[linux-2.6-block.git] / arch / x86 / mm / gup.c

/*
 * Lockless get_user_pages_fast for x86
 *
 * Copyright (C) 2008 Nick Piggin
 * Copyright (C) 2008 Novell Inc.
 */
#include <linux/sched.h>
#include <linux/mm.h>
#include <linux/vmstat.h>
#include <linux/highmem.h>

#include <asm/pgtable.h>

static inline pte_t gup_get_pte(pte_t *ptep)
{
#ifndef CONFIG_X86_PAE
	return ACCESS_ONCE(*ptep);
#else
	/*
	 * With get_user_pages_fast, we walk down the pagetables without taking
	 * any locks.  For this we would like to load the pointers atomically,
	 * but that is not possible (without expensive cmpxchg8b) on PAE.  What
	 * we do have is the guarantee that a pte will only either go from not
	 * present to present, or present to not present or both -- it will not
	 * switch to a completely different present page without a TLB flush in
	 * between; something that we are blocking by holding interrupts off.
	 *
	 * Setting ptes from not present to present goes:
	 * ptep->pte_high = h;
	 * smp_wmb();
	 * ptep->pte_low = l;
	 *
	 * And present to not present goes:
	 * ptep->pte_low = 0;
	 * smp_wmb();
	 * ptep->pte_high = 0;
	 *
	 * We must ensure here that the load of pte_low sees l iff pte_high
	 * sees h. We load pte_high *after* loading pte_low, which ensures we
	 * don't see an older value of pte_high.  *Then* we recheck pte_low,
	 * which ensures that we haven't picked up a changed pte high. We might
	 * have got rubbish values from pte_low and pte_high, but we are
	 * guaranteed that pte_low will not have the present bit set *unless*
	 * it is 'l'. And get_user_pages_fast only operates on present ptes, so
	 * we're safe.
	 *
	 * gup_get_pte should not be used or copied outside gup.c without being
	 * very careful -- it does not atomically load the pte or anything that
	 * is likely to be useful for you.
	 */
	pte_t pte;

retry:
	pte.pte_low = ptep->pte_low;
	smp_rmb();
	pte.pte_high = ptep->pte_high;
	smp_rmb();
	if (unlikely(pte.pte_low != ptep->pte_low))
		goto retry;

	return pte;
#endif
}

/*
 * The performance critical leaf functions are made noinline otherwise gcc
 * inlines everything into a single function which results in too much
 * register pressure.
 */
static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
		unsigned long end, int write, struct page **pages, int *nr)
{
	unsigned long mask;
	pte_t *ptep;

	mask = _PAGE_PRESENT|_PAGE_USER;
	if (write)
		mask |= _PAGE_RW;

	ptep = pte_offset_map(&pmd, addr);
	do {
		pte_t pte = gup_get_pte(ptep);
		struct page *page;

		if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) {
			pte_unmap(ptep);
			return 0;
		}
		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
		page = pte_page(pte);
		get_page(page);
		pages[*nr] = page;
		(*nr)++;

	} while (ptep++, addr += PAGE_SIZE, addr != end);
	pte_unmap(ptep - 1);

	return 1;
}

static inline void get_head_page_multiple(struct page *page, int nr)
{
	VM_BUG_ON(page != compound_head(page));
	VM_BUG_ON(page_count(page) == 0);
	atomic_add(nr, &page->_count);
}

static inline void get_huge_page_tail(struct page *page)
{
	/*
	 * __split_huge_page_refcount() cannot run
	 * from under us.
	 */
	VM_BUG_ON(atomic_read(&page->_count) < 0);
	atomic_inc(&page->_count);
}

static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
		unsigned long end, int write, struct page **pages, int *nr)
{
	unsigned long mask;
	pte_t pte = *(pte_t *)&pmd;
	struct page *head, *page;
	int refs;

	mask = _PAGE_PRESENT|_PAGE_USER;
	if (write)
		mask |= _PAGE_RW;
	if ((pte_flags(pte) & mask) != mask)
		return 0;
	/* hugepages are never "special" */
	VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
	VM_BUG_ON(!pfn_valid(pte_pfn(pte)));

