On Fri, 31 Aug 2018, Feng Tang wrote:
> On Thu, Aug 30, 2018 at 03:25:42PM +0200, Thomas Gleixner wrote:
> This panic happens as the earlycon's fixmap address has no
> pmd/pte ready, and __set_fixmap will try to allocate memory to
> setup the page table, and trigger panic due to no memory.
>
> x86 kernel actually prepares the page table for fixmap in head_64.S:
>
> NEXT_PAGE(level2_fixmap_pgt)
> .fill 506,8,0
> .quad level1_fixmap_pgt - __START_KERNEL_map +
> _PAGE_TABLE_NOENC
> /* 8MB reserved for vsyscalls + a 2MB hole = 4 + 1 entries */
> .fill 5,8,0
>
> and it expects the fixmap address is in [-12M, -10M] range, but
> current code in fixmap.h will break the expectation when
> X86_VSYSCALL_EMULATION=n
>
> #ifdef CONFIG_X86_VSYSCALL_EMULATION
> VSYSCALL_PAGE = (FIXADDR_TOP - VSYSCALL_ADDR) >> PAGE_SHIFT,
> #endif
>
> So removing the "#ifdef" will make the fixmap address space stable in
> [-12M, -10M] and fix the issue.
Why on earth are you not fixing the damned PTE setup which is the obvious
and correct thing to do?
Thanks,
tglx
