2 * Lockless get_user_pages_fast for x86
4 * Copyright (C) 2008 Nick Piggin
5 * Copyright (C) 2008 Novell Inc.
7 #include <linux/sched.h>
9 #include <linux/vmstat.h>
10 #include <linux/highmem.h>
11 #include <linux/swap.h>
12 #include <linux/memremap.h>
14 #include <asm/pgtable.h>
16 static inline pte_t
gup_get_pte(pte_t
*ptep
)
18 #ifndef CONFIG_X86_PAE
19 return READ_ONCE(*ptep
);
22 * With get_user_pages_fast, we walk down the pagetables without taking
23 * any locks. For this we would like to load the pointers atomically,
24 * but that is not possible (without expensive cmpxchg8b) on PAE. What
25 * we do have is the guarantee that a pte will only either go from not
26 * present to present, or present to not present or both -- it will not
27 * switch to a completely different present page without a TLB flush in
28 * between; something that we are blocking by holding interrupts off.
30 * Setting ptes from not present to present goes:
35 * And present to not present goes:
40 * We must ensure here that the load of pte_low sees l iff pte_high
41 * sees h. We load pte_high *after* loading pte_low, which ensures we
42 * don't see an older value of pte_high. *Then* we recheck pte_low,
43 * which ensures that we haven't picked up a changed pte high. We might
44 * have got rubbish values from pte_low and pte_high, but we are
45 * guaranteed that pte_low will not have the present bit set *unless*
46 * it is 'l'. And get_user_pages_fast only operates on present ptes, so
49 * gup_get_pte should not be used or copied outside gup.c without being
50 * very careful -- it does not atomically load the pte or anything that
51 * is likely to be useful for you.
56 pte
.pte_low
= ptep
->pte_low
;
58 pte
.pte_high
= ptep
->pte_high
;
60 if (unlikely(pte
.pte_low
!= ptep
->pte_low
))
67 static void undo_dev_pagemap(int *nr
, int nr_start
, struct page
**pages
)
69 while ((*nr
) - nr_start
) {
70 struct page
*page
= pages
[--(*nr
)];
72 ClearPageReferenced(page
);
78 * The performance critical leaf functions are made noinline otherwise gcc
79 * inlines everything into a single function which results in too much
82 static noinline
int gup_pte_range(pmd_t pmd
, unsigned long addr
,
83 unsigned long end
, int write
, struct page
**pages
, int *nr
)
85 struct dev_pagemap
*pgmap
= NULL
;
90 mask
= _PAGE_PRESENT
|_PAGE_USER
;
94 ptep
= pte_offset_map(&pmd
, addr
);
96 pte_t pte
= gup_get_pte(ptep
);
99 /* Similar to the PMD case, NUMA hinting must take slow path */
100 if (pte_protnone(pte
)) {
105 if (pte_devmap(pte
)) {
106 pgmap
= get_dev_pagemap(pte_pfn(pte
), pgmap
);
107 if (unlikely(!pgmap
)) {
108 undo_dev_pagemap(nr
, nr_start
, pages
);
112 } else if ((pte_flags(pte
) & (mask
| _PAGE_SPECIAL
)) != mask
) {
116 VM_BUG_ON(!pfn_valid(pte_pfn(pte
)));
117 page
= pte_page(pte
);
119 put_dev_pagemap(pgmap
);
120 SetPageReferenced(page
);
124 } while (ptep
++, addr
+= PAGE_SIZE
, addr
!= end
);
130 static inline void get_head_page_multiple(struct page
*page
, int nr
)
132 VM_BUG_ON_PAGE(page
!= compound_head(page
), page
);
