On Fri, Jul 20, 2018 at 12:27 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > On Fri, 20 Jul 2018, Andy Lutomirski wrote: >> > On Jul 20, 2018, at 6:22 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote: >> > >> > From: Joerg Roedel <jroedel@xxxxxxx> >> > >> > The ring-buffer is accessed in the NMI handler, so we better >> > avoid faulting on it. Sync the vmalloc range with all >> > page-tables in system to make sure everyone has it mapped. >> > >> > This fixes a WARN_ON_ONCE() that can be triggered with PTI >> > enabled on x86-32: >> > >> > WARNING: CPU: 4 PID: 0 at arch/x86/mm/fault.c:320 vmalloc_fault+0x220/0x230 >> > >> > This triggers because with PTI enabled on an PAE kernel the >> > PMDs are no longer shared between the page-tables, so the >> > vmalloc changes do not propagate automatically. >> >> It seems like it would be much more robust to fix the vmalloc_fault() >> code instead. > > Right, but now the obvious fix for the issue at hand is this. We surely > should revisit this. If you commit this under this reasoning, then please at least make it say: /* XXX: The vmalloc_fault() code is buggy on PTI+PAE systems, and this is a workaround. */ Let's not have code in the kernel that pretends to make sense but is actually voodoo magic that works around bugs elsewhere. It's no fun to maintain down the road.
