x86/mm: do not trigger a kernel warning if user-space disables interrupts and generates a page fault

Arjan reported a spike in the following bug pattern in v2.6.27:

   http://www.kerneloops.org/searchweek.php?search=lock_page

which happens because hwclock started triggering warnings due to
a (correct) might_sleep() check in the MM code.

The warning occurs because hwclock uses this dubious sequence of
code to run "atomic" code:

  static unsigned long
  atomic(const char *name, unsigned long (*op)(unsigned long),
         unsigned long arg)
  {
    unsigned long v;
    __asm__ volatile ("cli");
    v = (*op)(arg);
    __asm__ volatile ("sti");
    return v;
  }

Then it pagefaults in that "atomic" section, triggering the warning.

There is no way the kernel could provide "atomicity" in this path,
a page fault is a cannot-continue machine event so the kernel has to
wait for the page to be filled in.

Even if it was just a minor fault we'd have to take locks and might have
to spend quite a bit of time with interrupts disabled - not nice to irq
latencies in general.

So instead just enable interrupts in the pagefault path unconditionally
if we come from user-space, and handle the fault.

Also, while touching this code, unify some trivial parts of the x86
VM paths at the same time.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 11 insertions, 19 deletions

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index a742d753d5b0..ac2ad781da00 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -645,24 +645,23 @@ void __kprobes do_page_fault(struct pt_regs *regs, unsigned long error_code)
         }
 
 
-#ifdef CONFIG_X86_32
-        /* It's safe to allow irq's after cr2 has been saved and the vmalloc
-           fault has been handled. */
-        if (regs->flags & (X86_EFLAGS_IF | X86_VM_MASK))
-                local_irq_enable();
-
         /*
-         * If we're in an interrupt, have no user context or are running in an
-         * atomic region then we must not take the fault.
+         * It's safe to allow irq's after cr2 has been saved and the
+         * vmalloc fault has been handled.
+         *
+         * User-mode registers count as a user access even for any
+         * potential system fault or CPU buglet.
          */
-        if (in_atomic() || !mm)
-                goto bad_area_nosemaphore;
-#else /* CONFIG_X86_64 */
-        if (likely(regs->flags & X86_EFLAGS_IF))
+        if (user_mode_vm(regs)) {
+                local_irq_enable();
+                error_code |= PF_USER;
+        } else if (regs->flags & X86_EFLAGS_IF)
                 local_irq_enable();
 
+#ifdef CONFIG_X86_64
         if (unlikely(error_code & PF_RSVD))
                 pgtable_bad(address, regs, error_code);
+#endif
 
         /*
          * If we're in an interrupt, have no user context or are running in an
@@ -671,14 +670,7 @@ void __kprobes do_page_fault(struct pt_regs *regs, unsigned long error_code)
         if (unlikely(in_atomic() || !mm))
                 goto bad_area_nosemaphore;
 
-        /*
-         * User-mode registers count as a user access even for any
-         * potential system fault or CPU buglet.
-         */
-        if (user_mode_vm(regs))
-                error_code |= PF_USER;
 again:
-#endif
         /* When running in the kernel we expect faults to occur only to
          * addresses in user space.  All other faults represent errors in the
          * kernel and should generate an OOPS.  Unfortunately, in the case of an