133 VM_BUG_ON_PAGE(page_count(page
) == 0, page
);
134 atomic_add(nr
, &page
->_count
);
135 SetPageReferenced(page
);
138 static int __gup_device_huge_pmd(pmd_t pmd
, unsigned long addr
,
139 unsigned long end
, struct page
**pages
, int *nr
)
142 unsigned long pfn
= pmd_pfn(pmd
);
143 struct dev_pagemap
*pgmap
= NULL
;
145 pfn
+= (addr
& ~PMD_MASK
) >> PAGE_SHIFT
;
147 struct page
*page
= pfn_to_page(pfn
);
149 pgmap
= get_dev_pagemap(pfn
, pgmap
);
150 if (unlikely(!pgmap
)) {
151 undo_dev_pagemap(nr
, nr_start
, pages
);
154 SetPageReferenced(page
);
157 put_dev_pagemap(pgmap
);
160 } while (addr
+= PAGE_SIZE
, addr
!= end
);
164 static noinline
int gup_huge_pmd(pmd_t pmd
, unsigned long addr
,
165 unsigned long end
, int write
, struct page
**pages
, int *nr
)
168 struct page
*head
, *page
;
171 mask
= _PAGE_PRESENT
|_PAGE_USER
;
174 if ((pmd_flags(pmd
) & mask
) != mask
)
177 VM_BUG_ON(!pfn_valid(pmd_pfn(pmd
)));
179 return __gup_device_huge_pmd(pmd
, addr
, end
, pages
, nr
);
181 /* hugepages are never "special" */
182 VM_BUG_ON(pmd_flags(pmd
) & _PAGE_SPECIAL
);
185 head
= pmd_page(pmd
);
186 page
= head
+ ((addr
& ~PMD_MASK
) >> PAGE_SHIFT
);
188 VM_BUG_ON_PAGE(compound_head(page
) != head
, page
);
193 } while (addr
+= PAGE_SIZE
, addr
!= end
);
194 get_head_page_multiple(head
, refs
);
199 static int gup_pmd_range(pud_t pud
, unsigned long addr
, unsigned long end
,
200 int write
, struct page
**pages
, int *nr
)
205 pmdp
= pmd_offset(&pud
, addr
);
209 next
= pmd_addr_end(addr
, end
);
212 if (unlikely(pmd_large(pmd
) || !pmd_present(pmd
))) {
214 * NUMA hinting faults need to be handled in the GUP
215 * slowpath for accounting purposes and so that they
216 * can be serialised against THP migration.
218 if (pmd_protnone(pmd
))
220 if (!gup_huge_pmd(pmd
, addr
, next
, write
, pages
, nr
))
223 if (!gup_pte_range(pmd
, addr
, next
, write
, pages
, nr
))
226 } while (pmdp
++, addr
= next
, addr
!= end
);
231 static noinline
int gup_huge_pud(pud_t pud
, unsigned long addr
,
232 unsigned long end
, int write
, struct page
**pages
, int *nr
)
235 struct page
*head
, *page
;
238 mask
= _PAGE_PRESENT
|_PAGE_USER
;
241 if ((pud_flags(pud
) & mask
) != mask
)
243 /* hugepages are never "special" */
244 VM_BUG_ON(pud_flags(pud
) & _PAGE_SPECIAL
);
245 VM_BUG_ON(!pfn_valid(pud_pfn(pud
)));
248 head
= pud_page(pud
);
249 page
= head
+ ((addr
& ~PUD_MASK
) >> PAGE_SHIFT
);
251 VM_BUG_ON_PAGE(compound_head(page
) != head
, page
);
256 } while (addr
+= PAGE_SIZE
, addr
!= end
);
257 get_head_page_multiple(head
, refs
);
262 static int gup_pud_range(pgd_t pgd
, unsigned long addr
, unsigned long end
,
263 int write
, struct page
**pages
, int *nr
)
268 pudp
= pud_offset(&pgd
, addr
);
272 next
= pud_addr_end(addr
, end
);
275 if (unlikely(pud_large(pud
))) {
276 if (!gup_huge_pud(pud
, addr
, next
, write
, pages
, nr
))
279 if (!gup_pmd_range(pud
, addr
, next
, write
, pages
, nr
))
282 } while (pudp
++, addr
= next
, addr
!= end
);
288 * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
289 * back to the regular GUP.
