linux-kernel - Re: [PATCH] MIPS: reserve the memblock right after the kernel

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Date:   Tue, 10 Nov 2020 11:29:50 +0100
From:   Alexander Sverdlin <alexander.sverdlin@...ia.com>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     Jiaxun Yang <jiaxun.yang@...goat.com>, linux-mips@...r.kernel.org,
        Paul Burton <paulburton@...nel.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] MIPS: reserve the memblock right after the kernel

Hello Thomas,

On 10/11/2020 10:55, Thomas Bogendoerfer wrote:
>>>> Linux doesn't own the memory immediately after the kernel image. On Octeon
>>>> bootloader places a shared structure right close after the kernel _end,
>>>> refer to "struct cvmx_bootinfo *octeon_bootinfo" in cavium-octeon/setup.c.
>>>>
>>>> If check_kernel_sections_mem() rounds the PFNs up, first memblock_alloc()
>>>> inside early_init_dt_alloc_memory_arch() <= device_tree_init() returns
>>>> memory block overlapping with the above octeon_bootinfo structure, which
>>>> is being overwritten afterwards.
>>> as this special for Octeon how about added the memblock_reserve
>>> in octen specific code ?
>> while the shared structure which is being corrupted is indeed Octeon-specific,
>> the wrong assumption that the memory right after the kernel can be allocated by memblock
>> allocator and re-used somewhere in Linux is in MIPS-generic check_kernel_sections_mem().
> ok, I see your point. IMHO this whole check_kernel_sections_mem() should
> be removed. IMHO memory adding should only be done my memory detection code.
> 
> Could you send a patch, which removes check_kernel_section_mem completly ?

this will expose one issue:
platforms usually do it in a sane way, like it was done last 15 years, namely
add kernel image without non-complete pages on the boundaries.
This will lead to the situation, that request_resource() will fail at least
for .bss section of the kernel and it will not be properly displayed under
/proc/iomem (and probably same problem will appear, which initially motivated
the creation of check_kernel_section_mem()).

As I understood, the issue is that memblock API operates internally on the
page granularity (at least there are many ROUND_DOWN() inside for the size
or upper boundary), so for request_resource() to success one has to claim
the rest of the .bss last page. And with current memblock API
memblock_reserve() must appear somewhere, being this ARCH or platform code.

-- 
Best regards,
Alexander Sverdlin.