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