291 int __get_user_pages_fast(unsigned long start
, int nr_pages
, int write
,
294 struct mm_struct
*mm
= current
->mm
;
295 unsigned long addr
, len
, end
;
303 len
= (unsigned long) nr_pages
<< PAGE_SHIFT
;
305 if (unlikely(!access_ok(write
? VERIFY_WRITE
: VERIFY_READ
,
306 (void __user
*)start
, len
)))
310 * XXX: batch / limit 'nr', to avoid large irq off latency
311 * needs some instrumenting to determine the common sizes used by
312 * important workloads (eg. DB2), and whether limiting the batch size
313 * will decrease performance.
315 * It seems like we're in the clear for the moment. Direct-IO is
316 * the main guy that batches up lots of get_user_pages, and even
317 * they are limited to 64-at-a-time which is not so many.
320 * This doesn't prevent pagetable teardown, but does prevent
321 * the pagetables and pages from being freed on x86.
323 * So long as we atomically load page table pointers versus teardown
324 * (which we do on x86, with the above PAE exception), we can follow the
325 * address down to the the page and take a ref on it.
327 local_irq_save(flags
);
328 pgdp
= pgd_offset(mm
, addr
);
332 next
= pgd_addr_end(addr
, end
);
335 if (!gup_pud_range(pgd
, addr
, next
, write
, pages
, &nr
))
337 } while (pgdp
++, addr
= next
, addr
!= end
);
338 local_irq_restore(flags
);
344 * get_user_pages_fast() - pin user pages in memory
345 * @start: starting user address
346 * @nr_pages: number of pages from start to pin
347 * @write: whether pages will be written to
348 * @pages: array that receives pointers to the pages pinned.
349 * Should be at least nr_pages long.
351 * Attempt to pin user pages in memory without taking mm->mmap_sem.
352 * If not successful, it will fall back to taking the lock and
353 * calling get_user_pages().
355 * Returns number of pages pinned. This may be fewer than the number
356 * requested. If nr_pages is 0 or negative, returns 0. If no pages
357 * were pinned, returns -errno.
359 int get_user_pages_fast(unsigned long start
, int nr_pages
, int write
,
362 struct mm_struct
*mm
= current
->mm
;
363 unsigned long addr
, len
, end
;
370 len
= (unsigned long) nr_pages
<< PAGE_SHIFT
;
377 if (end
>> __VIRTUAL_MASK_SHIFT
)
382 * XXX: batch / limit 'nr', to avoid large irq off latency
383 * needs some instrumenting to determine the common sizes used by
384 * important workloads (eg. DB2), and whether limiting the batch size
385 * will decrease performance.
387 * It seems like we're in the clear for the moment. Direct-IO is
388 * the main guy that batches up lots of get_user_pages, and even
389 * they are limited to 64-at-a-time which is not so many.
392 * This doesn't prevent pagetable teardown, but does prevent
393 * the pagetables and pages from being freed on x86.
395 * So long as we atomically load page table pointers versus teardown
396 * (which we do on x86, with the above PAE exception), we can follow the
397 * address down to the the page and take a ref on it.
400 pgdp
= pgd_offset(mm
, addr
);
404 next
= pgd_addr_end(addr
, end
);
407 if (!gup_pud_range(pgd
, addr
, next
, write
, pages
, &nr
))
409 } while (pgdp
++, addr
= next
, addr
!= end
);
412 VM_BUG_ON(nr
!= (end
- start
) >> PAGE_SHIFT
);
421 /* Try to get the remaining pages with get_user_pages */
422 start
+= nr
<< PAGE_SHIFT
;
425 ret
= get_user_pages_unlocked(current
, mm
, start
,
426 (end
- start
) >> PAGE_SHIFT
,
429 /* Have to be a bit careful with return values */
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