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 #include <linux/swap.h>
12
13 #include <asm/pgtable.h>
14
gup_get_pte(pte_t * ptep)15 static inline pte_t gup_get_pte(pte_t *ptep)
16 {
17 #ifndef CONFIG_X86_PAE
18 return READ_ONCE(*ptep);
19 #else
20 /*
21 * With get_user_pages_fast, we walk down the pagetables without taking
22 * any locks. For this we would like to load the pointers atomically,
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 */
gup_pte_range(pmd_t pmd,unsigned long addr,unsigned long end,int write,struct page ** pages,int * nr)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
86 /* Similar to the PMD case, NUMA hinting must take slow path */
87 if (pte_protnone(pte)) {
88 pte_unmap(ptep);
89 return 0;
90 }
91
92 if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) {
93 pte_unmap(ptep);
94 return 0;
95 }
96 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
97 page = pte_page(pte);
98 get_page(page);
99 SetPageReferenced(page);
100 pages[*nr] = page;
101 (*nr)++;
102
103 } while (ptep++, addr += PAGE_SIZE, addr != end);
104 pte_unmap(ptep - 1);
105
106 return 1;
107 }
108
get_head_page_multiple(struct page * page,int nr)109 static inline void get_head_page_multiple(struct page *page, int nr)
110 {
111 VM_BUG_ON_PAGE(page != compound_head(page), page);
112 VM_BUG_ON_PAGE(page_count(page) == 0, page);
113 atomic_add(nr, &page->_count);
114 SetPageReferenced(page);
115 }
116
gup_huge_pmd(pmd_t pmd,unsigned long addr,unsigned long end,int write,struct page ** pages,int * nr)117 static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
118 unsigned long end, int write, struct page **pages, int *nr)
119 {
120 unsigned long mask;
121 pte_t pte = *(pte_t *)&pmd;
122 struct page *head, *page;
123 int refs;
124
125 mask = _PAGE_PRESENT|_PAGE_USER;
126 if (write)
127 mask |= _PAGE_RW;
128 if ((pte_flags(pte) & mask) != mask)
129 return 0;
130 /* hugepages are never "special" */
131 VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
132 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
133
134 refs = 0;
135 head = pte_page(pte);
136 page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
137 do {
138 VM_BUG_ON_PAGE(compound_head(page) != head, page);
139 pages[*nr] = page;
140 if (PageTail(page))
141 get_huge_page_tail(page);
142 (*nr)++;
143 page++;
144 refs++;
145 } while (addr += PAGE_SIZE, addr != end);
146 get_head_page_multiple(head, refs);
147
148 return 1;
149 }
150
gup_pmd_range(pud_t pud,unsigned long addr,unsigned long end,int write,struct page ** pages,int * nr)151 static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
152 int write, struct page **pages, int *nr)
153 {
154 unsigned long next;
155 pmd_t *pmdp;
156
157 pmdp = pmd_offset(&pud, addr);
158 do {
159 pmd_t pmd = *pmdp;
160
161 next = pmd_addr_end(addr, end);
162 /*
163 * The pmd_trans_splitting() check below explains why
164 * pmdp_splitting_flush has to flush the tlb, to stop
165 * this gup-fast code from running while we set the
166 * splitting bit in the pmd. Returning zero will take
167 * the slow path that will call wait_split_huge_page()
168 * if the pmd is still in splitting state. gup-fast
169 * can't because it has irq disabled and
170 * wait_split_huge_page() would never return as the
171 * tlb flush IPI wouldn't run.
172 */
173 if (pmd_none(pmd) || pmd_trans_splitting(pmd))
174 return 0;
175 if (unlikely(pmd_large(pmd) || !pmd_present(pmd))) {
176 /*
177 * NUMA hinting faults need to be handled in the GUP
178 * slowpath for accounting purposes and so that they
179 * can be serialised against THP migration.
180 */
181 if (pmd_protnone(pmd))
182 return 0;
183 if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
184 return 0;
185 } else {
186 if (!gup_pte_range(pmd, addr, next, write, pages, nr))
187 return 0;
188 }
189 } while (pmdp++, addr = next, addr != end);
190
191 return 1;
192 }
193
gup_huge_pud(pud_t pud,unsigned long addr,unsigned long end,int write,struct page ** pages,int * nr)194 static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
195 unsigned long end, int write, struct page **pages, int *nr)
196 {
197 unsigned long mask;
198 pte_t pte = *(pte_t *)&pud;
199 struct page *head, *page;
200 int refs;
201
202 mask = _PAGE_PRESENT|_PAGE_USER;
203 if (write)
204 mask |= _PAGE_RW;
205 if ((pte_flags(pte) & mask) != mask)
206 return 0;
207 /* hugepages are never "special" */
208 VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
209 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
210
211 refs = 0;
212 head = pte_page(pte);
213 page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
214 do {
215 VM_BUG_ON_PAGE(compound_head(page) != head, page);
216 pages[*nr] = page;
217 if (PageTail(page))
218 get_huge_page_tail(page);
219 (*nr)++;
220 page++;
221 refs++;
222 } while (addr += PAGE_SIZE, addr != end);
223 get_head_page_multiple(head, refs);
224
225 return 1;
226 }
227
gup_pud_range(pgd_t pgd,unsigned long addr,unsigned long end,int write,struct page ** pages,int * nr)228 static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
229 int write, struct page **pages, int *nr)
230 {
231 unsigned long next;
232 pud_t *pudp;
233
234 pudp = pud_offset(&pgd, addr);
235 do {
236 pud_t pud = *pudp;
237
238 next = pud_addr_end(addr, end);
239 if (pud_none(pud))
240 return 0;
241 if (unlikely(pud_large(pud))) {
242 if (!gup_huge_pud(pud, addr, next, write, pages, nr))
243 return 0;
244 } else {
245 if (!gup_pmd_range(pud, addr, next, write, pages, nr))
246 return 0;
247 }
248 } while (pudp++, addr = next, addr != end);
249
250 return 1;
251 }
252
253 /*
254 * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
255 * back to the regular GUP.
