Re: [PATCH v3 1/2] x86/setup: don't remove E820_TYPE_RAM for pfn 0

David Hildenbrand
2021-01-13T12:58:45Z

On 11.01.21 20:40, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
>
> The first 4Kb of memory is a BIOS owned area and to avoid its allocation
> for the kernel it was not listed in e820 tables as memory. As the result,
> pfn 0 was never recognised by the generic memory management and it is not a
> part of neither node 0 nor ZONE_DMA.
>
> If set_pfnblock_flags_mask() would be ever called for the pageblock
> corresponding to the first 2Mbytes of memory, having pfn 0 outside of
> ZONE_DMA would trigger
>
> VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
>
> Along with reserving the first 4Kb in e820 tables, several first pages are
> reserved with memblock in several places during setup_arch(). These
> reservations are enough to ensure the kernel does not touch the BIOS area
> and it is not necessary to remove E820_TYPE_RAM for pfn 0.
>
> Remove the update of e820 table that changes the type of pfn 0 and move the
> comment describing why it was done to trim_low_memory_range() that reserves
> the beginning of the memory.
>
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
> arch/x86/kernel/setup.c | 20 +++++++++-----------
> 1 file changed, 9 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 740f3bdb3f61..3412c4595efd 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -660,17 +660,6 @@ static void __init trim_platform_memory_ranges(void)
>
> static void __init trim_bios_range(void)
> {
> - /*
> - * A special case is the first 4Kb of memory;
> - * This is a BIOS owned area, not kernel ram, but generally
> - * not listed as such in the E820 table.
> - *
> - * This typically reserves additional memory (64KiB by default)
> - * since some BIOSes are known to corrupt low memory. See the
> - * Kconfig help text for X86_RESERVE_LOW.
> - */
> - e820__range_update(0, PAGE_SIZE, E820_TYPE_RAM, E820_TYPE_RESERVED);
> -
> /*
> * special case: Some BIOSes report the PC BIOS
> * area (640Kb -> 1Mb) as RAM even though it is not.
> @@ -728,6 +717,15 @@ early_param("reservelow", parse_reservelow);
>
> static void __init trim_low_memory_range(void)
> {
> + /*
> + * A special case is the first 4Kb of memory;
> + * This is a BIOS owned area, not kernel ram, but generally
> + * not listed as such in the E820 table.
> + *
> + * This typically reserves additional memory (64KiB by default)
> + * since some BIOSes are known to corrupt low memory. See the
> + * Kconfig help text for X86_RESERVE_LOW.
> + */
> memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE));
> }
>
>

The only somewhat-confusing thing is that in-between
e820__memblock_setup() and trim_low_memory_range(), we already have
memblock allocations. So [0..4095] might look like ordinary memory until
we reserve it later on.

E.g., reserve_real_mode() does a

mem = memblock_find_in_range(0, 1<<20, size, PAGE_SIZE);
...
memblock_reserve(mem, size);
set_real_mode_mem(mem);

which looks kind of suspicious to me. Most probably I am missing
something, just wanted to point that out. We might want to do such
trimming/adjustments before any kind of allocations.

--
Thanks,

David / dhildenb