linux-kernel - [patch V4 2/8] mm/highmem: Provide CONFIG_DEBUG_KMAP_LOCAL_FORCE

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Date:   Wed, 18 Nov 2020 20:48:40 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Linus Torvalds <torvalds@...uxfoundation.org>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: [patch V4 2/8] 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@...ux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
V4: New patch
---
 lib/Kconfig.debug |   14 ++++++++++++++
 mm/highmem.c      |   12 +++++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

--- 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
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -474,7 +474,12 @@ void *__kmap_local_page_prot(struct page
 {
 	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