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

[Baoquan He]

On 02/01/21 at 10:32am, David Hildenbrand wrote:
> On 30.01.21 23:10, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> >   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.

Sounds reasonable. Maybe we can change to get the real expected lowest
pfn from find_min_pfn_for_node() by iterating memblock.memory and
memblock.reserved and comparing.

> 
> 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