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Nick Piggin8174c432008-07-25 19:45:24 -07001/*
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>
Andrea Arcangeli8ee53822011-01-13 15:47:10 -080011#include <linux/swap.h>
Nick Piggin8174c432008-07-25 19:45:24 -070012
13#include <asm/pgtable.h>
14
15static inline pte_t gup_get_pte(pte_t *ptep)
16{
17#ifndef CONFIG_X86_PAE
Ingo Molnar0c871972009-06-15 11:35:01 +020018 return ACCESS_ONCE(*ptep);
Nick Piggin8174c432008-07-25 19:45:24 -070019#else
20 /*
21 * With get_user_pages_fast, we walk down the pagetables without taking
Andy Shevchenkoab098092010-02-02 14:38:12 -080022 * any locks. For this we would like to load the pointers atomically,
Nick Piggin8174c432008-07-25 19:45:24 -070023 * 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
54retry:
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 */
71static 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
Jan Beulich606ee442008-09-17 16:48:17 +010086 if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) {
Nick Piggin8174c432008-07-25 19:45:24 -070087 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);
Andrea Arcangeli8ee53822011-01-13 15:47:10 -080093 SetPageReferenced(page);
Nick Piggin8174c432008-07-25 19:45:24 -070094 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
103static 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);
Andrea Arcangeli8ee53822011-01-13 15:47:10 -0800108 SetPageReferenced(page);
Nick Piggin8174c432008-07-25 19:45:24 -0700109}
110
111static 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;
Jan Beulich606ee442008-09-17 16:48:17 +0100122 if ((pte_flags(pte) & mask) != mask)
Nick Piggin8174c432008-07-25 19:45:24 -0700123 return 0;
124 /* hugepages are never "special" */
Jan Beulich606ee442008-09-17 16:48:17 +0100125 VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
Nick Piggin8174c432008-07-25 19:45:24 -0700126 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
127
128 refs = 0;
129 head = pte_page(pte);
Nick Piggin652ea692008-07-25 19:45:27 -0700130 page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
Nick Piggin8174c432008-07-25 19:45:24 -0700131 do {
132 VM_BUG_ON(compound_head(page) != head);
133 pages[*nr] = page;
Andrea Arcangeli91807062011-01-13 15:46:32 -0800134 if (PageTail(page))
135 get_huge_page_tail(page);
Nick Piggin8174c432008-07-25 19:45:24 -0700136 (*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
145static 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);
Andrea Arcangeli64cc6ae2011-01-13 15:46:42 -0800156 /*
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))
Nick Piggin8174c432008-07-25 19:45:24 -0700168 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
Nick Piggin652ea692008-07-25 19:45:27 -0700181static 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;
Jan Beulich606ee442008-09-17 16:48:17 +0100192 if ((pte_flags(pte) & mask) != mask)
Nick Piggin652ea692008-07-25 19:45:27 -0700193 return 0;
194 /* hugepages are never "special" */
Jan Beulich606ee442008-09-17 16:48:17 +0100195 VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
Nick Piggin652ea692008-07-25 19:45:27 -0700196 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;
204 (*nr)++;
205 page++;
206 refs++;
207 } while (addr += PAGE_SIZE, addr != end);
208 get_head_page_multiple(head, refs);
209
210 return 1;
211}
212
Nick Piggin8174c432008-07-25 19:45:24 -0700213static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
214 int write, struct page **pages, int *nr)
215{
216 unsigned long next;
217 pud_t *pudp;
218
219 pudp = pud_offset(&pgd, addr);
220 do {
221 pud_t pud = *pudp;
222
223 next = pud_addr_end(addr, end);
224 if (pud_none(pud))
225 return 0;
Nick Piggin652ea692008-07-25 19:45:27 -0700226 if (unlikely(pud_large(pud))) {
227 if (!gup_huge_pud(pud, addr, next, write, pages, nr))
228 return 0;
229 } else {
230 if (!gup_pmd_range(pud, addr, next, write, pages, nr))
231 return 0;
232 }
Nick Piggin8174c432008-07-25 19:45:24 -0700233 } while (pudp++, addr = next, addr != end);
234
235 return 1;
236}
237
Peter Zijlstra465a4542009-06-15 12:31:37 +0200238/*
239 * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
240 * back to the regular GUP.
241 */
242int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
243 struct page **pages)
244{
245 struct mm_struct *mm = current->mm;
246 unsigned long addr, len, end;
247 unsigned long next;
248 unsigned long flags;
249 pgd_t *pgdp;
250 int nr = 0;
251
252 start &= PAGE_MASK;
253 addr = start;
254 len = (unsigned long) nr_pages << PAGE_SHIFT;
255 end = start + len;
256 if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
257 (void __user *)start, len)))
258 return 0;
259
260 /*
261 * XXX: batch / limit 'nr', to avoid large irq off latency
262 * needs some instrumenting to determine the common sizes used by
263 * important workloads (eg. DB2), and whether limiting the batch size
264 * will decrease performance.
