linux-kernel - [PATCH 3/4] dma-debug: Dynamically expand the dma_debug

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Date:   Mon,  3 Dec 2018 17:28:08 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     hch@....de
Cc:     m.szyprowski@...sung.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, cai@....us, salil.mehta@...wei.com,
        john.garry@...wei.com
Subject: [PATCH 3/4] dma-debug: Dynamically expand the dma_debug_entry pool

Certain drivers such as large multi-queue network adapters can use pools
of mapped DMA buffers larger than the default dma_debug_entry pool of
65536 entries, with the result that merely probing such a device can
cause DMA debug to disable itself during boot unless explicitly given an
appropriate "dma_debug_entries=..." option.

Developers trying to debug some other driver on such a system may not be
immediately aware of this, and at worst it can hide bugs if they fail to
realise that dma-debug has already disabled itself unexpectedly by the
time the code of interest gets to run. Even once they do realise, it can
be a bit of a pain to emprirically determine a suitable number of
preallocated entries to configure without massively over-allocating.

There's really no need for such a static limit, though, since we can
quite easily expand the pool at runtime in those rare cases that the
preallocated entries are insufficient, which is arguably the least
surprising and most useful behaviour.

Signed-off-by: Robin Murphy <robin.murphy@....com>
---
 kernel/dma/debug.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
index de5db800dbfc..46cc075aec99 100644
--- a/kernel/dma/debug.c
+++ b/kernel/dma/debug.c
@@ -47,6 +47,9 @@
 #ifndef PREALLOC_DMA_DEBUG_ENTRIES
 #define PREALLOC_DMA_DEBUG_ENTRIES (1 << 16)
 #endif
+/* If the pool runs out, try this many times to allocate this many new entries */
+#define DMA_DEBUG_DYNAMIC_ENTRIES 256
+#define DMA_DEBUG_DYNAMIC_RETRIES 2

 enum {
 	dma_debug_single,
@@ -702,12 +705,21 @@ static struct dma_debug_entry *dma_entry_alloc(void)
 {
 	struct dma_debug_entry *entry;
 	unsigned long flags;
+	int retry_count;

-	spin_lock_irqsave(&free_entries_lock, flags);
+	for (retry_count = 0; ; retry_count++) {
+		spin_lock_irqsave(&free_entries_lock, flags);
+
+		if (num_free_entries > 0)
+			break;

-	if (list_empty(&free_entries)) {
-		global_disable = true;
 		spin_unlock_irqrestore(&free_entries_lock, flags);
+
+		if (retry_count < DMA_DEBUG_DYNAMIC_RETRIES &&
+		    !prealloc_memory(DMA_DEBUG_DYNAMIC_ENTRIES))
+			continue;
+
+		global_disable = true;
 		pr_err("debugging out of memory - disabling\n");
 		return NULL;
 	}
-- 
2.19.1.dirty