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To:     Mike Rapoport <rppt@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
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References: <20210130221035.4169-1-rppt@kernel.org>
 <20210130221035.4169-2-rppt@kernel.org>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Subject: Re: [PATCH v4 1/2] x86/setup: always add the beginning of RAM as
 memblock.memory
Message-ID: <56e2c568-b121-8860-a6b0-274ace46d835@redhat.com>
Date:   Mon, 1 Feb 2021 10:32:44 +0100
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On 30.01.21 23:10, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> The physical memory on an x86 system starts at address 0, but this is not
> always reflected in e820 map. For example, the BIOS can have e820 entries
> like
> 
> [    0.000000] BIOS-provided physical RAM map:
> [    0.000000] BIOS-e820: [mem 0x0000000000001000-0x000000000009ffff] usable
> 
> or
> 
> [    0.000000] BIOS-provided physical RAM map:
> [    0.000000] BIOS-e820: [mem 0x0000000000000000-0x0000000000000fff] reserved
> [    0.000000] BIOS-e820: [mem 0x0000000000001000-0x0000000000057fff] usable
> 
> In either case, e820__memblock_setup() won't add the range 0x0000 - 0x1000
> to memblock.memory and later during memory map initialization this range is
> left outside any zone.
> 
> With SPARSEMEM=y there is always a struct page for pfn 0 and this struct
> page will have it's zone link wrong no matter what value will be set there.
> 
> To avoid this inconsistency, add the beginning of RAM to memblock.memory.
> Limit the added chunk size to match the reserved memory to avoid
> registering memory that may be used by the firmware but never reserved at
> e820__memblock_setup() time.
> 
> Fixes: bde9cfa3afe4 ("x86/setup: don't remove E820_TYPE_RAM for pfn 0")
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> Cc: stable@vger.kernel.org
> ---
>   arch/x86/kernel/setup.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 3412c4595efd..67c77ed6eef8 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -727,6 +727,14 @@ static void __init trim_low_memory_range(void)
>   	 * Kconfig help text for X86_RESERVE_LOW.
>   	 */
>   	memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE));
> +
> +	/*
> +	 * Even if the firmware does not report the memory at address 0 as
> +	 * usable, inform the generic memory management about its existence
> +	 * to ensure it is a part of ZONE_DMA and the memory map for it is
> +	 * properly initialized.
> +	 */
> +	memblock_add(0, ALIGN(reserve_low, PAGE_SIZE));
>   }
>   	
>   /*
> 

I think, to make that code more robust, and to not rely on archs to do 
the right thing, we should do something like

1) Make sure in free_area_init() that each PFN with a memmap (i.e., 
falls into a partial present section) is spanned by a zone; that would 
include PFN 0 in this case.

2) In init_zone_unavailable_mem(), similar to round_up(max_pfn, 
PAGES_PER_SECTION) handling, consider range
	[round_down(min_pfn, PAGES_PER_SECTION), min_pfn - 1]
which would handle in the x86-64 case [0..0] and, therefore, initialize 
PFN 0.

Also, I think the special-case of PFN 0 is analogous to the 
round_up(max_pfn, PAGES_PER_SECTION) handling in 
init_zone_unavailable_mem(): who guarantees that these PFN above the 
highest present PFN are actually spanned by a zone?

I'd suggest going through all zone ranges in free_area_init() first, 
dealing with zones that have "not section aligned start/end", clamping 
them up/down if required such that no holes within a section are left 
uncovered by a zone.

-- 
Thanks,

David / dhildenb