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

[Mike Rapoport]

From: Mike Rapoport <rppt@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
---
 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));
 }
 	
-- 
2.28.0