[PATCH] x86, mm: Reserver some memory for bootmem allocator for NO_BOOTMEM

[Feng Tang <feng.tang@intel.com>]

We hit a kernel panic when enabling earlycon for a platform, the
call trace is:

	  panic+0xd2/0x220
	  __alloc_bootmem+0x31/0x34
	  spp_getpage+0x60/0x8a
	  fill_pte+0x71/0x130
	  __set_pte_vaddr+0x1d/0x50
	  set_pte_vaddr+0x3c/0x60
	  __native_set_fixmap+0x23/0x30
	  native_set_fixmap+0x30/0x40
	  setup_earlycon+0x1e0/0x32f
	  param_setup_earlycon+0x13/0x22
	  do_early_param+0x5b/0x90
	  parse_args+0x1f7/0x300
	  parse_early_options+0x24/0x28
	  parse_early_param+0x65/0x73
	  setup_arch+0x31e/0x9f1
	  start_kernel+0x58/0x44e

The root cause is that when CONFIG_NO_BOOTMEM=y,  before
e820__memblock_setup() is called there is no memory for bootmem
to allocate, and any alloc_bootmem() in this time window will
trigger a panic. Seems this is a known issue as already
mentioned in arch/x86/kernel/setup.c:
	"
	/* after early param, so could get panic from serial */
	memblock_x86_reserve_range_setup_data();
	"
By reserving some memory in the linker script, small memory request
could be fulfilled by bootmem allocator. And this memory region
is not wasted, but usable after kernel boot.

Signed-off-by: Feng Tang <feng.tang@intel.com>
---
 arch/x86/include/asm/sections.h |  5 +++++
 arch/x86/kernel/setup.c         | 12 +++++++++++-
 arch/x86/kernel/vmlinux.lds.S   |  9 +++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/sections.h b/arch/x86/include/asm/sections.h
index 4a911a382ade..d568829510e4 100644
--- a/arch/x86/include/asm/sections.h
+++ b/arch/x86/include/asm/sections.h
@@ -9,6 +9,11 @@ extern char __brk_base[], __brk_limit[];
 extern struct exception_table_entry __stop___ex_table[];
 extern char __end_rodata_aligned[];
 
+#ifdef CONFIG_NO_BOOTMEM
+extern char __bootmem_start[], __bootmem_end[];
+#endif
+
+
 #if defined(CONFIG_X86_64)
 extern char __end_rodata_hpage_align[];
 extern char __entry_trampoline_start[], __entry_trampoline_end[];
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index b4866badb235..a4e3d9ec1a83 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -829,6 +829,17 @@ void __init setup_arch(char **cmdline_p)
 	 */
 	memblock_reserve(0, PAGE_SIZE);
 
+#ifdef CONFIG_NO_BOOTMEM
+	/*
+	 * There is small time window till e820__memblock_setup() is
+	 * called, in which __bootmem_alloc() has no available memory
+	 * to allocate and will trigger panic. Adding this revered bootmem
+	 * can alleviate the situation.
+	 */
+	memblock_add(__pa_symbol(__bootmem_start),
+			__bootmem_end - __bootmem_start);
+#endif
+
 	early_reserve_initrd();
 
 	/*
@@ -989,7 +1000,6 @@ void __init setup_arch(char **cmdline_p)
 
 	x86_report_nx();
 
-	/* after early param, so could get panic from serial */
 	memblock_x86_reserve_range_setup_data();
 
 	if (acpi_mps_check()) {
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 8bde0a419f86..fec1965d668f 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -367,6 +367,15 @@ SECTIONS
 		__brk_limit = .;
 	}
 
+#ifdef CONFIG_NO_BOOTMEM
+	. = ALIGN(PAGE_SIZE);
+	.bootmem : AT(ADDR(.bootmem) - LOAD_OFFSET) {
+		__bootmem_start = .;
+		. += 128 * 1024;		/* 128 KB for early boot mem */
+		__bootmem_end = .;
+	}
+#endif
+
 	. = ALIGN(PAGE_SIZE);		/* keep VO_INIT_SIZE page aligned */
 	_end = .;
 
-- 
2.14.1