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Subject: Re: [PATCH 02/14] MIPS: memblock: Surely map BSS kernel memory
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To:     Serge Semin <fancer.lancer@gmail.com>
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From:   Matt Redfearn <matt.redfearn@mips.com>
Message-ID: <8a26c7cd-966f-90d4-96c9-2f974808c2f4@mips.com>
Date:   Tue, 23 Jan 2018 11:03:27 +0000
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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@mips.com> 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@mips.com>
>>
>> Thanks,
>> Matt
>>
>>>
>>> Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
>>> ---
>>>   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();
>>>