Re: [PATCH 02/14] MIPS: memblock: Surely map BSS kernel memory section

[Matt Redfearn]

Hi Serge,

On 22/01/18 21:47, Serge Semin wrote:
> Hello Matt,
>
> On Mon, Jan 22, 2018 at 04:35:26PM +0000, Matt Redfearn <matt.redfearn@xxxxxxxx> wrote:
> > Hi Serge,
> >
> > On 17/01/18 22:23, Serge Semin wrote:
> > > The current MIPS code makes sure the kernel code/data/init
> > > sections are in the maps, but BSS should also be there.
> >
> > Quite right - it should. But this was protected against by reserving all
> > bootmem up to the _end symbol here:
> > http://elixir.free-electrons.com/linux/v4.15-rc8/source/arch/mips/kernel/setup.c#L388
> > Which you remove in the next patch in this series. I'm not sure it is worth
>
> Right. Missed that part. The old code just doesn't set the kernel memory free
> calling the free_bootmem() method for non-reserved parts below reserved_end.
>
> > disentangling the reserved_end stuff from the next patch to make this into a
> > single logical change of reserving just .bss rather than everything below
> > _end.
>
> Good point. I'll move this change into the "[PATCH 05/14] MIPS: memblock:
> Add reserved memory regions to memblock". It logically belongs to that place.
> Since basically by the arch_mem_addpart() calls we reserve all the kernel


Actually I was wrong - it's not this sequence of arch_mem_addpart's that 
reserves the kernels memory. At least on DT based systems, it's pretty 
likely that these regions will overlap with the system memory already added.
of_scan_flat_dt will look for the memory node and add it via 
early_init_dt_add_memory_arch.
These calls to add the kernel text, init and bss detect that they 
overlap with the already present system memory, so don't get added, here:
http://elixir.free-electrons.com/linux/v4.15-rc9/source/arch/mips/kernel/setup.c#L759

As such, when we print out the content of boot_mem_map, we only have a 
single entry:

[    0.000000] Determined physical RAM map:
[    0.000000]  memory: 10000000 @ 00000000 (usable)


> memory now I'd also merged them into a single call for the range [_text, _end].
> What do you think?


I think that this patch makes sense in case the .bss is for some reason 
not covered by an existing entry, but I would leave it as a separate patch.

Your [PATCH 05/14] MIPS: memblock: Add reserved memory regions to 
memblock is actually self-contained since it replaces reserving all 
memory up to _end with the single reservation of the kernel's whole size

+	size = __pa_symbol(&_end) - __pa_symbol(&_text);
+	memblock_reserve(__pa_symbol(&_text), size);


Which I think is definitely an improvement since it is much clearer.

Thanks,
Matt

> Regards,
> -Sergey
>
> > Reviewed-by: Matt Redfearn <matt.redfearn@xxxxxxxx>
> >
> > Thanks,
> > Matt
> >
> > > Signed-off-by: Serge Semin <fancer.lancer@xxxxxxxxx>
> > > ---
> > >   arch/mips/kernel/setup.c | 3 +++
> > >   1 file changed, 3 insertions(+)
> > >
> > > diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
> > > index 76e9e2075..0d21c9e04 100644
> > > --- a/arch/mips/kernel/setup.c
> > > +++ b/arch/mips/kernel/setup.c
> > > @@ -845,6 +845,9 @@ static void __init arch_mem_init(char **cmdline_p)
> > >   	arch_mem_addpart(PFN_UP(__pa_symbol(&__init_begin)) << PAGE_SHIFT,
> > >   			 PFN_DOWN(__pa_symbol(&__init_end)) << PAGE_SHIFT,
> > >   			 BOOT_MEM_INIT_RAM);
> > > +	arch_mem_addpart(PFN_DOWN(__pa_symbol(&__bss_start)) << PAGE_SHIFT,
> > > +			 PFN_UP(__pa_symbol(&__bss_stop)) << PAGE_SHIFT,
> > > +			 BOOT_MEM_RAM);
> > >   	pr_info("Determined physical RAM map:\n");
> > >   	print_memory_map();