Re: [PATCH RT] Revert "ARM: Initialize split page table locks for vector page"
From: Matthew Wilcox
Date: Tue Oct 15 2019 - 12:26:15 EST
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?