[tip: core/mm] mm/highmem: Provide CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the core/mm branch of tip:

Commit-ID:     0e91a0c6984c837a7c6760e3f28e8e1c532abf87
Gitweb:        https://git.kernel.org/tip/0e91a0c6984c837a7c6760e3f28e8e1c532abf87
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Wed, 18 Nov 2020 20:48:40 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 24 Nov 2020 14:42:08 +01:00

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@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20201118204007.028261233@xxxxxxxxxxxxx

---
 lib/Kconfig.debug | 14 ++++++++++++++
 mm/highmem.c      | 12 +++++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index f24fa15..e952f89 100644
--- a/lib/Kconfig.debug
+++ b/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
diff --git a/mm/highmem.c b/mm/highmem.c
index fab128d..39aaca1 100644
--- a/mm/highmem.c
+++ b/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