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
12 #include <asm/pgtable.h>
13
gup_get_pte(pte_t * ptep)14 static inline pte_t gup_get_pte(pte_t *ptep)
15 {
16 #ifndef CONFIG_X86_PAE
17 return *ptep;
18 #else
19 /*
20 * With get_user_pages_fast, we walk down the pagetables without taking
21 * any locks. For this we would like to load the pointers atoimcally,
22 * but that is not possible (without expensive cmpxchg8b) on PAE. What
23 * we do have is the guarantee that a pte will only either go from not
24 * present to present, or present to not present or both -- it will not
25 * switch to a completely different present page without a TLB flush in
26 * between; something that we are blocking by holding interrupts off.
27 *
28 * Setting ptes from not present to present goes:
29 * ptep->pte_high = h;
30 * smp_wmb();
31 * ptep->pte_low = l;
32 *
33 * And present to not present goes:
34 * ptep->pte_low = 0;
35 * smp_wmb();
36 * ptep->pte_high = 0;
37 *
38 * We must ensure here that the load of pte_low sees l iff pte_high
39 * sees h. We load pte_high *after* loading pte_low, which ensures we
40 * don't see an older value of pte_high. *Then* we recheck pte_low,
41 * which ensures that we haven't picked up a changed pte high. We might
42 * have got rubbish values from pte_low and pte_high, but we are
43 * guaranteed that pte_low will not have the present bit set *unless*
44 * it is 'l'. And get_user_pages_fast only operates on present ptes, so
45 * we're safe.
46 *
47 * gup_get_pte should not be used or copied outside gup.c without being
48 * very careful -- it does not atomically load the pte or anything that
49 * is likely to be useful for you.
50 */
51 pte_t pte;
52
53 retry:
54 pte.pte_low = ptep->pte_low;
55 smp_rmb();
56 pte.pte_high = ptep->pte_high;
57 smp_rmb();
58 if (unlikely(pte.pte_low != ptep->pte_low))
59 goto retry;
60
61 return pte;
62 #endif
63 }
64
65 /*
66 * The performance critical leaf functions are made noinline otherwise gcc
67 * inlines everything into a single function which results in too much
68 * register pressure.
69 */
gup_pte_range(pmd_t pmd,unsigned long addr,unsigned long end,int write,struct page ** pages,int * nr)70 static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
71 unsigned long end, int write, struct page **pages, int *nr)
72 {
73 unsigned long mask;
74 pte_t *ptep;
75
76 mask = _PAGE_PRESENT|_PAGE_USER;
77 if (write)
78 mask |= _PAGE_RW;
79
80 ptep = pte_offset_map(&pmd, addr);
81 do {
82 pte_t pte = gup_get_pte(ptep);
83 struct page *page;
84
85 if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) {
86 pte_unmap(ptep);
87 return 0;
88 }
89 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
90 page = pte_page(pte);
91 get_page(page);
92 pages[*nr] = page;
93 (*nr)++;
94
95 } while (ptep++, addr += PAGE_SIZE, addr != end);
96 pte_unmap(ptep - 1);
97
98 return 1;
99 }
100
get_head_page_multiple(struct page * page,int nr)101 static inline void get_head_page_multiple(struct page *page, int nr)
102 {
103 VM_BUG_ON(page != compound_head(page));
104 VM_BUG_ON(page_count(page) == 0);
105 atomic_add(nr, &page->_count);
106 }
107
gup_huge_pmd(pmd_t pmd,unsigned long addr,unsigned long end,int write,struct page ** pages,int * nr)108 static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
109 unsigned long end, int write, struct page **pages, int *nr)
110 {
111 unsigned long mask;
112 pte_t pte = *(pte_t *)&pmd;
113 struct page *head, *page;
114 int refs;
115
116 mask = _PAGE_PRESENT|_PAGE_USER;
117 if (write)
118 mask |= _PAGE_RW;
119 if ((pte_flags(pte) & mask) != mask)
120 return 0;
121 /* hugepages are never "special" */
122 VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
123 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
124
125 refs = 0;
126 head = pte_page(pte);
127 page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
128 do {
129 VM_BUG_ON(compound_head(page) != head);
130 pages[*nr] = page;
131 (*nr)++;
132 page++;
133 refs++;
134 } while (addr += PAGE_SIZE, addr != end);
135 get_head_page_multiple(head, refs);
136
137 return 1;
138 }
139
gup_pmd_range(pud_t pud,unsigned long addr,unsigned long end,int write,struct page ** pages,int * nr)140 static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
141 int write, struct page **pages, int *nr)
142 {
143 unsigned long next;
144 pmd_t *pmdp;
