From 891cffbd6bcba26409869c19c07ecd4bfc0c2460 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 12 Oct 2008 13:16:12 -0700 Subject: [PATCH] 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 Reported-by: Arjan van de Ven Signed-off-by: Ingo Molnar --- arch/x86/mm/fault.c | 30 +++++++++++------------------- 1 file changed, 11 insertions(+), 19 deletions(-) diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index a742d753d5b..ac2ad781da0 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 -- 2.41.1