[PATCH] mm: vmalloc: use VMALLOC_EARLY_START boundary for early vmap area

[Jia He <justin.he@arm.com>]

When VMALLOC_START is redefined to a new boundary, most subsystems
continue to function correctly. However, vm_area_register_early()
assumes the use of the global _vmlist_ structure before vmalloc_init()
is invoked. This assumption can lead to issues during early boot.

See the calltrace as follows:
	start_kernel()
		setup_per_cpu_areas()
			pcpu_page_first_chunk()
				vm_area_register_early()
		mm_core_init()
			vmalloc_init()

The early vm areas will be added to vmlist at declare_kernel_vmas()
->declare_vma(): 
ffff800080010000 T _stext
ffff800080da0000 D __start_rodata
ffff800081890000 T __inittext_begin
ffff800081980000 D __initdata_begin
ffff800081ee0000 D _data
The starting address of the early areas is tied to the *old* VMALLOC_START
(i.e. 0xffff800080000000 on an arm64 N2 server).

If VMALLOC_START is redefined, it can disrupt early VM area allocation,
particularly in like pcpu_page_first_chunk()->vm_area_register_early().

To address this potential risk on arm64, introduce a new boundary,
VMALLOC_EARLY_START, to avoid boot issues when VMALLOC_START is
occasionaly redefined.

Signed-off-by: Jia He <justin.he@arm.com>
---
 arch/arm64/include/asm/pgtable.h | 2 ++
 mm/vmalloc.c                     | 6 +++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 192d86e1cc76..91031912a906 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -18,9 +18,11 @@
  * VMALLOC range.
  *
  * VMALLOC_START: beginning of the kernel vmalloc space
+ * VMALLOC_EARLY_START: early vm area before vmalloc_init()
  * VMALLOC_END: extends to the available space below vmemmap
  */
 #define VMALLOC_START		(MODULES_END)
+#define VMALLOC_EARLY_START	(MODULES_END)
 #if VA_BITS == VA_BITS_MIN
 #define VMALLOC_END		(VMEMMAP_START - SZ_8M)
 #else
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 6dbcdceecae1..86ab1e99641a 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -50,6 +50,10 @@
 #include "internal.h"
 #include "pgalloc-track.h"
 
+#ifndef VMALLOC_EARLY_START
+#define VMALLOC_EARLY_START		VMALLOC_START
+#endif
+
 #ifdef CONFIG_HAVE_ARCH_HUGE_VMAP
 static unsigned int __ro_after_init ioremap_max_page_shift = BITS_PER_LONG - 1;
 
@@ -3126,7 +3130,7 @@ void __init vm_area_add_early(struct vm_struct *vm)
  */
 void __init vm_area_register_early(struct vm_struct *vm, size_t align)
 {
-	unsigned long addr = ALIGN(VMALLOC_START, align);
+	unsigned long addr = ALIGN(VMALLOC_EARLY_START, align);
 	struct vm_struct *cur, **p;
 
 	BUG_ON(vmap_initialized);
-- 
2.34.1