[PATCH v3 4/7] dma-debug: Make leak-like behaviour apparent

From: Robin Murphy
Date: Mon Dec 10 2018 - 09:00:52 EST

Now that we can dynamically allocate DMA debug entries to cope with
drivers maintaining excessively large numbers of live mappings, a driver
which *does* actually have a bug leaking mappings (and is not unloaded)
will no longer trigger the "DMA-API: debugging out of memory - disabling"
message until it gets to actual kernel OOM conditions, which means it
could go unnoticed for a while. To that end, let's inform the user each
time the pool has grown to a multiple of its initial size, which should
make it apparent that they either have a leak or might want to increase
the preallocation size.

Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>

v3: Add Christoph's review tag

Documentation/DMA-API.txt | 6 +++++-
kernel/dma/debug.c | 13 +++++++++++++
2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
index 0fcb7561af1e..7a7d8a415ce8 100644
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -747,7 +747,11 @@ driver afterwards. This filter can be disabled or changed later using debugfs.
When the code disables itself at runtime this is most likely because it ran
out of dma_debug_entries and was unable to allocate more on-demand. 65536
entries are preallocated at boot - if this is too low for you boot with
-'dma_debug_entries=<your_desired_number>' to overwrite the default.
+'dma_debug_entries=<your_desired_number>' to overwrite the default. The
+code will print to the kernel log each time it has dynamically allocated
+as many entries as were initially preallocated. This is to indicate that a
+larger preallocation size may be appropriate, or if it happens continually
+that a driver may be leaking mappings.


diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
index ef7c90b7a346..912c23f4c177 100644
--- a/kernel/dma/debug.c
+++ b/kernel/dma/debug.c
@@ -691,6 +691,18 @@ static struct dma_debug_entry *__dma_entry_alloc(void)
return entry;

+void __dma_entry_alloc_check_leak(void)
+ u32 tmp = nr_total_entries % nr_prealloc_entries;
+ /* Shout each time we tick over some multiple of the initial pool */
+ pr_info("dma_debug_entry pool grown to %u (%u00%%)\n",
+ nr_total_entries,
+ (nr_total_entries / nr_prealloc_entries));
+ }
/* struct dma_entry allocator
* The next two functions implement the allocator for
@@ -710,6 +722,7 @@ static struct dma_debug_entry *dma_entry_alloc(void)
pr_err("debugging out of memory - disabling\n");
return NULL;
+ __dma_entry_alloc_check_leak();

entry = __dma_entry_alloc();