dma-debug: introduce debug_dma_assert_idle()

Record actively mapped pages and provide an api for asserting a given
page is dma inactive before execution proceeds.  Placing
debug_dma_assert_idle() in cow_user_page() flagged the violation of the
dma-api in the NET_DMA implementation (see commit 77873803 "net_dma:
mark broken").

The implementation includes the capability to count, in a limited way,
repeat mappings of the same page that occur without an intervening
unmap.  This 'overlap' counter is limited to the few bits of tag space
in a radix tree.  This mechanism is added to mitigate false negative
cases where, for example, a page is dma mapped twice and
debug_dma_assert_idle() is called after the page is un-mapped once.
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: James Bottomley <JBottomley@Parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/dma-debug.h

@@ -85,6 +85,8 @@ extern void debug_dma_sync_sg_for_device(struct device *dev,
 
 extern void debug_dma_dump_mappings(struct device *dev);
 
+extern void debug_dma_assert_idle(struct page *page);
+
 #else /* CONFIG_DMA_API_DEBUG */
 
 static inline void dma_debug_add_bus(struct bus_type *bus)
@@ -183,6 +185,10 @@ static inline void debug_dma_dump_mappings(struct device *dev)
 {
 }
 
+static inline void debug_dma_assert_idle(struct page *page)
+{
+}
+
 #endif /* CONFIG_DMA_API_DEBUG */
 
 #endif /* __DMA_DEBUG_H */

lib/Kconfig.debug

@@ -1584,8 +1584,16 @@ config DMA_API_DEBUG
 	  With this option you will be able to detect common bugs in device
 	  drivers like double-freeing of DMA mappings or freeing mappings that
 	  were never allocated.
-	  This option causes a performance degredation.  Use only if you want
-	  to debug device drivers. If unsure, say N.
+
+	  This also attempts to catch cases where a page owned by DMA is
+	  accessed by the cpu in a way that could cause data corruption.  For
+	  example, this enables cow_user_page() to check that the source page is
+	  not undergoing DMA.
+
+	  This option causes a performance degradation.  Use only if you want to
+	  debug device drivers and dma interactions.
+
+	  If unsure, say N.
 
 source "samples/Kconfig"
 

mm/memory.c

@@ -59,6 +59,7 @@
 #include <linux/gfp.h>
 #include <linux/migrate.h>
 #include <linux/string.h>
+#include <linux/dma-debug.h>
 
 #include <asm/io.h>
 #include <asm/pgalloc.h>
@@ -2559,6 +2560,8 @@ static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
 
 static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
 {
+	debug_dma_assert_idle(src);
+
 	/*
 	 * If the source page was a PFN mapping, we don't have
 	 * a "struct page" for it. We do a best-effort copy by