256 */
__get_user_pages_fast(unsigned long start,int nr_pages,int write,struct page ** pages)257 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
258 struct page **pages)
259 {
260 struct mm_struct *mm = current->mm;
261 unsigned long addr, len, end;
262 unsigned long next;
263 unsigned long flags;
264 pgd_t *pgdp;
265 int nr = 0;
266
267 start &= PAGE_MASK;
268 addr = start;
269 len = (unsigned long) nr_pages << PAGE_SHIFT;
270 end = start + len;
271 if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
272 (void __user *)start, len)))
273 return 0;
274
275 /*
276 * XXX: batch / limit 'nr', to avoid large irq off latency
277 * needs some instrumenting to determine the common sizes used by
278 * important workloads (eg. DB2), and whether limiting the batch size
279 * will decrease performance.
280 *
281 * It seems like we're in the clear for the moment. Direct-IO is
282 * the main guy that batches up lots of get_user_pages, and even
283 * they are limited to 64-at-a-time which is not so many.
284 */
285 /*
286 * This doesn't prevent pagetable teardown, but does prevent
287 * the pagetables and pages from being freed on x86.
288 *
289 * So long as we atomically load page table pointers versus teardown
290 * (which we do on x86, with the above PAE exception), we can follow the
291 * address down to the the page and take a ref on it.
292 */
293 local_irq_save(flags);
294 pgdp = pgd_offset(mm, addr);
295 do {
296 pgd_t pgd = *pgdp;
297
298 next = pgd_addr_end(addr, end);
299 if (pgd_none(pgd))
300 break;
301 if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
302 break;
303 } while (pgdp++, addr = next, addr != end);
304 local_irq_restore(flags);
305
306 return nr;
307 }
308
309 /**
310 * get_user_pages_fast() - pin user pages in memory
311 * @start: starting user address
312 * @nr_pages: number of pages from start to pin
313 * @write: whether pages will be written to
314 * @pages: array that receives pointers to the pages pinned.
315 * Should be at least nr_pages long.
316 *
317 * Attempt to pin user pages in memory without taking mm->mmap_sem.
318 * If not successful, it will fall back to taking the lock and
319 * calling get_user_pages().
320 *
321 * Returns number of pages pinned. This may be fewer than the number
322 * requested. If nr_pages is 0 or negative, returns 0. If no pages
323 * were pinned, returns -errno.
324 */
get_user_pages_fast(unsigned long start,int nr_pages,int write,struct page ** pages)325 int get_user_pages_fast(unsigned long start, int nr_pages, int write,
326 struct page **pages)
327 {
328 struct mm_struct *mm = current->mm;
329 unsigned long addr, len, end;
330 unsigned long next;
331 pgd_t *pgdp;
332 int nr = 0;
333
334 start &= PAGE_MASK;
335 addr = start;
336 len = (unsigned long) nr_pages << PAGE_SHIFT;
337
338 end = start + len;
339 if (end < start)
340 goto slow_irqon;
341
342 #ifdef CONFIG_X86_64
343 if (end >> __VIRTUAL_MASK_SHIFT)
344 goto slow_irqon;
345 #endif
346
347 /*
348 * XXX: batch / limit 'nr', to avoid large irq off latency
349 * needs some instrumenting to determine the common sizes used by
350 * important workloads (eg. DB2), and whether limiting the batch size
351 * will decrease performance.
352 *
353 * It seems like we're in the clear for the moment. Direct-IO is
354 * the main guy that batches up lots of get_user_pages, and even
355 * they are limited to 64-at-a-time which is not so many.
356 */
357 /*
358 * This doesn't prevent pagetable teardown, but does prevent
359 * the pagetables and pages from being freed on x86.
360 *
361 * So long as we atomically load page table pointers versus teardown
362 * (which we do on x86, with the above PAE exception), we can follow the
363 * address down to the the page and take a ref on it.
364 */
365 local_irq_disable();
366 pgdp = pgd_offset(mm, addr);
367 do {
368 pgd_t pgd = *pgdp;
369
370 next = pgd_addr_end(addr, end);
371 if (pgd_none(pgd))
372 goto slow;
373 if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
374 goto slow;
375 } while (pgdp++, addr = next, addr != end);
376 local_irq_enable();
377
378 VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
379 return nr;
380
381 {
382 int ret;
383
384 slow:
385 local_irq_enable();
386 slow_irqon:
387 /* Try to get the remaining pages with get_user_pages */
388 start += nr << PAGE_SHIFT;
389 pages += nr;
390
391 ret = get_user_pages_unlocked(current, mm, start,
392 (end - start) >> PAGE_SHIFT,
393 write, 0, pages);
394
395 /* Have to be a bit careful with return values */
396 if (nr > 0) {
397 if (ret < 0)
398 ret = nr;
399 else
400 ret += nr;
401 }
402
403 return ret;
404 }
405 }
406