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

[Matt Redfearn <matt.redfearn@mips.com>]

Hi Serge,

On 23/01/18 19:27, Serge Semin wrote:
> Hello Matt,
> 
> On Tue, Jan 23, 2018 at 11:03:27AM +0000, Matt Redfearn <matt.redfearn@mips.com> wrote:
>> 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.
>>
> 
> Alright lets sum it up. First of all, yeah, you are right, arch_mem_addpart()
> is created to make sure the kernel memory is added to the memblock/bootmem pool.
> The previous arch code was leaving such the memory range non-freed since it was
> higher the reserved_end, so to make sure the early memory allocations wouldn't
> be made from the pages, where kernel actually resides.
> 
> In my code I still wanted to make sure the kernel memory is in the memblock pool.
> But I also noticed, that .bss memory range wouldn't be added to the pool if neither
> dts nor platform-specific code added any memory to the boot_mem_map pool. So I
> decided to fix it. The actual kernel memory reservation is performed after all
> the memory regions are declared by the code you cited. It's essential to do
> the [_text, _end] memory range reservation there, otherwise memblock may
> allocate from the memory range occupied by the kernel code/data.
> 
> Since you agree with leaving it in the separate patch, I'd only suggest to
> call the arch_mem_addpart() method for just one range [_text, _end] instead of
> doing it three times for a separate _text, _data and bss sections. What do you
> think?

I think it's best left as 3 separate reservations, mainly due to the 
different attribute used for the init section. So all in all, I like 
this patch as it is.

Thanks,
Matt

> 
> Regards,
> -Sergey
> 
>> 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();
>>>>>