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 77873803363c "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>

1 files changed, 10 insertions, 2 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 6982094a7e74..900b63c1e899 100644
--- a/lib/Kconfig.debug
+++ b/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"