Commit | Line | Data |
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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> | |
8ee53820 | 11 | #include <linux/swap.h> |
8174c430 NP |
12 | |
13 | #include <asm/pgtable.h> | |
14 | ||
15 | static inline pte_t gup_get_pte(pte_t *ptep) | |
16 | { | |
17 | #ifndef CONFIG_X86_PAE | |
0c871971 | 18 | return ACCESS_ONCE(*ptep); |
8174c430 NP |
19 | #else |
20 | /* | |
21 | * With get_user_pages_fast, we walk down the pagetables without taking | |
ab09809f | 22 | * any locks. For this we would like to load the pointers atomically, |
8174c430 NP |
23 | * but that is not possible (without expensive cmpxchg8b) on PAE. What |
24 | * we do have is the guarantee that a pte will only either go from not | |
25 | * present to present, or present to not present or both -- it will not | |
26 | * switch to a completely different present page without a TLB flush in | |
27 | * between; something that we are blocking by holding interrupts off. | |
28 | * | |
29 | * Setting ptes from not present to present goes: | |
30 | * ptep->pte_high = h; | |
31 | * smp_wmb(); | |
32 | * ptep->pte_low = l; | |
33 | * | |
34 | * And present to not present goes: | |
35 | * ptep->pte_low = 0; | |
36 | * smp_wmb(); | |
37 | * ptep->pte_high = 0; | |
38 | * | |
39 | * We must ensure here that the load of pte_low sees l iff pte_high | |
40 | * sees h. We load pte_high *after* loading pte_low, which ensures we | |
41 | * don't see an older value of pte_high. *Then* we recheck pte_low, | |
42 | * which ensures that we haven't picked up a changed pte high. We might | |
43 | * have got rubbish values from pte_low and pte_high, but we are | |
44 | * guaranteed that pte_low will not have the present bit set *unless* | |
45 | * it is 'l'. And get_user_pages_fast only operates on present ptes, so | |
46 | * we're safe. | |
47 | * | |
48 | * gup_get_pte should not be used or copied outside gup.c without being | |
49 | * very careful -- it does not atomically load the pte or anything that | |
50 | * is likely to be useful for you. | |
51 | */ | |
52 | pte_t pte; | |
53 | ||
54 | retry: | |
55 | pte.pte_low = ptep->pte_low; | |
56 | smp_rmb(); | |
57 | pte.pte_high = ptep->pte_high; | |
58 | smp_rmb(); | |
59 | if (unlikely(pte.pte_low != ptep->pte_low)) | |
60 | goto retry; | |
61 | ||
62 | return pte; | |
63 | #endif | |
64 | } | |
65 | ||
66 | /* | |
67 | * The performance critical leaf functions are made noinline otherwise gcc | |
68 | * inlines everything into a single function which results in too much | |
69 | * register pressure. | |
70 | */ | |
71 | static noinline int gup_pte_range(pmd_t pmd, unsigned long addr, | |
72 | unsigned long end, int write, struct page **pages, int *nr) | |
73 | { | |
74 | unsigned long mask; | |
75 | pte_t *ptep; | |
76 | ||
77 | mask = _PAGE_PRESENT|_PAGE_USER; | |
78 | if (write) | |
79 | mask |= _PAGE_RW; | |
80 | ||
81 | ptep = pte_offset_map(&pmd, addr); | |
82 | do { | |
83 | pte_t pte = gup_get_pte(ptep); | |
84 | struct page *page; | |
85 | ||
606ee44d | 86 | if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) { |
8174c430 NP |
87 | pte_unmap(ptep); |
88 | return 0; | |
89 | } | |
90 | VM_BUG_ON(!pfn_valid(pte_pfn(pte))); | |
91 | page = pte_page(pte); | |
92 | get_page(page); | |
8ee53820 | 93 | SetPageReferenced(page); |
8174c430 NP |
94 | pages[*nr] = page; |
95 | (*nr)++; | |
96 | ||
97 | } while (ptep++, addr += PAGE_SIZE, addr != end); | |
98 | pte_unmap(ptep - 1); | |
99 | ||
