On Wed, May 22, 2013 at 11:25:36AM +0200, Arnd Bergmann wrote: > On Thursday 16 May 2013, Michael S. Tsirkin wrote: > > This improves the might_fault annotations used > > by uaccess routines: > > > > 1. The only reason uaccess routines might sleep > > is if they fault. Make this explicit for > > all architectures. > > 2. Accesses (e.g through socket ops) to kernel memory > > with KERNEL_DS like net/sunrpc does will never sleep. > > Remove an unconditinal might_sleep in the inline > > might_fault in kernel.h > > (used when PROVE_LOCKING is not set). > > 3. Accesses with pagefault_disable return EFAULT > > but won't cause caller to sleep. > > Check for that and avoid might_sleep when > > PROVE_LOCKING is set. > > > > I'd like these changes to go in for the benefit of > > the vhost driver where we want to call socket ops > > under a spinlock, and fall back on slower thread handler > > on error. > > Hi Michael, > > I have recently stumbled over a related topic, which is the highly > inconsistent placement of might_fault() or might_sleep() in certain > classes of uaccess functions. Your patches seem completely reasonable, > but it would be good to also fix the other problem, at least on > the architectures we most care about. > > Given the most commonly used functions and a couple of architectures > I'm familiar with, these are the ones that currently call might_fault() > > x86-32 x86-64 arm arm64 powerpc s390 generic > copy_to_user - x - - - x x > copy_from_user - x - - - x x > put_user x x x x x x x > get_user x x x x x x x > __copy_to_user x x - - x - - > __copy_from_user x x - - x - - > __put_user - - x - x - - > __get_user - - x - x - - > > WTF? Yea. > Calling might_fault() for every __get_user/__put_user is rather expensive > because it turns what should be a single instruction (plus fixup) into an > external function call. You mean _cond_resched with CONFIG_PREEMPT_VOLUNTARY? Or do you mean when we build with PROVE_LOCKING? > My feeling is that we should do might_fault() only in access_ok() to get > the right balance. > > Arnd Well access_ok is currently non-blocking I think - we'd have to audit all callers. There are some 200 of these in drivers and some 1000 total so ... a bit risky. -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
