On Fri, Nov 21, 2014 at 01:34:08PM -0800, Andy Lutomirski wrote: > On Fri, Nov 21, 2014 at 1:32 PM, Frederic Weisbecker <fweis...@gmail.com> > wrote: > > On Fri, Nov 21, 2014 at 12:01:51PM -0500, Steven Rostedt wrote: > >> On Fri, Nov 21, 2014 at 11:25:06AM -0500, Tejun Heo wrote: > >> > > >> > * Static percpu areas wouldn't trigger fault lazily. Note that this > >> > is not necessarily because the first percpu chunk which contains the > >> > static area is embedded inside the kernel linear mapping. Depending > >> > on the memory layout and boot param, percpu allocator may choose to > >> > map the first chunk in vmalloc space too; however, this still works > >> > out fine because at that point there are no other page tables and > >> > the PUD entries covering the first chunk is faulted in before other > >> > pages tables are copied from the kernel one. > >> > >> That sounds correct. > >> > >> > > >> > * NMI used to be a problem because vmalloc fault handler couldn't > >> > safely nest inside NMI handler but this has been fixed since and it > >> > should work fine from NMI handlers now. > >> > >> Right. Of course "should work fine" does not excatly mean "will work fine". > >> > >> > >> > > >> > * Function tracers are problematic because they may end up nesting > >> > inside themselves through triggering a vmalloc fault while accessing > >> > dynamic percpu memory area. This may lead to recursive locking and > >> > other surprises. > >> > >> The function tracer infrastructure now has a recursive check that happens > >> rather early in the call. Unless the registered OPS specifically states > >> it handles recursions (FTRACE_OPS_FL_RECUSION_SAFE), ftrace will add the > >> necessary recursion checks. If a registered OPS lies about being recusion > >> safe, well we can't stop suicide. > > > > Same if the recursion state is based on per cpu memory. > > > >> > >> Looking at kernel/trace/trace_functions.c: function_trace_call() which is > >> registered with RECURSION_SAFE, I see that the recursion check is done > >> before the per_cpu_ptr() call to the dynamically allocated per_cpu data. > >> > >> It looks OK, but... > >> > >> Oh! but if we trace the page fault handler, and we fault here too > >> we just nuked the cr2 register. Not good. > > > > If we fault in the page fault handler, we double fault and apparently > > recovering from that isn't quite expected anyway. > > Not quite. We only double fault if we fault while pushing the > hardware part of the state onto the stack. That happens even before > the entry asm gets run. > > Otherwise if we have a page fault inside do_page_fault, it's just a > nested page fault.
Oh ok! But we still have the cr2 issue that Steve talked about. > > --Andy > > > -- > Andy Lutomirski > AMA Capital Management, LLC -- 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/