100 | return 1; | |
101 | } | |
102 | ||
103 | static inline void get_head_page_multiple(struct page *page, int nr) | |
104 | { | |
105 | VM_BUG_ON(page != compound_head(page)); | |
106 | VM_BUG_ON(page_count(page) == 0); | |
107 | atomic_add(nr, &page->_count); | |
8ee53820 | 108 | SetPageReferenced(page); |
8174c430 NP |
109 | } |
110 | ||
111 | static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr, | |
112 | unsigned long end, int write, struct page **pages, int *nr) | |
113 | { | |
114 | unsigned long mask; | |
115 | pte_t pte = *(pte_t *)&pmd; | |
116 | struct page *head, *page; | |
117 | int refs; | |
118 | ||
119 | mask = _PAGE_PRESENT|_PAGE_USER; | |
120 | if (write) | |
121 | mask |= _PAGE_RW; | |
606ee44d | 122 | if ((pte_flags(pte) & mask) != mask) |
8174c430 NP |
123 | return 0; |
124 | /* hugepages are never "special" */ | |
606ee44d | 125 | VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL); |
8174c430 NP |
126 | VM_BUG_ON(!pfn_valid(pte_pfn(pte))); |
127 | ||
128 | refs = 0; | |
129 | head = pte_page(pte); | |
652ea695 | 130 | page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT); |
8174c430 NP |
131 | do { |
132 | VM_BUG_ON(compound_head(page) != head); | |
133 | pages[*nr] = page; | |
91807063 AA |
134 | if (PageTail(page)) |
135 | get_huge_page_tail(page); | |
8174c430 NP |
136 | (*nr)++; |
137 | page++; | |
138 | refs++; | |
139 | } while (addr += PAGE_SIZE, addr != end); | |
140 | get_head_page_multiple(head, refs); | |
141 | ||
142 | return 1; | |
143 | } | |
144 | ||
145 | static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end, | |
146 | int write, struct page **pages, int *nr) | |
147 | { | |
148 | unsigned long next; | |
149 | pmd_t *pmdp; | |
150 | ||
151 | pmdp = pmd_offset(&pud, addr); | |
152 | do { | |
153 | pmd_t pmd = *pmdp; | |
154 | ||
155 | next = pmd_addr_end(addr, end); | |
64cc6ae0 AA |
156 | /* |
157 | * The pmd_trans_splitting() check below explains why | |
158 | * pmdp_splitting_flush has to flush the tlb, to stop | |
159 | * this gup-fast code from running while we set the | |
160 | * splitting bit in the pmd. Returning zero will take | |
161 | * the slow path that will call wait_split_huge_page() | |
162 | * if the pmd is still in splitting state. gup-fast | |
163 | * can't because it has irq disabled and | |
164 | * wait_split_huge_page() would never return as the | |
165 | * tlb flush IPI wouldn't run. | |
166 | */ | |
167 | if (pmd_none(pmd) || pmd_trans_splitting(pmd)) | |
8174c430 NP |
168 | return 0; |
169 | if (unlikely(pmd_large(pmd))) { | |
170 | if (!gup_huge_pmd(pmd, addr, next, write, pages, nr)) | |
171 | return 0; | |
172 | } else { | |
173 | if (!gup_pte_range(pmd, addr, next, write, pages, nr)) | |
174 | return 0; | |
175 | } | |
176 | } while (pmdp++, addr = next, addr != end); | |
177 | ||
178 | return 1; | |
179 | } | |
180 | ||
652ea695 NP |
181 | static noinline int gup_huge_pud(pud_t pud, unsigned long addr, |
182 | unsigned long end, int write, struct page **pages, int *nr) | |
183 | { | |
184 | unsigned long mask; | |
185 | pte_t pte = *(pte_t *)&pud; | |
186 | struct page *head, *page; | |
187 | int refs; | |
188 | ||
189 | mask = _PAGE_PRESENT|_PAGE_USER; | |
190 | if (write) | |
191 | mask |= _PAGE_RW; | |
606ee44d | 192 | if ((pte_flags(pte) & mask) != mask) |
652ea695 NP |
193 | return 0; |
194 | /* hugepages are never "special" */ | |
606ee44d | 195 | VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL); |
652ea695 NP |
