Commit 891cffbd authored by Linus Torvalds's avatar Linus Torvalds Committed by Ingo Molnar
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x86/mm: do not trigger a kernel warning if user-space disables interrupts and... 

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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Reported-by: default avatarArjan van de Ven <arjan@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 4480f15b

arch/x86/mm/fault.c

+11 −19
Original line number Original line Diff line number Diff line
@@ -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

	 * It's safe to allow irq's after cr2 has been saved and the

	 * atomic region then we must not take the fault.

	 * 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)

	if (user_mode_vm(regs)) {

		goto bad_area_nosemaphore;

		local_irq_enable();

#else /* CONFIG_X86_64 */

		error_code |= PF_USER;

	if (likely(regs->flags & X86_EFLAGS_IF))

	} else if (regs->flags & X86_EFLAGS_IF)

		local_irq_enable();

		local_irq_enable();





#ifdef CONFIG_X86_64

	if (unlikely(error_code & PF_RSVD))

	if (unlikely(error_code & PF_RSVD))

		pgtable_bad(address, regs, error_code);

		pgtable_bad(address, regs, error_code);

#endif





	/*

	/*

	 * If we're in an interrupt, have no user context or are running in an

	 * 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))

	if (unlikely(in_atomic() || !mm))

		goto bad_area_nosemaphore;

		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:

again:

#endif

	/* When running in the kernel we expect faults to occur only to

	/* When running in the kernel we expect faults to occur only to

	 * addresses in user space.  All other faults represent errors in the

	 * addresses in user space.  All other faults represent errors in the

	 * kernel and should generate an OOPS.  Unfortunately, in the case of an

	 * kernel and should generate an OOPS.  Unfortunately, in the case of an

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