	refs = 0;
	head = pte_page(pte);
	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
	do {
		VM_BUG_ON(compound_head(page) != head);
		pages[*nr] = page;
		if (PageTail(page))
			get_huge_page_tail(page);
		(*nr)++;
		page++;
		refs++;
	} while (addr += PAGE_SIZE, addr != end);
	get_head_page_multiple(head, refs);

	return 1;
}

static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
		int write, struct page **pages, int *nr)
{
	unsigned long next;
	pmd_t *pmdp;

	pmdp = pmd_offset(&pud, addr);
	do {
		pmd_t pmd = *pmdp;

		next = pmd_addr_end(addr, end);
		if (pmd_none(pmd))
			return 0;
		if (unlikely(pmd_large(pmd))) {
			if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
				return 0;
		} else {
			if (!gup_pte_range(pmd, addr, next, write, pages, nr))
				return 0;
		}
	} while (pmdp++, addr = next, addr != end);

	return 1;
}

static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
		unsigned long end, int write, struct page **pages, int *nr)
{
	unsigned long mask;
	pte_t pte = *(pte_t *)&pud;
	struct page *head, *page;
	int refs;

	mask = _PAGE_PRESENT|_PAGE_USER;
	if (write)
		mask |= _PAGE_RW;
	if ((pte_flags(pte) & mask) != mask)
		return 0;
	/* hugepages are never "special" */
	VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
	VM_BUG_ON(!pfn_valid(pte_pfn(pte)));

	refs = 0;
	head = pte_page(pte);
	page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
	do {
		VM_BUG_ON(compound_head(page) != head);
		pages[*nr] = page;
		(*nr)++;
		page++;
		refs++;
	} while (addr += PAGE_SIZE, addr != end);
	get_head_page_multiple(head, refs);

	return 1;
}

static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
			int write, struct page **pages, int *nr)
{
	unsigned long next;
	pud_t *pudp;

	pudp = pud_offset(&pgd, addr);
	do {
		pud_t pud = *pudp;

		next = pud_addr_end(addr, end);
		if (pud_none(pud))
			return 0;
		if (unlikely(pud_large(pud))) {
			if (!gup_huge_pud(pud, addr, next, write, pages, nr))
				return 0;
		} else {
			if (!gup_pmd_range(pud, addr, next, write, pages, nr))
				return 0;
		}
	} while (pudp++, addr = next, addr != end);

	return 1;
}

/*
 * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
 * back to the regular GUP.
 */
int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
			  struct page **pages)
{
	struct mm_struct *mm = current->mm;
	unsigned long addr, len, end;
	unsigned long next;
	unsigned long flags;
	pgd_t *pgdp;
	int nr = 0;

	start &= PAGE_MASK;
	addr = start;
	len = (unsigned long) nr_pages << PAGE_SHIFT;
	end = start + len;
	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
					(void __user *)start, len)))
		return 0;

	/*
	 * XXX: batch / limit 'nr', to avoid large irq off latency
	 * needs some instrumenting to determine the common sizes used by
	 * important workloads (eg. DB2), and whether limiting the batch size
	 * will decrease performance.
	 *
	 * It seems like we're in the clear for the moment. Direct-IO is
	 * the main guy that batches up lots of get_user_pages, and even
	 * they are limited to 64-at-a-time which is not so many.
	 */
	/*
	 * This doesn't prevent pagetable teardown, but does prevent
	 * the pagetables and pages from being freed on x86.
	 *
	 * So long as we atomically load page table pointers versus teardown
	 * (which we do on x86, with the above PAE exception), we can follow the
	 * address down to the the page and take a ref on it.
	 */
	local_irq_save(flags);
	pgdp = pgd_offset(mm, addr);
	do {
		pgd_t pgd = *pgdp;

		next = pgd_addr_end(addr, end);
		if (pgd_none(pgd))
			break;
		if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
			break;
	} while (pgdp++, addr = next, addr != end);
	local_irq_restore(flags);