196 | VM_BUG_ON(!pfn_valid(pte_pfn(pte))); |
197 | ||
198 | refs = 0; | |
199 | head = pte_page(pte); | |
200 | page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT); | |
201 | do { | |
202 | VM_BUG_ON(compound_head(page) != head); | |
203 | pages[*nr] = page; | |
b6999b19 YS |
204 | if (PageTail(page)) |
205 | get_huge_page_tail(page); | |
652ea695 NP |
206 | (*nr)++; |
207 | page++; | |
208 | refs++; | |
209 | } while (addr += PAGE_SIZE, addr != end); | |
210 | get_head_page_multiple(head, refs); | |
211 | ||
212 | return 1; | |
213 | } | |
214 | ||
8174c430 NP |
215 | static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end, |
216 | int write, struct page **pages, int *nr) | |
217 | { | |
218 | unsigned long next; | |
219 | pud_t *pudp; | |
220 | ||
221 | pudp = pud_offset(&pgd, addr); | |
222 | do { | |
223 | pud_t pud = *pudp; | |
224 | ||
225 | next = pud_addr_end(addr, end); | |
226 | if (pud_none(pud)) | |
227 | return 0; | |
652ea695 NP |
228 | if (unlikely(pud_large(pud))) { |
229 | if (!gup_huge_pud(pud, addr, next, write, pages, nr)) | |
230 | return 0; | |
231 | } else { | |
232 | if (!gup_pmd_range(pud, addr, next, write, pages, nr)) | |
233 | return 0; | |
234 | } | |
8174c430 NP |
235 | } while (pudp++, addr = next, addr != end); |
236 | ||
237 | return 1; | |
238 | } | |
239 | ||
465a454f PZ |
240 | /* |
241 | * Like get_user_pages_fast() except its IRQ-safe in that it won't fall | |
242 | * back to the regular GUP. | |
243 | */ | |
244 | int __get_user_pages_fast(unsigned long start, int nr_pages, int write, | |
245 | struct page **pages) | |
246 | { | |
247 | struct mm_struct *mm = current->mm; | |
248 | unsigned long addr, len, end; | |
249 | unsigned long next; | |
250 | unsigned long flags; | |
251 | pgd_t *pgdp; | |
252 | int nr = 0; | |
253 | ||
254 | start &= PAGE_MASK; | |
255 | addr = start; | |
256 | len = (unsigned long) nr_pages << PAGE_SHIFT; | |
257 | end = start + len; | |
258 | if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ, | |
259 | (void __user *)start, len))) | |
260 | return 0; | |
261 | ||
262 | /* | |
263 | * XXX: batch / limit 'nr', to avoid large irq off latency | |
264 | * needs some instrumenting to determine the common sizes used by | |
265 | * important workloads (eg. DB2), and whether limiting the batch size | |
266 | * will decrease performance. | |
267 | * | |
268 | * It seems like we're in the clear for the moment. Direct-IO is | |
269 | * the main guy that batches up lots of get_user_pages, and even | |
270 | * they are limited to 64-at-a-time which is not so many. | |
271 | */ | |
272 | /* | |
273 | * This doesn't prevent pagetable teardown, but does prevent | |
274 | * the pagetables and pages from being freed on x86. | |
275 | * | |
276 | * So long as we atomically load page table pointers versus teardown | |
277 | * (which we do on x86, with the above PAE exception), we can follow the | |
278 | * address down to the the page and take a ref on it. | |
279 | */ | |
280 | local_irq_save(flags); | |
281 | pgdp = pgd_offset(mm, addr); | |
282 | do { | |
283 | pgd_t pgd = *pgdp; | |
284 | ||
285 | next = pgd_addr_end(addr, end); | |
286 | if (pgd_none(pgd)) | |
287 | break; | |
288 | if (!gup_pud_range(pgd, addr, next, write, pages, &nr)) | |
289 | break; | |
290 | } while (pgdp++, addr = next, addr != end); | |
291 | local_irq_restore(flags); | |
292 | ||
293 | return nr; | |
294 | } | |
295 | ||
a0d22f48 AG |
296 | /** |
297 | * get_user_pages_fast() - pin user pages in memory | |
