On Sat, Oct 25, 2014 at 10:49:25PM -0700, Andy Lutomirski wrote: > 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.
So dereferencing stack[0] is probably enough for 8KB worth of stack. I think we have vmalloc_sync_all() but I heard this only work on x86-64. Too bad we don't have a universal solution, I have that problem with per cpu allocated memory faulting at random places. I hit at least two places where it got harmful: context tracking and perf callchains. We fixed the latter using open-coded per cpu allocation. I still haven't found a solution for context tracking. -- 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/
