> On Dec 3, 2020, at 9:29 AM, Florian Weimer <fweimer@xxxxxxxxxx> wrote: > > * Andy Lutomirski: > >> If you want a 4GB allocation to succeed, you can only divide the >> address space into 32k fragments. Or, a little more precisely, if you >> want a randomly selected 4GB region to be empty, any other allocation >> has a 1/32k chance of being in the way. (Rough numbers — I’m ignoring >> effects of the beginning and end of the address space, and I’m >> ignoring the size of a potential conflicting allocation.). > > I think the probability distribution is way more advantageous than that > because it is unlikely that 32K allocations are all exactly spaced 4 GB > apart. (And with 32K allocations, you are close to the VMA limit anyway.) I’m assuming the naive algorithm of choosing an address and trying it. Actually looking for a big enough gap would be more reliable. I suspect that something much more clever could be done in which the heap is divided up into a few independently randomized sections and heap pages are randomized within the sections might do much better. There should certainly be a lot of room for something between what we have now and a fully randomized scheme. It might also be worth looking at what other OSes do.
