arch/x86/mm/gup.c - kernel/quantenna - Git at Google

 /*
  * 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 <linux/swap.h>
 #include <linux/memremap.h>

 #include <asm/mmu_context.h>
 #include <asm/pgtable.h>

 static inline pte_t gup_get_pte(pte_t *ptep)
 {
 #ifndef CONFIG_X86_PAE
 	return READ_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
 }

 static void undo_dev_pagemap(int *nr, int nr_start, struct page **pages)
 {
 	while ((*nr) - nr_start) {
 		struct page *page = pages[--(*nr)];

 		ClearPageReferenced(page);
 		put_page(page);
 	}
 }

 /*
  * 'pteval' can come from a pte, pmd or pud.  We only check
  * _PAGE_PRESENT, _PAGE_USER, and _PAGE_RW in here which are the
  * same value on all 3 types.
  */
 static inline int pte_allows_gup(unsigned long pteval, int write)
 {
 	unsigned long need_pte_bits = _PAGE_PRESENT|_PAGE_USER;

 	if (write)
 		need_pte_bits |= _PAGE_RW;

 	if ((pteval & need_pte_bits) != need_pte_bits)
 		return 0;

 	/* Check memory protection keys permissions. */
 	if (!__pkru_allows_pkey(pte_flags_pkey(pteval), write))
 		return 0;

 	return 1;
 }

 /*
  * 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)
 {
 	struct dev_pagemap *pgmap = NULL;
 	int nr_start = *nr;
 	pte_t *ptep;

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

 		/* Similar to the PMD case, NUMA hinting must take slow path */
 		if (pte_protnone(pte)) {
 			pte_unmap(ptep);
 			return 0;
 		}

 		if (pte_devmap(pte)) {
 			pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
 			if (unlikely(!pgmap)) {
 				undo_dev_pagemap(nr, nr_start, pages);
 				pte_unmap(ptep);
 				return 0;
 			}
 		} else if (!pte_allows_gup(pte_val(pte), write) ||
 			   pte_special(pte)) {
 			pte_unmap(ptep);
 			return 0;
 		}
 		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 		page = pte_page(pte);
 		get_page(page);
 		put_dev_pagemap(pgmap);
 		SetPageReferenced(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(page != compound_head(page), page);
 	VM_BUG_ON_PAGE(page_count(page) == 0, page);
 	page_ref_add(page, nr);
 	SetPageReferenced(page);
 }

 static int __gup_device_huge_pmd(pmd_t pmd, unsigned long addr,
 		unsigned long end, struct page **pages, int *nr)
 {
 	int nr_start = *nr;
 	unsigned long pfn = pmd_pfn(pmd);
 	struct dev_pagemap *pgmap = NULL;

 	pfn += (addr & ~PMD_MASK) >> PAGE_SHIFT;
 	do {
 		struct page *page = pfn_to_page(pfn);

 		pgmap = get_dev_pagemap(pfn, pgmap);
 		if (unlikely(!pgmap)) {
 			undo_dev_pagemap(nr, nr_start, pages);
 			return 0;
 		}
 		SetPageReferenced(page);
 		pages[*nr] = page;
 		get_page(page);
 		put_dev_pagemap(pgmap);
 		(*nr)++;
 		pfn++;
 	} while (addr += PAGE_SIZE, addr != end);
 	return 1;
 }

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

 	if (!pte_allows_gup(pmd_val(pmd), write))
 		return 0;

 	VM_BUG_ON(!pfn_valid(pmd_pfn(pmd)));
 	if (pmd_devmap(pmd))
 		return __gup_device_huge_pmd(pmd, addr, end, pages, nr);

 	/* hugepages are never "special" */
 	VM_BUG_ON(pmd_flags(pmd) & _PAGE_SPECIAL);

 	refs = 0;
 	head = pmd_page(pmd);
 	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 	do {
 		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		pages[*nr] = 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) || !pmd_present(pmd))) {
 			/*
 			 * NUMA hinting faults need to be handled in the GUP
 			 * slowpath for accounting purposes and so that they
 			 * can be serialised against THP migration.
 			 */
 			if (pmd_protnone(pmd))
 				return 0;
 			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)
 {
 	struct page *head, *page;
 	int refs;

 	if (!pte_allows_gup(pud_val(pud), write))
 		return 0;
 	/* hugepages are never "special" */
 	VM_BUG_ON(pud_flags(pud) & _PAGE_SPECIAL);
 	VM_BUG_ON(!pfn_valid(pud_pfn(pud)));

 	refs = 0;
 	head = pud_page(pud);
 	page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 	do {
 		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		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;

 		ret = get_user_pages_unlocked(start,
 					      (end - start) >> PAGE_SHIFT,
 					      write, 0, pages);

