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. -- 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/
