LoongArch: Add HIGHMEM (PKMAP and FIX_KMAP) support

Add HIGHMEM (High Memory) support for LoongArch, mostly needed by 32BIT
kernel because the size of kernel virtual memory space is only 512MB and
the size of usable physical memory is only 256MB in this case.

HIGHMEM adds permanent kernel mapping (PKMAP) and fixed kernel mapping
(FIX_KMAP), which increase usable physical memory up to 2.25GB (2304MB).

We can just use the generic copy_user_highpage(), so remove the custom
version.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Kconfig

@@ -348,6 +348,11 @@ config RUSTC_HAS_ANNOTATE_TABLEJUMP
 
 source "kernel/Kconfig.hz"
 
+config HIGHMEM
+	bool "High Memory Support"
+	depends on 32BIT
+	select KMAP_LOCAL
+
 choice
 	prompt "Page Table Layout"
 	default 16KB_2LEVEL if 32BIT

arch/loongarch/include/asm/fixmap.h

@@ -8,10 +8,19 @@
 #ifndef _ASM_FIXMAP_H
 #define _ASM_FIXMAP_H
 
+#ifdef CONFIG_HIGHMEM
+#include <linux/threads.h>
+#include <asm/kmap_size.h>
+#endif
+
 #define NR_FIX_BTMAPS 64
 
 enum fixed_addresses {
 	FIX_HOLE,
+#ifdef CONFIG_HIGHMEM
+	FIX_KMAP_BEGIN,
+	FIX_KMAP_END = FIX_KMAP_BEGIN + (KM_MAX_IDX * NR_CPUS) - 1,
+#endif
 	FIX_EARLYCON_MEM_BASE,
 	__end_of_fixed_addresses
 };
@@ -25,4 +34,9 @@ extern void __set_fixmap(enum fixed_addresses idx,
 
 #include <asm-generic/fixmap.h>
 
+/*
+ * Called from pagetable_init()
+ */
+extern void fixrange_init(unsigned long start, unsigned long end, pgd_t *pgd_base);
+
 #endif

arch/loongarch/include/asm/highmem.h

@@ -0,0 +1,43 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * highmem.h: virtual kernel memory mappings for high memory
+ *
+ * Used in CONFIG_HIGHMEM systems for memory pages which
+ * are not addressable by direct kernel virtual addresses.
+ *
+ * Copyright (C) 2025 Loongson Technology Corporation Limited
+ */
+#ifndef _ASM_HIGHMEM_H
+#define _ASM_HIGHMEM_H
+
+#ifdef __KERNEL__
+
+#include <asm/kmap_size.h>
+
+#ifndef __ASSEMBLER__
+
+extern pte_t *pkmap_page_table;
+
+#define ARCH_HAS_KMAP_FLUSH_TLB
+void kmap_flush_tlb(unsigned long addr);
+
+#endif /* !__ASSEMBLER__ */
+
+/*
+ * Right now we initialize only a single pte table. It can be extended
+ * easily, subsequent pte tables have to be allocated in one physical
+ * chunk of RAM.
+ */
+#define LAST_PKMAP 1024
+#define LAST_PKMAP_MASK (LAST_PKMAP - 1)
+#define PKMAP_NR(virt)	((virt - PKMAP_BASE) >> PAGE_SHIFT)
+#define PKMAP_ADDR(nr)	(PKMAP_BASE + ((nr) << PAGE_SHIFT))
+
+#define flush_cache_kmaps() do {} while (0)
+
+#define arch_kmap_local_post_map(vaddr, pteval)	local_flush_tlb_one(vaddr)
+#define arch_kmap_local_post_unmap(vaddr)	local_flush_tlb_one(vaddr)
+
+#endif /* __KERNEL__ */
+
+#endif /* _ASM_HIGHMEM_H */

arch/loongarch/include/asm/page.h

@@ -36,10 +36,6 @@ extern unsigned long shm_align_mask;
 
 struct page;
 struct vm_area_struct;
-void copy_user_highpage(struct page *to, struct page *from,
-	      unsigned long vaddr, struct vm_area_struct *vma);
-
-#define __HAVE_ARCH_COPY_USER_HIGHPAGE
 
 typedef struct { unsigned long pte; } pte_t;
 #define pte_val(x)	((x).pte)

arch/loongarch/include/asm/pgtable.h

@@ -23,6 +23,10 @@
 #include <asm-generic/pgtable-nop4d.h>
 #endif
 
+#ifdef CONFIG_HIGHMEM
+#include <asm/highmem.h>
+#endif
+
 #if CONFIG_PGTABLE_LEVELS == 2
 #define PGDIR_SHIFT	(PAGE_SHIFT + (PAGE_SHIFT - PTRLOG))
 #elif CONFIG_PGTABLE_LEVELS == 3
@@ -86,7 +90,15 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
 #ifdef CONFIG_32BIT
 
 #define VMALLOC_START	(vm_map_base + PCI_IOSIZE + (2 * PAGE_SIZE))
+
+#ifdef CONFIG_HIGHMEM
+#define VMALLOC_END	(PKMAP_BASE - (2 * PAGE_SIZE))
+#else
 #define VMALLOC_END	(FIXADDR_START - (2 * PAGE_SIZE))
+#endif
+
+#define PKMAP_BASE	(PKMAP_END - (PAGE_SIZE * LAST_PKMAP))
+#define PKMAP_END	((FIXADDR_START) & ~((LAST_PKMAP << PAGE_SHIFT)-1))
 
 #endif
 

arch/loongarch/mm/Makefile

@@ -7,6 +7,7 @@ obj-y				+= init.o cache.o tlb.o tlbex.o extable.o \
 				   fault.o ioremap.o maccess.o mmap.o pgtable.o \
 				   page.o pageattr.o
 
