mm: constify highmem related functions for improved const-correctness

Lots of functions in mm/highmem.c do not write to the given pointers and
do not call functions that take non-const pointers and can therefore be
constified.

This includes functions like kunmap() which might be implemented in a way
that writes to the pointer (e.g.  to update reference counters or mapping
fields), but currently are not.

kmap() on the other hand cannot be made const because it calls
set_page_address() which is non-const in some
architectures/configurations.

[akpm@linux-foundation.org: "fix" folio_page() build failure]
Link: https://lkml.kernel.org/r/20250901205021.3573313-13-max.kellermann@ionos.com
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christian Zankel <chris@zankel.net>
Cc: David Rientjes <rientjes@google.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Helge Deller <deller@gmx.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Bottomley <james.bottomley@HansenPartnership.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "Nysal Jan K.A" <nysal@linux.ibm.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russel King <linux@armlinux.org.uk>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: MaxKellermann

arch/arm/include/asm/highmem.h

@@ -46,9 +46,9 @@ extern pte_t *pkmap_page_table;
 #endif
 
 #ifdef ARCH_NEEDS_KMAP_HIGH_GET
-extern void *kmap_high_get(struct page *page);
+extern void *kmap_high_get(const struct page *page);
 
-static inline void *arch_kmap_local_high_get(struct page *page)
+static inline void *arch_kmap_local_high_get(const struct page *page)
 {
 	if (IS_ENABLED(CONFIG_DEBUG_HIGHMEM) && !cache_is_vivt())
 		return NULL;
@@ -57,7 +57,7 @@ static inline void *arch_kmap_local_high_get(struct page *page)
 #define arch_kmap_local_high_get arch_kmap_local_high_get
 
 #else /* ARCH_NEEDS_KMAP_HIGH_GET */
-static inline void *kmap_high_get(struct page *page)
+static inline void *kmap_high_get(const struct page *page)
 {
 	return NULL;
 }

arch/xtensa/include/asm/highmem.h

@@ -29,7 +29,7 @@
 
 #if DCACHE_WAY_SIZE > PAGE_SIZE
 #define get_pkmap_color get_pkmap_color
-static inline int get_pkmap_color(struct page *page)
+static inline int get_pkmap_color(const struct page *page)
 {
 	return DCACHE_ALIAS(page_to_phys(page));
 }

include/linux/highmem-internal.h

@@ -7,7 +7,7 @@
  */
 #ifdef CONFIG_KMAP_LOCAL
 void *__kmap_local_pfn_prot(unsigned long pfn, pgprot_t prot);
-void *__kmap_local_page_prot(struct page *page, pgprot_t prot);
+void *__kmap_local_page_prot(const struct page *page, pgprot_t prot);
 void kunmap_local_indexed(const void *vaddr);
 void kmap_local_fork(struct task_struct *tsk);
 void __kmap_local_sched_out(void);
@@ -33,7 +33,7 @@ static inline void kmap_flush_tlb(unsigned long addr) { }
 #endif
 
 void *kmap_high(struct page *page);
-void kunmap_high(struct page *page);
+void kunmap_high(const struct page *page);
 void __kmap_flush_unused(void);
 struct page *__kmap_to_page(void *addr);
 
@@ -50,7 +50,7 @@ static inline void *kmap(struct page *page)
 	return addr;
 }
 
-static inline void kunmap(struct page *page)
+static inline void kunmap(const struct page *page)
 {
 	might_sleep();
 	if (!PageHighMem(page))
@@ -68,26 +68,26 @@ static inline void kmap_flush_unused(void)
 	__kmap_flush_unused();
 }
 
-static inline void *kmap_local_page(struct page *page)
+static inline void *kmap_local_page(const struct page *page)
 {
 	return __kmap_local_page_prot(page, kmap_prot);
 }
 
-static inline void *kmap_local_page_try_from_panic(struct page *page)
+static inline void *kmap_local_page_try_from_panic(const struct page *page)
 {
 	if (!PageHighMem(page))
 		return page_address(page);
 	/* If the page is in HighMem, it's not safe to kmap it.*/
 	return NULL;
 }
 
-static inline void *kmap_local_folio(struct folio *folio, size_t offset)
+static inline void *kmap_local_folio(const struct folio *folio, size_t offset)
 {
-	struct page *page = folio_page(folio, offset / PAGE_SIZE);
+	const struct page *page = folio_page(folio, offset / PAGE_SIZE);
 	return __kmap_local_page_prot(page, kmap_prot) + offset % PAGE_SIZE;
 }
 
-static inline void *kmap_local_page_prot(struct page *page, pgprot_t prot)
+static inline void *kmap_local_page_prot(const struct page *page, pgprot_t prot)
 {
 	return __kmap_local_page_prot(page, prot);
 }
@@ -102,7 +102,7 @@ static inline void __kunmap_local(const void *vaddr)
 	kunmap_local_indexed(vaddr);
 }
 
-static inline void *kmap_atomic_prot(struct page *page, pgprot_t prot)
+static inline void *kmap_atomic_prot(const struct page *page, pgprot_t prot)
 {
 	if (IS_ENABLED(CONFIG_PREEMPT_RT))
 		migrate_disable();
@@ -113,7 +113,7 @@ static inline void *kmap_atomic_prot(struct page *page, pgprot_t prot)
 	return __kmap_local_page_prot(page, prot);
 }
 
-static inline void *kmap_atomic(struct page *page)
+static inline void *kmap_atomic(const struct page *page)
 {
 	return kmap_atomic_prot(page, kmap_prot);
 }
@@ -173,32 +173,32 @@ static inline void *kmap(struct page *page)
 	return page_address(page);
 }
 
