x86/setup: don't remove E820_TYPE_RAM for pfn 0

Patch series "mm: fix initialization of struct page for holes in  memory layout", v3.

Commit 73a6e474 ("mm: memmap_init: iterate over memblock regions
rather that check each PFN") exposed several issues with the memory map
initialization and these patches fix those issues.

Initially there were crashes during compaction that Qian Cai reported
back in April [1].  It seemed back then that the problem was fixed, but
a few weeks ago Andrea Arcangeli hit the same bug [2] and there was an
additional discussion at [3].

[1] https://lore.kernel.org/lkml/8C537EB7-85EE-4DCF-943E-3CC0ED0DF56D@lca.pw
[2] https://lore.kernel.org/lkml/20201121194506.13464-1-aarcange@redhat.com
[3] https://lore.kernel.org/mm-commits/20201206005401.qKuAVgOXr%akpm@linux-foundation.org

This patch (of 2):

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.

Link: https://lkml.kernel.org/r/20210111194017.22696-2-rppt@kernel.org


Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: David Hildenbrand <david@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qian Cai <cai@lca.pw>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>