	return nr;
}

/**
 * get_user_pages_fast() - pin user pages in memory
 * @start:	starting user address
 * @nr_pages:	number of pages from start to pin
 * @write:	whether pages will be written to
 * @pages:	array that receives pointers to the pages pinned.
 * 		Should be at least nr_pages long.
 *
 * Attempt to pin user pages in memory without taking mm->mmap_sem.
 * If not successful, it will fall back to taking the lock and
 * calling get_user_pages().
 *
 * Returns number of pages pinned. This may be fewer than the number
 * requested. If nr_pages is 0 or negative, returns 0. If no pages
 * were pinned, returns -errno.
 */
int get_user_pages_fast(unsigned long start, int nr_pages, int write,
			struct page **pages)
{
	struct mm_struct *mm = current->mm;
	unsigned long addr, len, end;
	unsigned long next;
	pgd_t *pgdp;
	int nr = 0;

	start &= PAGE_MASK;
	addr = start;
	len = (unsigned long) nr_pages << PAGE_SHIFT;

	end = start + len;
	if (end < start)
		goto slow_irqon;

#ifdef CONFIG_X86_64
	if (end >> __VIRTUAL_MASK_SHIFT)
		goto slow_irqon;
#endif

	/*
	 * XXX: batch / limit 'nr', to avoid large irq off latency
	 * needs some instrumenting to determine the common sizes used by
	 * important workloads (eg. DB2), and whether limiting the batch size
	 * will decrease performance.
	 *
	 * It seems like we're in the clear for the moment. Direct-IO is
	 * the main guy that batches up lots of get_user_pages, and even
	 * they are limited to 64-at-a-time which is not so many.
	 */
	/*
	 * This doesn't prevent pagetable teardown, but does prevent
	 * the pagetables and pages from being freed on x86.
	 *
	 * So long as we atomically load page table pointers versus teardown
	 * (which we do on x86, with the above PAE exception), we can follow the
	 * address down to the the page and take a ref on it.
	 */
	local_irq_disable();
	pgdp = pgd_offset(mm, addr);
	do {
		pgd_t pgd = *pgdp;

		next = pgd_addr_end(addr, end);
		if (pgd_none(pgd))
			goto slow;
		if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
			goto slow;
	} while (pgdp++, addr = next, addr != end);
	local_irq_enable();

	VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
	return nr;

	{
		int ret;

slow:
		local_irq_enable();
slow_irqon:
		/* Try to get the remaining pages with get_user_pages */
		start += nr << PAGE_SHIFT;
		pages += nr;

		down_read(&mm->mmap_sem);
		ret = get_user_pages(current, mm, start,
			(end - start) >> PAGE_SHIFT, write, 0, pages, NULL);
		up_read(&mm->mmap_sem);

		/* Have to be a bit careful with return values */
		if (nr > 0) {
			if (ret < 0)
				ret = nr;
			else
				ret += nr;
		}

