Re: [PATCH] RISC-V: Fix Maximum Physical Memory 2GiB option for 64bit systems

[Anup Patel <anup@brainfault.org>]

On Tue, Apr 2, 2019 at 2:05 PM Mike Rapoport <rppt@linux.ibm.com> wrote:
>
> On Tue, Apr 02, 2019 at 06:02:38AM +0000, Anup Patel wrote:
> > The Maximum Physical Memory 2GiB option for 64bit systems is currently
> > broken because kernel hangs at boot-time when this option is enabled
> > and the underlying system has more than 2GiB memory.
> >
> > This issue can be easily reproduced on SiFive Unleashed board where
> > we have 8GiB of memory.
> >
> > This patch fixes above issue by reserving unusable memory region in
> > setup_bootmem().
> >
> > Signed-off-by: Anup Patel <anup.patel@wdc.com>
> > ---
> >  arch/riscv/mm/init.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index 5fd8c922e1c2..6b063f20a9d0 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -121,6 +121,14 @@ void __init setup_bootmem(void)
> >                        */
> >                       memblock_reserve(reg->base, vmlinux_end - reg->base);
> >                       mem_size = min(reg->size, (phys_addr_t)-PAGE_OFFSET);
> > +
> > +                     /*
> > +                      * Reserve from the end of usable area to the end of
> > +                      * region
> > +                      */
> > +                     if ((reg->base + mem_size) < end)
> > +                             memblock_reserve(reg->base + mem_size,
> > +                                              end - reg->base - mem_size);
>
> The memory above MAXPHYSMEM should not be reserved. It should be either
> removed from memblock with memblock_remove or not added at the first place.

Sure, I will try memblock_remove() instead of memblock_reserve().

>
> Frankly, I fail to understand the logic behind setting PAGE_OFFSET to
> MAXPHYSMEM and then using PAGE_OFFSET as the limit for accessible physical
> memory. Still, as it is there, you can set MAX_MEMBLOCK_ADDR=PAGE_OFFSET in
> arch/riscv/include/page.h and then early_init_dt_add_memory_arch() will
> simply ignore the memory above PAGE_OFFSET.
>
> More sustainable fix for the long term, IMHO, would be to break
> PAGE_OFFSET and MAXPHYSMEM interdependency.

I agree with you. There is too much dependency on PAGE_OFFSET value.

Another thing that I had attempted in my setup_vm() patches, is to reduce
initial page tables memory consumption by making page table array size
independent of -PAGE_OFFSET.

Regards,
Anup