-static inline void kunmap_high(struct page *page) { }
+static inline void kunmap_high(const struct page *page) { }
 static inline void kmap_flush_unused(void) { }
 
-static inline void kunmap(struct page *page)
+static inline void kunmap(const struct page *page)
 {
 #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
 	kunmap_flush_on_unmap(page_address(page));
 #endif
 }
 
-static inline void *kmap_local_page(struct page *page)
+static inline void *kmap_local_page(const struct page *page)
 {
 	return page_address(page);
 }
 
-static inline void *kmap_local_page_try_from_panic(struct page *page)
+static inline void *kmap_local_page_try_from_panic(const struct page *page)
 {
 	return page_address(page);
 }
 
-static inline void *kmap_local_folio(struct folio *folio, size_t offset)
+static inline void *kmap_local_folio(const struct folio *folio, size_t offset)
 {
 	return folio_address(folio) + offset;
 }
 
-static inline void *kmap_local_page_prot(struct page *page, pgprot_t prot)
+static inline void *kmap_local_page_prot(const struct page *page, pgprot_t prot)
 {
 	return kmap_local_page(page);
 }
@@ -215,7 +215,7 @@ static inline void __kunmap_local(const void *addr)
 #endif
 }
 
-static inline void *kmap_atomic(struct page *page)
+static inline void *kmap_atomic(const struct page *page)
 {
 	if (IS_ENABLED(CONFIG_PREEMPT_RT))
 		migrate_disable();
@@ -225,7 +225,7 @@ static inline void *kmap_atomic(struct page *page)
 	return page_address(page);
 }
 
-static inline void *kmap_atomic_prot(struct page *page, pgprot_t prot)
+static inline void *kmap_atomic_prot(const struct page *page, pgprot_t prot)
 {
 	return kmap_atomic(page);
 }

include/linux/highmem.h

@@ -43,7 +43,7 @@ static inline void *kmap(struct page *page);
  * Counterpart to kmap(). A NOOP for CONFIG_HIGHMEM=n and for mappings of
  * pages in the low memory area.
  */
-static inline void kunmap(struct page *page);
+static inline void kunmap(const struct page *page);
 
 /**
  * kmap_to_page - Get the page for a kmap'ed address
@@ -93,7 +93,7 @@ static inline void kmap_flush_unused(void);
  * disabling migration in order to keep the virtual address stable across
  * preemption. No caller of kmap_local_page() can rely on this side effect.
  */
-static inline void *kmap_local_page(struct page *page);
+static inline void *kmap_local_page(const struct page *page);
 
 /**
  * kmap_local_folio - Map a page in this folio for temporary usage
@@ -129,7 +129,7 @@ static inline void *kmap_local_page(struct page *page);
  * Context: Can be invoked from any context.
  * Return: The virtual address of @offset.
  */
-static inline void *kmap_local_folio(struct folio *folio, size_t offset);
+static inline void *kmap_local_folio(const struct folio *folio, size_t offset);
 
 /**
  * kmap_atomic - Atomically map a page for temporary usage - Deprecated!
@@ -176,7 +176,7 @@ static inline void *kmap_local_folio(struct folio *folio, size_t offset);
  * kunmap_atomic(vaddr2);
  * kunmap_atomic(vaddr1);
  */
-static inline void *kmap_atomic(struct page *page);
+static inline void *kmap_atomic(const struct page *page);
 
 /* Highmem related interfaces for management code */
 static inline unsigned long nr_free_highpages(void);

include/linux/page-flags.h

@@ -316,9 +316,9 @@ static __always_inline unsigned long _compound_head(const struct page *page)
  * check that the page number lies within @folio; the caller is presumed
  * to have a reference to the page.
  */
-static inline struct page *folio_page(struct folio *folio, unsigned long n)
+static inline struct page *folio_page(const struct folio *folio, unsigned long n)
 {
-	return &folio->page + n;
+	return (struct page *)(&folio->page + n);
 }
 
 static __always_inline int PageTail(const struct page *page)

mm/highmem.c

@@ -61,7 +61,7 @@ static inline int kmap_local_calc_idx(int idx)
 /*
  * Determine color of virtual address where the page should be mapped.
  */
-static inline unsigned int get_pkmap_color(struct page *page)
+static inline unsigned int get_pkmap_color(const struct page *page)
 {
 	return 0;
 }
@@ -334,7 +334,7 @@ EXPORT_SYMBOL(kmap_high);
  *
  * This can be called from any context.
  */
-void *kmap_high_get(struct page *page)
+void *kmap_high_get(const struct page *page)
 {
 	unsigned long vaddr, flags;
 
@@ -356,7 +356,7 @@ void *kmap_high_get(struct page *page)
  * If ARCH_NEEDS_KMAP_HIGH_GET is not defined then this may be called
  * only from user context.
  */
-void kunmap_high(struct page *page)
+void kunmap_high(const struct page *page)
 {
 	unsigned long vaddr;
 	unsigned long nr;
@@ -508,7 +508,7 @@ static inline void kmap_local_idx_pop(void)
 #endif
 
 #ifndef arch_kmap_local_high_get
-static inline void *arch_kmap_local_high_get(struct page *page)
+static inline void *arch_kmap_local_high_get(const struct page *page)
 {
 	return NULL;
 }
@@ -572,7 +572,7 @@ void *__kmap_local_pfn_prot(unsigned long pfn, pgprot_t prot)
 }
 EXPORT_SYMBOL_GPL(__kmap_local_pfn_prot);
 
-void *__kmap_local_page_prot(struct page *page, pgprot_t prot)
+void *__kmap_local_page_prot(const struct page *page, pgprot_t prot)
 {
 	void *kmap;