		return ret;
	}
}
Commit	Line	Data
8174c430 NP	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
	14	static inline pte_t gup_get_pte(pte_t *ptep)
	15	{
	16	#ifndef CONFIG_X86_PAE
0c871971	17	return ACCESS_ONCE(*ptep);
8174c430 NP	18	#else
	19	/*
	20	* With get_user_pages_fast, we walk down the pagetables without taking
ab09809f	21	* any locks. For this we would like to load the pointers atomically,
8174c430 NP	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	*/
	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
606ee44d	85	if ((pte_flags(pte) & (mask \| _PAGE_SPECIAL)) != mask) {
8174c430 NP	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
	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
91807063 AA	108	static inline void get_huge_page_tail(struct page *page)
	109	{
	110	/*
	111	* __split_huge_page_refcount() cannot run
	112	* from under us.
	113	*/
	114	VM_BUG_ON(atomic_read(&page->_count) < 0);
	115	atomic_inc(&page->_count);
	116	}
	117
8174c430 NP	118	static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
	119	unsigned long end, int write, struct page *pages, int nr)
	120	{
	121	unsigned long mask;
	122	pte_t pte = (pte_t )&pmd;
	123	struct page head, page;
	124	int refs;
	125
	126	mask = _PAGE_PRESENT\|_PAGE_USER;
	127	if (write)
	128	mask \|= _PAGE_RW;
606ee44d	129	if ((pte_flags(pte) & mask) != mask)
8174c430 NP	130	return 0;
8174c430 NP	131	/* hugepages are never "special" */
606ee44d	132	VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
8174c430 NP	133	VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
	134
	135	refs = 0;
	136	head = pte_page(pte);
652ea695	137	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
8174c430 NP	138	do {
	139	VM_BUG_ON(compound_head(page) != head);
	140	pages[*nr] = page;
91807063 AA	141	if (PageTail(page))
91807063 AA	142	get_huge_page_tail(page);
8174c430 NP	143	(*nr)++;
	144	page++;
	145	refs++;
	146	} while (addr += PAGE_SIZE, addr != end);
	147	get_head_page_multiple(head, refs);
	148
	149	return 1;
	150	}
	151
	152	static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
	153	int write, struct page *pages, int nr)
	154	{
	155	unsigned long next;
	156	pmd_t *pmdp;
	157
	158	pmdp = pmd_offset(&pud, addr);
	159	do {
	160	pmd_t pmd = *pmdp;
	161
	162	next = pmd_addr_end(addr, end);
	163	if (pmd_none(pmd))
	164	return 0;
	165	if (unlikely(pmd_large(pmd))) {
	166	if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
	167	return 0;
	168	} else {
	169	if (!gup_pte_range(pmd, addr, next, write, pages, nr))
	170	return 0;
	171	}
	172	} while (pmdp++, addr = next, addr != end);
	173
	174	return 1;
	175	}
	176
652ea695 NP	177	static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
	178	unsigned long end, int write, struct page *pages, int nr)
	179	{
	180	unsigned long mask;
	181	pte_t pte = (pte_t )&pud;
	182	struct page head, page;
	183	int refs;
	184
	185	mask = _PAGE_PRESENT\|_PAGE_USER;
	186	if (write)
	187	mask \|= _PAGE_RW;
606ee44d	188	if ((pte_flags(pte) & mask) != mask)
652ea695 NP	189	return 0;
652ea695 NP	190	/* hugepages are never "special" */
606ee44d	191	VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
652ea695 NP	192	VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
	193
	194	refs = 0;
	195	head = pte_page(pte);
	196	page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
	197	do {
	198	VM_BUG_ON(compound_head(page) != head);
	199	pages[*nr] = page;
	200	(*nr)++;
	201	page++;
	202	refs++;
	203	} while (addr += PAGE_SIZE, addr != end);
	204	get_head_page_multiple(head, refs);
	205
	206	return 1;
	207	}
	208
8174c430 NP	209	static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
	210	int write, struct page *pages, int nr)
	211	{
	212	unsigned long next;
	213	pud_t *pudp;
	214
	215	pudp = pud_offset(&pgd, addr);
	216	do {
	217	pud_t pud = *pudp;
	218
	219	next = pud_addr_end(addr, end);
	220	if (pud_none(pud))
	221	return 0;
652ea695 NP	222	if (unlikely(pud_large(pud))) {
	223	if (!gup_huge_pud(pud, addr, next, write, pages, nr))
	224	return 0;
	225	} else {
	226	if (!gup_pmd_range(pud, addr, next, write, pages, nr))
	227	return 0;
	228	}
8174c430 NP	229	} while (pudp++, addr = next, addr != end);
	230
	231	return 1;
	232	}
	233
465a454f PZ	234	/*
	235	* Like get_user_pages_fast() except its IRQ-safe in that it won't fall
	236	* back to the regular GUP.
	237	*/
	238	int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
	239	struct page **pages)
	240	{
	241	struct mm_struct *mm = current->mm;
	242	unsigned long addr, len, end;
	243	unsigned long next;
