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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 <mathieu.desnoyers@polymtl.ca> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Nick Piggin <npiggin@suse.de> Cc: Pekka Enberg <penberg@cs.helsinki.fi> Cc: Vegard Nossum <vegard.nossum@gmail.com> Cc: Jeremy Fitzhardinge <jeremy@goop.org> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk> LKML-Reference: <20090616030522.GA22162@Krystal> Signed-off-by: Ingo Molnar <mingo@elte.hu> |
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