On Mon, Oct 14, 2019 at 06:02:38PM +0200, Sebastian Andrzej Siewior wrote: > I'm dropping this patch, with its original description: > > |ARM: Initialize split page table locks for vector page > | > |Without this patch, ARM can not use SPLIT_PTLOCK_CPUS if > |PREEMPT_RT_FULL=y because vectors_user_mapping() creates a > |VM_ALWAYSDUMP mapping of the vector page (address 0xffff0000), but no > |ptl->lock has been allocated for the page. An attempt to coredump > |that page will result in a kernel NULL pointer dereference when > |follow_page() attempts to lock the page. > | > |The call tree to the NULL pointer dereference is: > | > | do_notify_resume() > | get_signal_to_deliver() > | do_coredump() > | elf_core_dump() > | get_dump_page() > | __get_user_pages() > | follow_page() > | pte_offset_map_lock() <----- a #define > | ... > | rt_spin_lock() > | > |The underlying problem is exposed by mm-shrink-the-page-frame-to-rt-size.patch. > > The patch named mm-shrink-the-page-frame-to-rt-size.patch was dropped > from the RT queue once the SPLIT_PTLOCK_CPUS feature (in a slightly > different shape) went upstream (somewhere between v3.12 and v3.14). > > I can see that the patch still allocates a lock which wasn't there > before. However I can't trigger a kernel oops like described in the > patch by triggering a coredump. Did your test build have ALLOC_SPLIT_PTLOCKS defined?