145
146 pmdp = pmd_offset(&pud, addr);
147 do {
148 pmd_t pmd = *pmdp;
149
150 next = pmd_addr_end(addr, end);
151 if (pmd_none(pmd))
152 return 0;
153 if (unlikely(pmd_large(pmd))) {
154 if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
155 return 0;
156 } else {
157 if (!gup_pte_range(pmd, addr, next, write, pages, nr))
158 return 0;
159 }
160 } while (pmdp++, addr = next, addr != end);
161
162 return 1;
163 }
164
gup_huge_pud(pud_t pud,unsigned long addr,unsigned long end,int write,struct page ** pages,int * nr)165 static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
166 unsigned long end, int write, struct page **pages, int *nr)
167 {
168 unsigned long mask;
169 pte_t pte = *(pte_t *)&pud;
170 struct page *head, *page;
171 int refs;
172
173 mask = _PAGE_PRESENT|_PAGE_USER;
174 if (write)
175 mask |= _PAGE_RW;
176 if ((pte_flags(pte) & mask) != mask)
177 return 0;
178 /* hugepages are never "special" */
179 VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
180 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
181
182 refs = 0;
183 head = pte_page(pte);
184 page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
185 do {
186 VM_BUG_ON(compound_head(page) != head);
187 pages[*nr] = page;
188 (*nr)++;
189 page++;
190 refs++;
191 } while (addr += PAGE_SIZE, addr != end);
192 get_head_page_multiple(head, refs);
193
194 return 1;
195 }
196
gup_pud_range(pgd_t pgd,unsigned long addr,unsigned long end,int write,struct page ** pages,int * nr)197 static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
198 int write, struct page **pages, int *nr)
199 {
200 unsigned long next;
201 pud_t *pudp;
202
203 pudp = pud_offset(&pgd, addr);
204 do {
205 pud_t pud = *pudp;
206
207 next = pud_addr_end(addr, end);
208 if (pud_none(pud))
209 return 0;
210 if (unlikely(pud_large(pud))) {
211 if (!gup_huge_pud(pud, addr, next, write, pages, nr))
212 return 0;
213 } else {
214 if (!gup_pmd_range(pud, addr, next, write, pages, nr))
215 return 0;
216 }
217 } while (pudp++, addr = next, addr != end);
218
219 return 1;
220 }
221
get_user_pages_fast(unsigned long start,int nr_pages,int write,struct page ** pages)222 int get_user_pages_fast(unsigned long start, int nr_pages, int write,
223 struct page **pages)
224 {
225 struct mm_struct *mm = current->mm;
226 unsigned long addr, len, end;
227 unsigned long next;
228 pgd_t *pgdp;
229 int nr = 0;
230
231 start &= PAGE_MASK;
232 addr = start;
233 len = (unsigned long) nr_pages << PAGE_SHIFT;
234 end = start + len;
235 if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
236 (void __user *)start, len)))
237 goto slow_irqon;
238
239 /*
240 * XXX: batch / limit 'nr', to avoid large irq off latency
241 * needs some instrumenting to determine the common sizes used by
242 * important workloads (eg. DB2), and whether limiting the batch size
243 * will decrease performance.
244 *
245 * It seems like we're in the clear for the moment. Direct-IO is
246 * the main guy that batches up lots of get_user_pages, and even
247 * they are limited to 64-at-a-time which is not so many.
248 */
249 /*
250 * This doesn't prevent pagetable teardown, but does prevent
251 * the pagetables and pages from being freed on x86.
252 *
253 * So long as we atomically load page table pointers versus teardown
254 * (which we do on x86, with the above PAE exception), we can follow the
255 * address down to the the page and take a ref on it.
256 */
257 local_irq_disable();
258 pgdp = pgd_offset(mm, addr);
259 do {
260 pgd_t pgd = *pgdp;
261
262 next = pgd_addr_end(addr, end);
263 if (pgd_none(pgd))
264 goto slow;
265 if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
266 goto slow;
267 } while (pgdp++, addr = next, addr != end);
268 local_irq_enable();
269
270 VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
271 return nr;
272
273 {
274 int ret;
275
276 slow:
277 local_irq_enable();
278 slow_irqon:
279 /* Try to get the remaining pages with get_user_pages */
280 start += nr << PAGE_SHIFT;
281 pages += nr;
282
283 down_read(&mm->mmap_sem);
284 ret = get_user_pages(current, mm, start,
285 (end - start) >> PAGE_SHIFT, write, 0, pages, NULL);
286 up_read(&mm->mmap_sem);
287
288 /* Have to be a bit careful with return values */
289 if (nr > 0) {
290 if (ret < 0)
291 ret = nr;
292 else
293 ret += nr;
294 }
295
296 return ret;
297 }
298 }
299