kmap_atomic: make kunmap_atomic() harder to misuse

kunmap_atomic() is currently at level -4 on Rusty's "Hard To Misuse"
list[1] ("Follow common convention and you'll get it wrong"), except in
some architectures when CONFIG_DEBUG_HIGHMEM is set[2][3].

kunmap() takes a pointer to a struct page; kunmap_atomic(), however, takes
takes a pointer to within the page itself.  This seems to once in a while
trip people up (the convention they are following is the one from
kunmap()).

Make it much harder to misuse, by moving it to level 9 on Rusty's list[4]
("The compiler/linker won't let you get it wrong").  This is done by
refusing to build if the type of its first argument is a pointer to a
struct page.

The real kunmap_atomic() is renamed to kunmap_atomic_notypecheck()
(which is what you would call in case for some strange reason calling it
with a pointer to a struct page is not incorrect in your code).

The previous version of this patch was compile tested on x86-64.

[1] http://ozlabs.org/~rusty/index.cgi/tech/2008-04-01.html
[2] In these cases, it is at level 5, "Do it right or it will always
    break at runtime."
[3] At least mips and powerpc look very similar, and sparc also seems to
    share a common ancestor with both; there seems to be quite some
    degree of copy-and-paste coding here. The include/asm/highmem.h file
    for these three archs mention x86 CPUs at its top.
[4] http://ozlabs.org/~rusty/index.cgi/tech/2008-03-30.html
[5] As an aside, could someone tell me why mn10300 uses unsigned long as
    the first parameter of kunmap_atomic() instead of void *?

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Russell King <linux@arm.linux.org.uk> (arch/arm)
Cc: Ralf Baechle <ralf@linux-mips.org> (arch/mips)
Cc: David Howells <dhowells@redhat.com> (arch/frv, arch/mn10300)
Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com> (arch/mn10300)
Cc: Kyle McMartin <kyle@mcmartin.ca> (arch/parisc)
Cc: Helge Deller <deller@gmx.de> (arch/parisc)
Cc: "James E.J. Bottomley" <jejb@parisc-linux.org> (arch/parisc)
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> (arch/powerpc)
Cc: Paul Mackerras <paulus@samba.org> (arch/powerpc)
Cc: "David S. Miller" <davem@davemloft.net> (arch/sparc)
Cc: Thomas Gleixner <tglx@linutronix.de> (arch/x86)
Cc: Ingo Molnar <mingo@redhat.com> (arch/x86)
Cc: "H. Peter Anvin" <hpa@zytor.com> (arch/x86)
Cc: Arnd Bergmann <arnd@arndb.de> (include/asm-generic)
Cc: Rusty Russell <rusty@rustcorp.com.au> ("Hard To Misuse" list)
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 4 insertions, 4 deletions

diff --git a/arch/mips/include/asm/highmem.h b/arch/mips/include/asm/highmem.h
index 25adfb02923d..75753ca73bfd 100644
--- a/arch/mips/include/asm/highmem.h
+++ b/arch/mips/include/asm/highmem.h
@@ -48,14 +48,14 @@ extern void kunmap_high(struct page *page);
 extern void *__kmap(struct page *page);
 extern void __kunmap(struct page *page);
 extern void *__kmap_atomic(struct page *page, enum km_type type);
-extern void __kunmap_atomic(void *kvaddr, enum km_type type);
+extern void __kunmap_atomic_notypecheck(void *kvaddr, enum km_type type);
 extern void *kmap_atomic_pfn(unsigned long pfn, enum km_type type);
 extern struct page *__kmap_atomic_to_page(void *ptr);
 
 #define kmap                    __kmap
 #define kunmap                  __kunmap
 #define kmap_atomic             __kmap_atomic
-#define kunmap_atomic           __kunmap_atomic
+#define kunmap_atomic_notypecheck               __kunmap_atomic_notypecheck
 #define kmap_atomic_to_page     __kmap_atomic_to_page
 
 #define flush_cache_kmaps()     flush_cache_all()
diff --git a/arch/mips/mm/highmem.c b/arch/mips/mm/highmem.c
index 127d732474bf..6a2b1bf9ef11 100644
--- a/arch/mips/mm/highmem.c
+++ b/arch/mips/mm/highmem.c
@@ -64,7 +64,7 @@ void *__kmap_atomic(struct page *page, enum km_type type)
 }
 EXPORT_SYMBOL(__kmap_atomic);
 
-void __kunmap_atomic(void *kvaddr, enum km_type type)
+void __kunmap_atomic_notypecheck(void *kvaddr, enum km_type type)
 {
 #ifdef CONFIG_DEBUG_HIGHMEM
         unsigned long vaddr = (unsigned long) kvaddr & PAGE_MASK;
@@ -87,7 +87,7 @@ void __kunmap_atomic(void *kvaddr, enum km_type type)
 
         pagefault_enable();
 }
-EXPORT_SYMBOL(__kunmap_atomic);
+EXPORT_SYMBOL(__kunmap_atomic_notypecheck);
 
 /*
  * This is the same as kmap_atomic() but can map memory that doesn't