Date:   Wed, 24 Jan 2018 13:03:58 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Matt Redfearn <matt.redfearn@...s.com>
Cc:     ralf@...ux-mips.org, miodrag.dinic@...s.com, jhogan@...nel.org,
        goran.ferenc@...s.com, david.daney@...ium.com,
        paul.gortmaker@...driver.com, paul.burton@...s.com,
        alex.belits@...ium.com, Steven.Hill@...ium.com,
        alexander.sverdlin@...ia.com, kumba@...too.org,
        marcin.nowakowski@...s.com, James.hogan@...s.com,
        Peter.Wotton@...s.com, Sergey.Semin@...latforms.ru,
        linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/14] MIPS: memblock: Surely map BSS kernel memory
 section

Hello Matt,

On Wed, Jan 24, 2018 at 09:49:31AM +0000, Matt Redfearn <matt.redfearn@...s.com> wrote:
> 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@...s.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@...s.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.
> 

Alright. I'll leave as is. Lets see what others think about it.

Regards,
-Sergey

> Thanks,
> Matt
> 
> >
> >Regards,
> >-Sergey
> >
> >>Thanks,
> >>Matt
> >>
> >>>
> >>>Regards,
> >>>-Sergey
> >>>
> >>>>
> >>>>Reviewed-by: Matt Redfearn <matt.redfearn@...s.com>
> >>>>
> >>>>Thanks,
> >>>>Matt
> >>>>
> >>>>>
> >>>>>Signed-off-by: Serge Semin <fancer.lancer@...il.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();
> >>>>>

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