On Wed, Oct 30, 2013 at 12:49:05PM -0700, Linus Torvalds wrote: > On Wed, Oct 30, 2013 at 7:16 AM, Russell King - ARM Linux > <linux@xxxxxxxxxxxxxxxx> wrote: > > > > So, if *nr_to_walk was zero when this function was entered, that means > > we're wanting to operate on (~0UL)+1 objects - which might as well be > > infinite. > > > > Clearly this is not correct behaviour. If we think about the behaviour > > of this function when *nr_to_walk is 1, then clearly it's wrong - we > > decrement first and then test for zero - which results in us doing > > nothing at all. A post-decrement would give the desired behaviour - > > we'd try to walk one object and one object only if *nr_to_walk were > > one. > > > > It also gives the correct behaviour for zero - we exit at this point. > > Good analysis. > > HOWEVER. > > I actually think even your version is very dangerous, because we pass > in the *address* to that count, and the only real reason to do that is > because we might call it in a loop, and we want the function to update > that count. > > And even your version still underflows from 0 to really-large-count. > It *returns* when underflow happens, but you end up with the counter > updated to a large value, and then anybody who uses it later would be > screwed. > > See, for example, the inline list_lru_walk() function in <linux/list_lru.h> > > So I think we should either change that "unsigned long" to just > "long", and then check for "<= 0" (like list_lru_walk() already does), > or we should do > > if (!*nr_to_walk) > break; > --*nr_to_walk; > > to make sure that we never do that underflow. Yup, I missed that case. Thanks for finding and fixing it. > I will modify your patch to do the latter, since it's the smaller > change, but I suspect we should think about making that thing signed. Yeah, I'll look into it. The shrinker API itself only ever feeds shrinkctl->batch to it so we shouldn't ever have overflow problems from that perspective... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
