arm64: mm: move vmemmap region right below the linear region

This moves the vmemmap region right below PAGE_OFFSET, aka the start
of the linear region, and redefines its size to be a power of two.
Due to the placement of PAGE_OFFSET in the middle of the address space,
whose size is a power of two as well, this guarantees that virt to
page conversions and vice versa can be implemented efficiently, by
masking and shifting rather than ordinary arithmetic.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/memory.h

@@ -39,6 +39,21 @@
  */
 #define PCI_IO_SIZE		SZ_16M
 
+/*
+ * Log2 of the upper bound of the size of a struct page. Used for sizing
+ * the vmemmap region only, does not affect actual memory footprint.
+ * We don't use sizeof(struct page) directly since taking its size here
+ * requires its definition to be available at this point in the inclusion
+ * chain, and it may not be a power of 2 in the first place.
+ */
+#define STRUCT_PAGE_MAX_SHIFT	6
+
+/*
+ * VMEMMAP_SIZE - allows the whole linear region to be covered by
+ *                a struct page array
+ */
+#define VMEMMAP_SIZE (UL(1) << (VA_BITS - PAGE_SHIFT - 1 + STRUCT_PAGE_MAX_SHIFT))
+
 /*
  * PAGE_OFFSET - the virtual address of the start of the kernel image (top
  *		 (VA_BITS - 1))
@@ -54,7 +69,8 @@
 #define MODULES_END		(MODULES_VADDR + MODULES_VSIZE)
 #define MODULES_VADDR		(VA_START + KASAN_SHADOW_SIZE)
 #define MODULES_VSIZE		(SZ_128M)
-#define PCI_IO_END		(PAGE_OFFSET - SZ_2M)
+#define VMEMMAP_START		(PAGE_OFFSET - VMEMMAP_SIZE)
+#define PCI_IO_END		(VMEMMAP_START - SZ_2M)
 #define PCI_IO_START		(PCI_IO_END - PCI_IO_SIZE)
 #define FIXADDR_TOP		(PCI_IO_START - SZ_2M)
 #define TASK_SIZE_64		(UL(1) << VA_BITS)

arch/arm64/include/asm/pgtable.h

@@ -24,20 +24,15 @@
 #include <asm/pgtable-prot.h>
 
 /*
- * VMALLOC and SPARSEMEM_VMEMMAP ranges.
+ * VMALLOC range.
  *
- * VMEMAP_SIZE: allows the whole linear region to be covered by a struct page array
- *	(rounded up to PUD_SIZE).
  * VMALLOC_START: beginning of the kernel vmalloc space
- * VMALLOC_END: extends to the available space below vmmemmap, PCI I/O space,
+ * VMALLOC_END: extends to the available space below vmmemmap, PCI I/O space
- *	fixed mappings and modules
+ *	and fixed mappings
  */
-#define VMEMMAP_SIZE		ALIGN((1UL << (VA_BITS - PAGE_SHIFT - 1)) * sizeof(struct page), PUD_SIZE)
-
 #define VMALLOC_START		(MODULES_END)
 #define VMALLOC_END		(PAGE_OFFSET - PUD_SIZE - VMEMMAP_SIZE - SZ_64K)
 
-#define VMEMMAP_START		(VMALLOC_END + SZ_64K)
 #define vmemmap			((struct page *)VMEMMAP_START - (memstart_addr >> PAGE_SHIFT))
 
 #define FIRST_USER_ADDRESS	0UL

arch/arm64/mm/dump.c

@@ -37,14 +37,14 @@ enum address_markers_idx {
 	MODULES_END_NR,
 	VMALLOC_START_NR,
 	VMALLOC_END_NR,
-#ifdef CONFIG_SPARSEMEM_VMEMMAP
-	VMEMMAP_START_NR,
-	VMEMMAP_END_NR,
-#endif
 	FIXADDR_START_NR,
 	FIXADDR_END_NR,
 	PCI_START_NR,
 	PCI_END_NR,
+#ifdef CONFIG_SPARSEMEM_VMEMMAP
+	VMEMMAP_START_NR,
+	VMEMMAP_END_NR,
+#endif
 	KERNEL_SPACE_NR,
 };
 
@@ -53,14 +53,14 @@ static struct addr_marker address_markers[] = {
 	{ MODULES_END,		"Modules end" },
 	{ VMALLOC_START,	"vmalloc() Area" },
 	{ VMALLOC_END,		"vmalloc() End" },
-#ifdef CONFIG_SPARSEMEM_VMEMMAP
-	{ 0,			"vmemmap start" },
-	{ 0,			"vmemmap end" },
-#endif
 	{ FIXADDR_START,	"Fixmap start" },
 	{ FIXADDR_TOP,		"Fixmap end" },
 	{ PCI_IO_START,		"PCI I/O start" },
 	{ PCI_IO_END,		"PCI I/O end" },
+#ifdef CONFIG_SPARSEMEM_VMEMMAP
+	{ 0,			"vmemmap start" },
+	{ 0,			"vmemmap end" },
+#endif
 	{ PAGE_OFFSET,		"Linear Mapping" },
 	{ -1,			NULL },
 };

arch/arm64/mm/init.c

@@ -412,6 +412,10 @@ void __init mem_init(void)
 		MLK_ROUNDUP(__start_rodata, _etext),
 		MLK_ROUNDUP(__init_begin, __init_end),
 		MLK_ROUNDUP(_sdata, _edata));
+	pr_cont("    fixed   : 0x%16lx - 0x%16lx   (%6ld KB)\n",
+		MLK(FIXADDR_START, FIXADDR_TOP));
+	pr_cont("    PCI I/O : 0x%16lx - 0x%16lx   (%6ld MB)\n",
+		MLM(PCI_IO_START, PCI_IO_END));
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
 	pr_cont("    vmemmap : 0x%16lx - 0x%16lx   (%6ld GB maximum)\n"
 		"              0x%16lx - 0x%16lx   (%6ld MB actual)\n",
@@ -420,10 +424,6 @@ void __init mem_init(void)
 		MLM((unsigned long)phys_to_page(memblock_start_of_DRAM()),
 		    (unsigned long)virt_to_page(high_memory)));
 #endif
-	pr_cont("    fixed   : 0x%16lx - 0x%16lx   (%6ld KB)\n",
-		MLK(FIXADDR_START, FIXADDR_TOP));
-	pr_cont("    PCI I/O : 0x%16lx - 0x%16lx   (%6ld MB)\n",
-		MLM(PCI_IO_START, PCI_IO_END));
 	pr_cont("    memory  : 0x%16lx - 0x%16lx   (%6ld MB)\n",
 		MLM(__phys_to_virt(memblock_start_of_DRAM()),
 		    (unsigned long)high_memory));
@@ -440,6 +440,12 @@ void __init mem_init(void)
 	BUILD_BUG_ON(TASK_SIZE_32			> TASK_SIZE_64);
 #endif
 
+	/*
+	 * Make sure we chose the upper bound of sizeof(struct page)
+	 * correctly.
+	 */
+	BUILD_BUG_ON(sizeof(struct page) > (1 << STRUCT_PAGE_MAX_SHIFT));
+
 	if (PAGE_SIZE >= 16384 && get_num_physpages() <= 128) {
 		extern int sysctl_overcommit_memory;
 		/*