x86, mm: Add __get_user_pages_fast()

Introduce a gup_fast() variant which is usable from IRQ/NMI context.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Nick Piggin <npiggin@suse.de>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 56 insertions, 0 deletions

diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
index 6340cef6798a..697d5727c119 100644
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -219,6 +219,62 @@ static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long 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