	244	unsigned long flags;
	245	pgd_t *pgdp;
	246	int nr = 0;
	247
	248	start &= PAGE_MASK;
	249	addr = start;
	250	len = (unsigned long) nr_pages << PAGE_SHIFT;
	251	end = start + len;
	252	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
	253	(void __user *)start, len)))
	254	return 0;
	255
	256	/*
	257	* XXX: batch / limit 'nr', to avoid large irq off latency
	258	* needs some instrumenting to determine the common sizes used by
	259	* important workloads (eg. DB2), and whether limiting the batch size
	260	* will decrease performance.
	261	*
	262	* It seems like we're in the clear for the moment. Direct-IO is
	263	* the main guy that batches up lots of get_user_pages, and even
	264	* they are limited to 64-at-a-time which is not so many.
	265	*/
	266	/*
	267	* This doesn't prevent pagetable teardown, but does prevent
	268	* the pagetables and pages from being freed on x86.
	269	*
	270	* So long as we atomically load page table pointers versus teardown
	271	* (which we do on x86, with the above PAE exception), we can follow the
	272	* address down to the the page and take a ref on it.
	273	*/
	274	local_irq_save(flags);
	275	pgdp = pgd_offset(mm, addr);
	276	do {
	277	pgd_t pgd = *pgdp;
	278
	279	next = pgd_addr_end(addr, end);
	280	if (pgd_none(pgd))
	281	break;
	282	if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
	283	break;
	284	} while (pgdp++, addr = next, addr != end);
	285	local_irq_restore(flags);
	286
	287	return nr;
	288	}
	289
a0d22f48 AG	290	/**
	291	* get_user_pages_fast() - pin user pages in memory
	292	* @start: starting user address
	293	* @nr_pages: number of pages from start to pin
	294	* @write: whether pages will be written to
	295	* @pages: array that receives pointers to the pages pinned.
	296	* Should be at least nr_pages long.
	297	*
	298	* Attempt to pin user pages in memory without taking mm->mmap_sem.
	299	* If not successful, it will fall back to taking the lock and
	300	* calling get_user_pages().
	301	*
	302	* Returns number of pages pinned. This may be fewer than the number
	303	* requested. If nr_pages is 0 or negative, returns 0. If no pages
	304	* were pinned, returns -errno.
	305	*/
8174c430 NP	306	int get_user_pages_fast(unsigned long start, int nr_pages, int write,
	307	struct page **pages)
	308	{
	309	struct mm_struct *mm = current->mm;
9b79022c	310	unsigned long addr, len, end;
8174c430 NP	311	unsigned long next;
	312	pgd_t *pgdp;
	313	int nr = 0;
	314
9b79022c LT	315	start &= PAGE_MASK;
	316	addr = start;
	317	len = (unsigned long) nr_pages << PAGE_SHIFT;
7f818906	318
9b79022c	319	end = start + len;
7f818906 LT	320	if (end < start)
	321	goto slow_irqon;
	322
	323	#ifdef CONFIG_X86_64
	324	if (end >> __VIRTUAL_MASK_SHIFT)
8174c430	325	goto slow_irqon;
7f818906	326	#endif
8174c430 NP	327
	328	/*
	329	* XXX: batch / limit 'nr', to avoid large irq off latency
	330	* needs some instrumenting to determine the common sizes used by
	331	* important workloads (eg. DB2), and whether limiting the batch size
	332	* will decrease performance.
	333	*
	334	* It seems like we're in the clear for the moment. Direct-IO is
	335	* the main guy that batches up lots of get_user_pages, and even
	336	* they are limited to 64-at-a-time which is not so many.
	337	*/
	338	/*
	339	* This doesn't prevent pagetable teardown, but does prevent
	340	* the pagetables and pages from being freed on x86.
	341	*
	342	* So long as we atomically load page table pointers versus teardown
	343	* (which we do on x86, with the above PAE exception), we can follow the
	344	* address down to the the page and take a ref on it.
	345	*/
	346	local_irq_disable();
	347	pgdp = pgd_offset(mm, addr);
	348	do {
	349	pgd_t pgd = *pgdp;
	350
	351	next = pgd_addr_end(addr, end);
	352	if (pgd_none(pgd))
	353	goto slow;
	354	if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
	355	goto slow;
	356	} while (pgdp++, addr = next, addr != end);
	357	local_irq_enable();
	358
	359	VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
	360	return nr;
	361
	362	{
	363	int ret;
	364
	365	slow:
	366	local_irq_enable();
	367	slow_irqon:
	368	/* Try to get the remaining pages with get_user_pages */
	369	start += nr << PAGE_SHIFT;
	370	pages += nr;
	371
	372	down_read(&mm->mmap_sem);
	373	ret = get_user_pages(current, mm, start,
	374	(end - start) >> PAGE_SHIFT, write, 0, pages, NULL);
	375	up_read(&mm->mmap_sem);
	376
	377	/* Have to be a bit careful with return values */
	378	if (nr > 0) {
	379	if (ret < 0)
	380	ret = nr;
	381	else
	382	ret += nr;
	383	}
	384
	385	return ret;
	386	}
	387	}