+obj-$(CONFIG_HIGHMEM)		+= highmem.o
 obj-$(CONFIG_HUGETLB_PAGE)	+= hugetlbpage.o
 obj-$(CONFIG_KASAN)		+= kasan_init.o
 

arch/loongarch/mm/highmem.c

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/init.h>
+#include <linux/export.h>
+#include <linux/highmem.h>
+#include <asm/fixmap.h>
+#include <asm/tlbflush.h>
+
+void kmap_flush_tlb(unsigned long addr)
+{
+	flush_tlb_one(addr);
+}
+EXPORT_SYMBOL(kmap_flush_tlb);

arch/loongarch/mm/init.c

@@ -39,20 +39,6 @@
 unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)] __page_aligned_bss;
 EXPORT_SYMBOL(empty_zero_page);
 
-void copy_user_highpage(struct page *to, struct page *from,
-	unsigned long vaddr, struct vm_area_struct *vma)
-{
-	void *vfrom, *vto;
-
-	vfrom = kmap_local_page(from);
-	vto = kmap_local_page(to);
-	copy_page(vto, vfrom);
-	kunmap_local(vfrom);
-	kunmap_local(vto);
-	/* Make sure this page is cleared on other CPU's too before using it */
-	smp_wmb();
-}
-
 int __ref page_is_ram(unsigned long pfn)
 {
 	unsigned long addr = PFN_PHYS(pfn);
@@ -66,6 +52,9 @@ void __init arch_zone_limits_init(unsigned long *max_zone_pfns)
 	max_zone_pfns[ZONE_DMA32] = MAX_DMA32_PFN;
 #endif
 	max_zone_pfns[ZONE_NORMAL] = max_low_pfn;
+#ifdef CONFIG_HIGHMEM
+	max_zone_pfns[ZONE_HIGHMEM] = max_pfn;
+#endif
 }
 
 void __ref free_initmem(void)
@@ -73,6 +62,50 @@ void __ref free_initmem(void)
 	free_initmem_default(POISON_FREE_INITMEM);
 }
 
+#ifdef CONFIG_HIGHMEM
+
+void __init fixrange_init(unsigned long start, unsigned long end, pgd_t *pgd_base)
+{
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *pte;
+	int i, j, k;
+	int ptrs_per_pgd;
+	unsigned long vaddr;
+
+	vaddr = start;
+	i = pgd_index(vaddr);
+	j = pud_index(vaddr);
+	k = pmd_index(vaddr);
+	pgd = pgd_base + i;
+	ptrs_per_pgd = min((1 << (BITS_PER_LONG - PGDIR_SHIFT)), PTRS_PER_PGD);
+
+	for ( ; (i < ptrs_per_pgd) && (vaddr < end); pgd++, i++) {
+		pud = (pud_t *)pgd;
+		for ( ; (j < PTRS_PER_PUD) && (vaddr < end); pud++, j++) {
+			pmd = (pmd_t *)pud;
+			for (; (k < PTRS_PER_PMD) && (vaddr < end); pmd++, k++) {
+				if (pmd_none(*pmd)) {
+					pte = (pte_t *) memblock_alloc_low(PAGE_SIZE, PAGE_SIZE);
+					if (!pte)
+						panic("%s: Failed to allocate %lu bytes align=%lx\n",
+						      __func__, PAGE_SIZE, PAGE_SIZE);
+
+					kernel_pte_init(pte);
+					set_pmd(pmd, __pmd((unsigned long)pte));
+					BUG_ON(pte != pte_offset_kernel(pmd, 0));
+				}
+				vaddr += PMD_SIZE;
+			}
+			k = 0;
+		}
+		j = 0;
+	}
+}
+
+#endif
+
 #ifdef CONFIG_MEMORY_HOTPLUG
 int arch_add_memory(int nid, u64 start, u64 size, struct mhp_params *params)
 {

arch/loongarch/mm/pgtable.c

@@ -5,6 +5,7 @@
 #include <linux/init.h>
 #include <linux/export.h>
 #include <linux/mm.h>
+#include <asm/fixmap.h>
 #include <asm/pgalloc.h>
 #include <asm/pgtable.h>
 #include <asm/tlbflush.h>
@@ -144,6 +145,15 @@ void set_pmd_at(struct mm_struct *mm, unsigned long addr,
 
 void __init pagetable_init(void)
 {
+#ifdef CONFIG_HIGHMEM
+	unsigned long vaddr;
+	pgd_t *pgd;
+	p4d_t *p4d;
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *pte;
+#endif
+
 	/* Initialize the entire pgd.  */
 	pgd_init(swapper_pg_dir);
 	pgd_init(invalid_pg_dir);
@@ -153,4 +163,21 @@ void __init pagetable_init(void)
 #ifndef __PAGETABLE_PMD_FOLDED
 	pmd_init(invalid_pmd_table);
 #endif
+
+#ifdef CONFIG_HIGHMEM
+	/* Permanent kmaps */
+	vaddr = PKMAP_BASE;
+	fixrange_init(vaddr & PMD_MASK, vaddr + PAGE_SIZE * LAST_PKMAP, swapper_pg_dir);
+
+	pgd = swapper_pg_dir + pgd_index(vaddr);
+	p4d = p4d_offset(pgd, vaddr);
+	pud = pud_offset(p4d, vaddr);
+	pmd = pmd_offset(pud, vaddr);
+	pte = pte_offset_kernel(pmd, vaddr);
+	pkmap_page_table = pte;
+
+	/* Fixed mappings */
+	vaddr = __fix_to_virt(__end_of_fixed_addresses - 1);
+	fixrange_init(vaddr & PMD_MASK, vaddr + FIXADDR_SIZE, swapper_pg_dir);
+#endif
 }