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

[Anup Patel <anup@brainfault.org>]

On Mon, Sep 14, 2020 at 5:18 AM Atish Patra <atishp@atishpatra.org> wrote:
>
> On Sat, Sep 12, 2020 at 3:45 AM 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.
> >
>
> IIRC, the original intention was to fix MAXPHYSMEM_2GB option for RV64
> for the medlow model.
> That's why the patch removed any memory beyond -PAGE_OFFSET.
>
> > For that you can use memblock_enforce_memory_limit(-PAGE_OFFSET).
> >
> Thanks. I think we can just call memblock_enforce_memory_limit without
> tracking the total memory size
> and whether maximum memory described in DT is greater than  -PAGE_OFFSET.
>
> @Anup Patel Was there any other reason for this change originally?

No other reason. We just wanted to ensure that amount of memory addressable
by kernel (i.e. -PAGE_OFFSET) is also considered when removing memblock.

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.
> >
> > _______________________________________________
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> > linux-riscv@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv
>
>
>
> --
> Regards,
> Atish