On Sun, Oct 26, 2014 at 1:29 PM, Frederic Weisbecker <fweis...@gmail.com> wrote: > 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. >
I have no desire to do this for 32-bit. But we don't need vmalloc_sync_all -- we just need to sync the ony required entry. > 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. In principle, we could pre-populate all top-level pgd entries at boot, but that would cost up to 256 pages of memory, I think. --Andy -- 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/