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

 		return ret;
 	}
 }
	/*
	* 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 <linux/swap.h>
	#include <linux/memremap.h>

	#include <asm/mmu_context.h>
	#include <asm/pgtable.h>

	static inline pte_t gup_get_pte(pte_t *ptep)
	{
	#ifndef CONFIG_X86_PAE
	return READ_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
	}

	static void undo_dev_pagemap(int nr, int nr_start, struct page *pages)
	{
	while ((*nr) - nr_start) {
	struct page page = pages[--(nr)];

	ClearPageReferenced(page);
	put_page(page);
	}
	}

	/*
	* 'pteval' can come from a pte, pmd or pud. We only check
	* _PAGE_PRESENT, _PAGE_USER, and _PAGE_RW in here which are the
	* same value on all 3 types.
	*/
	static inline int pte_allows_gup(unsigned long pteval, int write)
	{
	unsigned long need_pte_bits = _PAGE_PRESENT\|_PAGE_USER;

	if (write)
	need_pte_bits \|= _PAGE_RW;

	if ((pteval & need_pte_bits) != need_pte_bits)
	return 0;

	/* Check memory protection keys permissions. */
	if (!__pkru_allows_pkey(pte_flags_pkey(pteval), write))
	return 0;

	return 1;
	}

	/*
	* 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)
	{
	struct dev_pagemap *pgmap = NULL;
	int nr_start = *nr;
	pte_t *ptep;

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

	/* Similar to the PMD case, NUMA hinting must take slow path */
	if (pte_protnone(pte)) {
	pte_unmap(ptep);
	return 0;
	}

	if (pte_devmap(pte)) {
	pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
	if (unlikely(!pgmap)) {
	undo_dev_pagemap(nr, nr_start, pages);
	pte_unmap(ptep);
	return 0;
	}
	} else if (!pte_allows_gup(pte_val(pte), write) \|\|
	pte_special(pte)) {
	pte_unmap(ptep);
	return 0;
	}
	VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
	page = pte_page(pte);
	get_page(page);
	put_dev_pagemap(pgmap);
	SetPageReferenced(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(page != compound_head(page), page);
	VM_BUG_ON_PAGE(page_count(page) == 0, page);
	page_ref_add(page, nr);
	SetPageReferenced(page);
	}

	static int __gup_device_huge_pmd(pmd_t pmd, unsigned long addr,
	unsigned long end, struct page *pages, int nr)
	{
	int nr_start = *nr;
	unsigned long pfn = pmd_pfn(pmd);
	struct dev_pagemap *pgmap = NULL;

	pfn += (addr & ~PMD_MASK) >> PAGE_SHIFT;
	do {
	struct page *page = pfn_to_page(pfn);

	pgmap = get_dev_pagemap(pfn, pgmap);
	if (unlikely(!pgmap)) {
	undo_dev_pagemap(nr, nr_start, pages);
	return 0;
	}
	SetPageReferenced(page);
	pages[*nr] = page;
	get_page(page);
	put_dev_pagemap(pgmap);
	(*nr)++;
	pfn++;
	} while (addr += PAGE_SIZE, addr != end);
	return 1;
	}

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

	if (!pte_allows_gup(pmd_val(pmd), write))
	return 0;

	VM_BUG_ON(!pfn_valid(pmd_pfn(pmd)));
	if (pmd_devmap(pmd))
	return __gup_device_huge_pmd(pmd, addr, end, pages, nr);

	/* hugepages are never "special" */
	VM_BUG_ON(pmd_flags(pmd) & _PAGE_SPECIAL);

	refs = 0;
	head = pmd_page(pmd);
	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
	do {
	VM_BUG_ON_PAGE(compound_head(page) != head, page);
	pages[*nr] = 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) \|\| !pmd_present(pmd))) {
	/*
	* NUMA hinting faults need to be handled in the GUP
	* slowpath for accounting purposes and so that they
	* can be serialised against THP migration.
	*/
	if (pmd_protnone(pmd))
	return 0;
	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)
	{
	struct page head, page;
	int refs;

	if (!pte_allows_gup(pud_val(pud), write))
	return 0;
	/* hugepages are never "special" */
	VM_BUG_ON(pud_flags(pud) & _PAGE_SPECIAL);
	VM_BUG_ON(!pfn_valid(pud_pfn(pud)));

	refs = 0;
	head = pud_page(pud);
	page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
	do {
	VM_BUG_ON_PAGE(compound_head(page) != head, page);
	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;

	ret = get_user_pages_unlocked(start,
	(end - start) >> PAGE_SHIFT,
	write, 0, pages);

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

	return ret;
	}
	}