Re: [PATCH v4 1/2] x86/setup: always add the beginning of RAM as memblock.memory

[David Hildenbrand <david@redhat.com>]

On 01.02.21 15:30, Mike Rapoport wrote:
> On Mon, Feb 01, 2021 at 10:32:44AM +0100, David Hildenbrand wrote:
>> 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.
> 
> I thought about changing the way zone extents are calculated so that zone
> start/end will be always on a section boundary, but zone->zone_start_pfn
> depends on node->node_start_pfn which is defined by hardware and expanding
> a node to make its start pfn aligned at the section boundary might violate
> the HW addressing scheme.
> 
> Maybe this could never happen, or maybe it's not really important as the
> pages there will be reserved anyway, but I'm not sure I can estimate all
> the implications.
> 

I'm suggesting to let zone (+node?) ranges cover memory holes with a 
valid memmap. Not to move actual memory between nodes/zones.

-- 
Thanks,

David / dhildenb