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

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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>
This commit is contained in:
Linus Torvalds 2008-10-12 13:16:12 -07:00 committed by Ingo Molnar
parent 4480f15b33
commit 891cffbd6b
1 changed files with 11 additions and 19 deletions
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30
arch/x86/mm/fault.c
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@ -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