On Oct 25, 2014 9:11 PM, "Frederic Weisbecker" <fweis...@gmail.com> wrote: > > 2014-10-25 2:22 GMT+02:00 Andy Lutomirski <l...@amacapital.net>: > > Is there any good reason not to use vmalloc for x86_64 stacks? > > > > The tricky bits I've thought of are: > > > > - On any context switch, we probably need to probe the new stack > > before switching to it. That way, if it's going to fault due to an > > out-of-sync pgd, we still have a stack available to handle the fault. > > Would that prevent from any further fault on a vmalloc'ed kernel > stack? We would need to ensure that pre-faulting, say the first byte, > is enough to sync the whole new stack entirely otherwise we risk > another future fault and some places really aren't safely faulted. >
I think so. The vmalloc faults only happen when the entire top-level page table entry is missing, and those cover giant swaths of address space. I don't know whether the vmalloc code guarantees not to span a pmd (pud? why couldn't these be called pte0, pte1, pte2, etc.?) boundary. --Andy > > > > - Any time we change cr3, we may need to check that the pgd > > corresponding to rsp is there. If now, we need to sync it over. > > > > - For simplicity, we probably want all stack ptes to be present all > > the time. This is fine; vmalloc already works that way. > > > > - If we overrun the stack, we double-fault. This should be easy to > > detect: any double-fault where rsp is less than 20 bytes from the > > bottom of the stack is a failure to deliver a non-IST exception due to > > a stack overflow. The question is: what do we do if this happens? > > We could just panic (guaranteed to work). We could also try to > > recover by killing the offending task, but that might be a bit > > challenging, since we're in IST context. We could do something truly > > awful: increment RSP by a few hundred bytes, point RIP at do_exit, and > > return from the double fault. > > > > Thoughts? This shouldn't be all that much code. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
