x86: lockless get_user_pages_fast() (8174c430) · Commits · 戴 / test

arch/x86/Kconfig

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		@@ -22,6 +22,7 @@ config X86
		select HAVE_IDE
		select HAVE_OPROFILE
		select HAVE_IOREMAP_PROT
		select HAVE_GET_USER_PAGES_FAST
		select HAVE_KPROBES
		select ARCH_WANT_OPTIONAL_GPIOLIB if !X86_RDC321X
		select HAVE_KRETPROBES

arch/x86/mm/Makefile

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Original line number	Diff line number	Diff line
		obj-y := init_$(BITS).o fault.o ioremap.o extable.o pageattr.o mmap.o \
		pat.o pgtable.o

		obj-$(CONFIG_HAVE_GET_USER_PAGES_FAST) += gup.o
		obj-$(CONFIG_X86_32) += pgtable_32.o

		obj-$(CONFIG_HUGETLB_PAGE) += hugetlbpage.o

arch/x86/mm/gup.c

0 → 100644

+258 −0

Original line number	Diff line number	Diff line
		/*
		* 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 *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 atoimcally,
		* 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_val(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 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_val(pte) & mask) != mask)
		return 0;
		/* hugepages are never "special" */
		VM_BUG_ON(pte_val(pte) & _PAGE_SPECIAL);
		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));

		refs = 0;
		head = pte_page(pte);
		page = head + ((addr & ~HPAGE_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_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 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 (!gup_pmd_range(pud, addr, next, write, pages, nr))
		return 0;
		} while (pudp++, addr = next, addr != end);

		return 1;
		}

		int get_user_pages_fast(unsigned long start, int nr_pages, int write,
		struct page **pages)
		{
		struct mm_struct *mm = current->mm;
		unsigned long end = start + (nr_pages << PAGE_SHIFT);
		unsigned long addr = start;
		unsigned long next;
		pgd_t *pgdp;
		int nr = 0;

		if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
		start, nr_pages*PAGE_SIZE)))
		goto slow_irqon;

		/*
		* 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;
		}
		}

include/asm-x86/uaccess.h

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Original line number	Diff line number	Diff line
		@@ -451,3 +451,4 @@ extern struct movsl_mask {
		#endif

		#endif

mm/Kconfig

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Original line number	Diff line number	Diff line
		@@ -77,6 +77,9 @@ config FLAT_NODE_MEM_MAP
		def_bool y
		depends on !SPARSEMEM

		config HAVE_GET_USER_PAGES_FAST
		bool

		#
		# Both the NUMA code and DISCONTIGMEM use arrays of pg_data_t's
		# to represent different areas of memory. This variable allows

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