298 | * @start: starting user address | |
299 | * @nr_pages: number of pages from start to pin | |
300 | * @write: whether pages will be written to | |
301 | * @pages: array that receives pointers to the pages pinned. | |
302 | * Should be at least nr_pages long. | |
303 | * | |
304 | * Attempt to pin user pages in memory without taking mm->mmap_sem. | |
305 | * If not successful, it will fall back to taking the lock and | |
306 | * calling get_user_pages(). | |
307 | * | |
308 | * Returns number of pages pinned. This may be fewer than the number | |
309 | * requested. If nr_pages is 0 or negative, returns 0. If no pages | |
310 | * were pinned, returns -errno. | |
311 | */ | |
8174c430 NP |
312 | int get_user_pages_fast(unsigned long start, int nr_pages, int write, |
313 | struct page **pages) | |
314 | { | |
315 | struct mm_struct *mm = current->mm; | |
9b79022c | 316 | unsigned long addr, len, end; |
8174c430 NP |
317 | unsigned long next; |
318 | pgd_t *pgdp; | |
319 | int nr = 0; | |
320 | ||
9b79022c LT |
321 | start &= PAGE_MASK; |
322 | addr = start; | |
323 | len = (unsigned long) nr_pages << PAGE_SHIFT; | |
7f818906 | 324 | |
9b79022c | 325 | end = start + len; |
7f818906 LT |
326 | if (end < start) |
327 | goto slow_irqon; | |
328 | ||
329 | #ifdef CONFIG_X86_64 | |
330 | if (end >> __VIRTUAL_MASK_SHIFT) | |
8174c430 | 331 | goto slow_irqon; |
7f818906 | 332 | #endif |
8174c430 NP |
333 | |
334 | /* | |
335 | * XXX: batch / limit 'nr', to avoid large irq off latency | |
336 | * needs some instrumenting to determine the common sizes used by | |
337 | * important workloads (eg. DB2), and whether limiting the batch size | |
338 | * will decrease performance. | |
339 | * | |
340 | * It seems like we're in the clear for the moment. Direct-IO is | |
341 | * the main guy that batches up lots of get_user_pages, and even | |
342 | * they are limited to 64-at-a-time which is not so many. | |
343 | */ | |
344 | /* | |
345 | * This doesn't prevent pagetable teardown, but does prevent | |
346 | * the pagetables and pages from being freed on x86. | |
347 | * | |
348 | * So long as we atomically load page table pointers versus teardown | |
349 | * (which we do on x86, with the above PAE exception), we can follow the | |
350 | * address down to the the page and take a ref on it. | |
351 | */ | |
352 | local_irq_disable(); | |
353 | pgdp = pgd_offset(mm, addr); | |
354 | do { | |
355 | pgd_t pgd = *pgdp; | |
356 | ||
357 | next = pgd_addr_end(addr, end); | |
358 | if (pgd_none(pgd)) | |
359 | goto slow; | |
360 | if (!gup_pud_range(pgd, addr, next, write, pages, &nr)) | |
361 | goto slow; | |
362 | } while (pgdp++, addr = next, addr != end); | |
363 | local_irq_enable(); | |
364 | ||
365 | VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT); | |
366 | return nr; | |
367 | ||
368 | { | |
369 | int ret; | |
370 | ||
371 | slow: | |
372 | local_irq_enable(); | |
373 | slow_irqon: | |
374 | /* Try to get the remaining pages with get_user_pages */ | |
375 | start += nr << PAGE_SHIFT; | |
376 | pages += nr; | |
377 | ||
378 | down_read(&mm->mmap_sem); | |
379 | ret = get_user_pages(current, mm, start, | |
380 | (end - start) >> PAGE_SHIFT, write, 0, pages, NULL); | |
381 | up_read(&mm->mmap_sem); | |
382 | ||
383 | /* Have to be a bit careful with return values */ | |
384 | if (nr > 0) { | |
385 | if (ret < 0) | |
386 | ret = nr; | |
387 | else | |
388 | ret += nr; | |
389 | } | |
390 | ||
391 | return ret; | |
392 | } | |
393 | } |