linux-kernel - [PATCH] highmem: Make __kunmap_{local,atomic}() take "const void *"

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Date:   Tue, 14 Jun 2022 16:25:31 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>, linux-kernel@...r.kernel.org
Cc:     David Sterba <dsterba@...e.cz>
Subject: [PATCH] highmem: Make __kunmap_{local,atomic}() take "const void *"

Sometimes __kunmap_{local,atomic}() should take pointers to const void.
Currently their prototypes take pointers to void, therefore compilations
break in cases like the above-mentioned.

This is the output of GCC-12 when it encounters one those cases:

./linux/fs/btrfs/zstd.c:547:33: warning: passing
argument 1 of '__kunmap_local' discards 'const' qualifier from pointer
target type [-Wdiscarded-qualifiers]
  547 |   kunmap_local(workspace->in_buf.src);
      |                ~~~~~~~~~~~~~~~~~^~~~
./linux/include/linux/highmem-internal.h:284:17:
note: in definition of macro 'kunmap_local'
  284 |  __kunmap_local(__addr);     \
      |                 ^~~~~~
./linux/include/linux/highmem-internal.h:92:41:
note: expected 'void *' but argument is of type 'const void *'
   92 | static inline void __kunmap_local(void *vaddr)
      |                                   ~~~~~~^~~~~

Make __kunmap_{local,atomic}() take pointers to const void.

Suggested-by: David Sterba <dsterba@...e.cz>
Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---
 include/linux/highmem-internal.h | 10 +++++-----
 mm/highmem.c                     |  2 +-
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
index cddb42ff0473..034b1106d022 100644
--- a/include/linux/highmem-internal.h
+++ b/include/linux/highmem-internal.h
@@ -8,7 +8,7 @@
 #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 kunmap_local_indexed(void *vaddr);
+void kunmap_local_indexed(const void *vaddr);
 void kmap_local_fork(struct task_struct *tsk);
 void __kmap_local_sched_out(void);
 void __kmap_local_sched_in(void);
@@ -89,7 +89,7 @@ static inline void *kmap_local_pfn(unsigned long pfn)
 	return __kmap_local_pfn_prot(pfn, kmap_prot);
 }
 
-static inline void __kunmap_local(void *vaddr)
+static inline void __kunmap_local(const void *vaddr)
 {
 	kunmap_local_indexed(vaddr);
 }
@@ -121,7 +121,7 @@ static inline void *kmap_atomic_pfn(unsigned long pfn)
 	return __kmap_local_pfn_prot(pfn, kmap_prot);
 }
 
-static inline void __kunmap_atomic(void *addr)
+static inline void __kunmap_atomic(const void *addr)
 {
 	kunmap_local_indexed(addr);
 	pagefault_enable();
@@ -197,7 +197,7 @@ static inline void *kmap_local_pfn(unsigned long pfn)
 	return kmap_local_page(pfn_to_page(pfn));
 }
 
-static inline void __kunmap_local(void *addr)
+static inline void __kunmap_local(const void *addr)
 {
 #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
 	kunmap_flush_on_unmap(addr);
@@ -224,7 +224,7 @@ static inline void *kmap_atomic_pfn(unsigned long pfn)
 	return kmap_atomic(pfn_to_page(pfn));
 }
 
-static inline void __kunmap_atomic(void *addr)
+static inline void __kunmap_atomic(const void *addr)
 {
 #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
 	kunmap_flush_on_unmap(addr);
diff --git a/mm/highmem.c b/mm/highmem.c
index 1a692997fac4..e32083e4ce0d 100644
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -561,7 +561,7 @@ void *__kmap_local_page_prot(struct page *page, pgprot_t prot)
 }
 EXPORT_SYMBOL(__kmap_local_page_prot);
 
-void kunmap_local_indexed(void *vaddr)
+void kunmap_local_indexed(const void *vaddr)
 {
 	unsigned long addr = (unsigned long) vaddr & PAGE_MASK;
 	pte_t *kmap_pte;
-- 
2.36.1