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Linux/arch/x86/mm/gup.c

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  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 
 15 static inline pte_t gup_get_pte(pte_t *ptep)
 16 {
 17 #ifndef CONFIG_X86_PAE
 18         return ACCESS_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  */
 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_numa(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 
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 
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 
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_numa(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 
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 
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  */
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  */
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                 down_read(&mm->mmap_sem);
392                 ret = get_user_pages(current, mm, start,
393                         (end - start) >> PAGE_SHIFT, write, 0, pages, NULL);
394                 up_read(&mm->mmap_sem);
395 
396                 /* Have to be a bit careful with return values */
397                 if (nr > 0) {
398                         if (ret < 0)
399                                 ret = nr;
400                         else
401                                 ret += nr;
402                 }
403 
404                 return ret;
405         }
406 }
407 

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