Re: [PATCH] RISC-V: Consider sparse memory while removing unusable memory

[Anup Patel <anup@brainfault.org>]

On Sat, Sep 12, 2020 at 4:15 PM Mike Rapoport <rppt@kernel.org> wrote:
>
> Hello Atish,
>
> On Fri, Sep 11, 2020 at 05:23:41PM -0700, Atish Patra wrote:
> > Currently, any usable memory area beyond page_offset is removed by adding the
> > memory sizes from each memblock. That may not work for sparse memory
> > as memory regions can be very far apart resulting incorrect removal of some
> > usable memory.
>
> If I understand correctly, the memory with physical addresses larger
> than (-PAGE_OFFSET) cannot be used. Since it was aready
> memblock_add()'ed during device tree parsing, you need to remove it from
> memblock.

-PAGE_OFFSET represents the size of memory addressable by the kernel and
not the last physical address.

mem_start + (-PAGE_OFFSET) will be the last physical address usable by kernel.

>
> For that you can use memblock_enforce_memory_limit(-PAGE_OFFSET).

I think we should use memblock_enforce_memory_limit(mem_start - PAGE_OFFSET).

Regards,
Anup

>
> > Just use the start of the first memory block and the end of the last memory
> > block to compute the size of the total memory that can be used.
> >
> > Signed-off-by: Atish Patra <atish.patra@wdc.com>
> > ---
> >  arch/riscv/mm/init.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index 787c75f751a5..188281fc2816 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -147,7 +147,6 @@ void __init setup_bootmem(void)
> >  {
> >       struct memblock_region *reg;
> >       phys_addr_t mem_size = 0;
> > -     phys_addr_t total_mem = 0;
> >       phys_addr_t mem_start, end = 0;
> >       phys_addr_t vmlinux_end = __pa_symbol(&_end);
> >       phys_addr_t vmlinux_start = __pa_symbol(&_start);
> > @@ -155,18 +154,17 @@ void __init setup_bootmem(void)
> >       /* Find the memory region containing the kernel */
> >       for_each_memblock(memory, reg) {
> >               end = reg->base + reg->size;
> > -             if (!total_mem)
> > +             if (!mem_start)
> >                       mem_start = reg->base;
> >               if (reg->base <= vmlinux_start && vmlinux_end <= end)
> >                       BUG_ON(reg->size == 0);
> > -             total_mem = total_mem + reg->size;
> >       }
> >
> >       /*
> >        * Remove memblock from the end of usable area to the
> >        * end of region
> >        */
> > -     mem_size = min(total_mem, (phys_addr_t)-PAGE_OFFSET);
> > +     mem_size = min(end - mem_start, (phys_addr_t)-PAGE_OFFSET);
> >       if (mem_start + mem_size < end)
> >               memblock_remove(mem_start + mem_size,
> >                               end - mem_start - mem_size);
> > --
> > 2.24.0
> >
>
> --
> Sincerely yours,
> Mike.