | pudp = pud_offset(&pgd, addr); | |
172 | do { | |
173 | pud_t pud = *pudp; | |
174 | ||
175 | next = pud_addr_end(addr, end); | |
176 | if (pud_none(pud)) | |
177 | return 0; | |
178 | if (!gup_pmd_range(pud, addr, next, write, pages, nr)) | |
179 | return 0; | |
180 | } while (pudp++, addr = next, addr != end); | |
181 | ||
182 | return 1; | |
183 | } | |
184 | ||
185 | int get_user_pages_fast(unsigned long start, int nr_pages, int write, | |
186 | struct page **pages) | |
187 | { | |
188 | struct mm_struct *mm = current->mm; | |
189 | unsigned long end = start + (nr_pages << PAGE_SHIFT); | |
190 | unsigned long addr = start; | |
191 | unsigned long next; | |
192 | pgd_t *pgdp; | |
193 | int nr = 0; | |
194 | ||
195 | if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ, | |
196 | start, nr_pages*PAGE_SIZE))) | |
197 | goto slow_irqon; | |
198 | ||
199 | /* | |
200 | * XXX: batch / limit 'nr', to avoid large irq off latency | |
201 | * needs some instrumenting to determine the common sizes used by | |
202 | * important workloads (eg. DB2), and whether limiting the batch size | |
203 | * will decrease performance. | |
204 | * | |
205 | * It seems like we're in the clear for the moment. Direct-IO is | |
206 | * the main guy that batches up lots of get_user_pages, and even | |
207 | * they are limited to 64-at-a-time which is not so many. | |
208 | */ | |
209 | /* | |
210 | * This doesn't prevent pagetable teardown, but does prevent | |
211 | * the pagetables and pages from being freed on x86. | |
212 | * | |
213 | * So long as we atomically load page table pointers versus teardown | |
214 | * (which we do on x86, with the above PAE exception), we can follow the | |
215 | * address down to the the page and take a ref on it. | |
216 | */ | |
217 | local_irq_disable(); | |
218 | pgdp = pgd_offset(mm, addr); | |
219 | do { | |
220 | pgd_t pgd = *pgdp; | |
221 | ||
222 | next = pgd_addr_end(addr, end); | |
223 | if (pgd_none(pgd)) | |
224 | goto slow; | |
225 | if (!gup_pud_range(pgd, addr, next, write, pages, &nr)) | |
226 | goto slow; | |
227 | } while (pgdp++, addr = next, addr != end); | |
228 | local_irq_enable(); | |
229 | ||
230 | VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT); | |
231 | return nr; | |
232 | ||
233 | { | |
234 | int ret; | |
235 | ||
236 | slow: | |
237 | local_irq_enable(); | |
238 | slow_irqon: | |
239 | /* Try to get the remaining pages with get_user_pages */ | |
240 | start += nr << PAGE_SHIFT; | |
241 | pages += nr; | |
242 | ||
243 | down_read(&mm->mmap_sem); | |
244 | ret = get_user_pages(current, mm, start, | |
245 | (end - start) >> PAGE_SHIFT, write, 0, pages, NULL); | |
246 | up_read(&mm->mmap_sem); | |
247 | ||
248 | /* Have to be a bit careful with return values */ | |
249 | if (nr > 0) { | |
250 | if (ret < 0) | |
251 | ret = nr; | |
252 | else | |
253 | ret += nr; | |
254 | } | |
255 | ||
256 | return ret; | |
257 | } | |
258 | } |