265 *
266 * It seems like we're in the clear for the moment. Direct-IO is
267 * the main guy that batches up lots of get_user_pages, and even
268 * they are limited to 64-at-a-time which is not so many.
269 */
270 /*
271 * This doesn't prevent pagetable teardown, but does prevent
272 * the pagetables and pages from being freed on x86.
273 *
274 * So long as we atomically load page table pointers versus teardown
275 * (which we do on x86, with the above PAE exception), we can follow the
276 * address down to the the page and take a ref on it.
277 */
278 local_irq_save(flags);
279 pgdp = pgd_offset(mm, addr);
280 do {
281 pgd_t pgd = *pgdp;
282
283 next = pgd_addr_end(addr, end);
284 if (pgd_none(pgd))
285 break;
286 if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
287 break;
288 } while (pgdp++, addr = next, addr != end);
289 local_irq_restore(flags);
290
291 return nr;
292}
293
Andy Grovera0d22f42009-04-09 16:45:29 -0700294/**
295 * get_user_pages_fast() - pin user pages in memory
296 * @start: starting user address
297 * @nr_pages: number of pages from start to pin
298 * @write: whether pages will be written to
299 * @pages: array that receives pointers to the pages pinned.
300 * Should be at least nr_pages long.
301 *
302 * Attempt to pin user pages in memory without taking mm->mmap_sem.
303 * If not successful, it will fall back to taking the lock and
304 * calling get_user_pages().
305 *
306 * Returns number of pages pinned. This may be fewer than the number
307 * requested. If nr_pages is 0 or negative, returns 0. If no pages
308 * were pinned, returns -errno.
309 */
Nick Piggin8174c432008-07-25 19:45:24 -0700310int get_user_pages_fast(unsigned long start, int nr_pages, int write,
311 struct page **pages)
312{
313 struct mm_struct *mm = current->mm;
Linus Torvalds9b790222008-07-28 17:54:21 -0700314 unsigned long addr, len, end;
Nick Piggin8174c432008-07-25 19:45:24 -0700315 unsigned long next;
316 pgd_t *pgdp;
317 int nr = 0;
318
Linus Torvalds9b790222008-07-28 17:54:21 -0700319 start &= PAGE_MASK;
320 addr = start;
321 len = (unsigned long) nr_pages << PAGE_SHIFT;
Linus Torvalds7f818902009-06-20 09:52:27 -0700322
Linus Torvalds9b790222008-07-28 17:54:21 -0700323 end = start + len;
Linus Torvalds7f818902009-06-20 09:52:27 -0700324 if (end < start)
Nick Piggin8174c432008-07-25 19:45:24 -0700325 goto slow_irqon;
326
Linus Torvalds7f818902009-06-20 09:52:27 -0700327#ifdef CONFIG_X86_64
328 if (end >> __VIRTUAL_MASK_SHIFT)
329 goto slow_irqon;
330#endif
331
Nick Piggin8174c432008-07-25 19:45:24 -0700332 /*
333 * XXX: batch / limit 'nr', to avoid large irq off latency
334 * needs some instrumenting to determine the common sizes used by
335 * important workloads (eg. DB2), and whether limiting the batch size
336 * will decrease performance.
337 *
338 * It seems like we're in the clear for the moment. Direct-IO is
339 * the main guy that batches up lots of get_user_pages, and even
340 * they are limited to 64-at-a-time which is not so many.
341 */
342 /*
343 * This doesn't prevent pagetable teardown, but does prevent
344 * the pagetables and pages from being freed on x86.
345 *
346 * So long as we atomically load page table pointers versus teardown
347 * (which we do on x86, with the above PAE exception), we can follow the
348 * address down to the the page and take a ref on it.
349 */
350 local_irq_disable();
351 pgdp = pgd_offset(mm, addr);
352 do {
353 pgd_t pgd = *pgdp;
354
355 next = pgd_addr_end(addr, end);
356 if (pgd_none(pgd))
357 goto slow;
358 if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
359 goto slow;
360 } while (pgdp++, addr = next, addr != end);
361 local_irq_enable();
362
363 VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
364 return nr;
365
366 {
367 int ret;
368
369slow:
370 local_irq_enable();
371slow_irqon:
372 /* Try to get the remaining pages with get_user_pages */
373 start += nr << PAGE_SHIFT;
374 pages += nr;
375
376 down_read(&mm->mmap_sem);
377 ret = get_user_pages(current, mm, start,
378 (end - start) >> PAGE_SHIFT, write, 0, pages, NULL);
379 up_read(&mm->mmap_sem);
380
381 /* Have to be a bit careful with return values */
382 if (nr > 0) {
383 if (ret < 0)
384 ret = nr;
385 else
386 ret += nr;
387 }
388
389 return ret;
390 }
391}