From 5dfaf90f8052327c92fbe3c470a2e6634be296c0 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Tue, 16 Jun 2009 10:23:32 +0200 Subject: [PATCH] x86: mm: Read cr2 before prefetching the mmap_lock Prefetch instructions can generate spurious faults on certain models of older CPUs. The faults themselves cannot be stopped and they can occur pretty much anywhere - so the way we solve them is that we detect certain patterns and ignore the fault. There is one small path of code where we must not take faults though: the #PF handler execution leading up to the reading of the CR2 (the faulting address). If we take a fault there then we destroy the CR2 value (with that of the prefetching instruction's) and possibly mishandle user-space or kernel-space pagefaults. It turns out that in current upstream we do exactly that: prefetchw(&mm->mmap_sem); /* Get the faulting address: */ address = read_cr2(); This is not good. So turn around the order: first read the cr2 then prefetch the lock address. Reading cr2 is plenty fast (2 cycles) so delaying the prefetch by this amount shouldnt be a big issue performance-wise. [ And this might explain a mystery fault.c warning that sometimes occurs on one an old AMD/Semptron based test-system i have - which does have such prefetch problems. ] Cc: Mathieu Desnoyers Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Nick Piggin Cc: Pekka Enberg Cc: Vegard Nossum Cc: Jeremy Fitzhardinge Cc: Hugh Dickins LKML-Reference: <20090616030522.GA22162@Krystal> Signed-off-by: Ingo Molnar --- arch/x86/mm/fault.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index c6acc6326374..0482fa649738 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -951,11 +951,11 @@ do_page_fault(struct pt_regs *regs, unsigned long error_code) tsk = current; mm = tsk->mm; - prefetchw(&mm->mmap_sem); - /* Get the faulting address: */ address = read_cr2(); + prefetchw(&mm->mmap_sem); + if (unlikely(kmmio_fault(regs, address))) return; -- 2.39.5