mm/highmem: Provide CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP

CONFIG_DEBUG_KMAP_LOCAL, which is selected by CONFIG_DEBUG_HIGHMEM is only
providing guard pages, but does not provide a mechanism to enforce the
usage of the kmap_local() infrastructure.

Provide CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP which forces the temporary
mapping even for lowmem pages. This needs to be a seperate config switch
because this only works on architectures which do not have cache aliasing
problems.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20201118204007.028261233@linutronix.de

lib/Kconfig.debug

@@ -856,9 +856,23 @@ config DEBUG_KMAP_LOCAL
 	  This option enables additional error checking for the kmap_local
 	  infrastructure.  Disable for production use.
 
+config ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP
+	bool
+
+config DEBUG_KMAP_LOCAL_FORCE_MAP
+	bool "Enforce kmap_local temporary mappings"
+	depends on DEBUG_KERNEL && ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP
+	select KMAP_LOCAL
+	select DEBUG_KMAP_LOCAL
+	help
+	  This option enforces temporary mappings through the kmap_local
+	  mechanism for non-highmem pages and on non-highmem systems.
+	  Disable this for production systems!
+
 config DEBUG_HIGHMEM
 	bool "Highmem debugging"
 	depends on DEBUG_KERNEL && HIGHMEM
+	select DEBUG_KMAP_LOCAL_FORCE_MAP if ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP
 	select DEBUG_KMAP_LOCAL
 	help
 	  This option enables additional error checking for high memory

mm/highmem.c

@@ -474,7 +474,12 @@ void *__kmap_local_page_prot(struct page *page, pgprot_t prot)
 {
 	void *kmap;
 
-	if (!PageHighMem(page))
+	/*
+	 * To broaden the usage of the actual kmap_local() machinery always map
+	 * pages when debugging is enabled and the architecture has no problems
+	 * with alias mappings.
+	 */
+	if (!IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP) && !PageHighMem(page))
 		return page_address(page);
 
 	/* Try kmap_high_get() if architecture has it enabled */
@@ -494,6 +499,11 @@ void kunmap_local_indexed(void *vaddr)
 
 	if (addr < __fix_to_virt(FIX_KMAP_END) ||
 	    addr > __fix_to_virt(FIX_KMAP_BEGIN)) {
+		if (IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP)) {
+			/* This _should_ never happen! See above. */
+			WARN_ON_ONCE(1);
+			return;
+		}
 		/*
 		 * Handle mappings which were obtained by kmap_high_get()
 		 * first as the virtual address of such mappings is below