/* * 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_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 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; (*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; } 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 (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ, (void __user *)start, len))) 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; } }