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>

diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
index 6340cef..697d572 100644 (file)
--- 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

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ad613ed..b457bc0 100644 (file)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -863,6 +863,12 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
                        struct page **pages);
 
 /*
+ * doesn't attempt to fault and will return short.
+ */
+int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
+                         struct page **pages);
+
+/*
  * A callback you can register to apply pressure to ageable caches.
  *
  * 'shrink' is passed a count 'nr_to_scan' and a 'gfpmask'.  It should