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

[Serge Semin]

Hello Matt,

On Tue, Jan 23, 2018 at 11:03:27AM +0000, Matt Redfearn <matt.redfearn@xxxxxxxx> 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@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.
> 

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?

Regards,
-Sergey

> 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();
> >>>