/*
   * 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 <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;
  
  		/* Similar to the PMD case, NUMA hinting must take slow path */
  		if (pte_numa(pte)) {
  			pte_unmap(ptep);
  			return 0;
  		}
  
  		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);
  		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);
  	atomic_add(nr, &page->_count);
  	SetPageReferenced(page);
  }
  
  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_PAGE(compound_head(page) != head, page);
  		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);
  		/*
  		 * The pmd_trans_splitting() check below explains why
  		 * pmdp_splitting_flush has to flush the tlb, to stop
  		 * this gup-fast code from running while we set the
  		 * splitting bit in the pmd. Returning zero will take
  		 * the slow path that will call wait_split_huge_page()
  		 * if the pmd is still in splitting state. gup-fast
  		 * can't because it has irq disabled and
  		 * wait_split_huge_page() would never return as the
  		 * tlb flush IPI wouldn't run.
  		 */
  		if (pmd_none(pmd) || pmd_trans_splitting(pmd))
  			return 0;
  		if (unlikely(pmd_large(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_numa(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)
  {
  	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_PAGE(compound_head(page) != head, page);
  		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_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;
  	}
  }