Commit 2b9d9ac0 authored by Robin Murphy's avatar Robin Murphy Committed by Christoph Hellwig
Browse files

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 their 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, short of 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. To that end, refactor the
prealloc_memory() logic a little bit to generalise it for runtime
reallocations as well.

Signed-off-by: default avatarRobin Murphy <robin.murphy@arm.com>
Tested-by: default avatarQian Cai <cai@lca.pw>
Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
parent f737b095

Documentation/DMA-API.txt

+5 −6
Original line number Diff line number Diff line
@@ -717,8 +717,8 @@ dma-api/num_errors The number in this file shows how many 
dma-api/min_free_entries	This read-only file can be read to get the

				minimum number of free dma_debug_entries the

				allocator has ever seen. If this value goes

				down to zero the code will disable itself

				because it is not longer reliable.

				down to zero the code will attempt to increase

				nr_total_entries to compensate.



dma-api/num_free_entries	The current number of free dma_debug_entries

				in the allocator.
@@ -745,10 +745,9 @@ driver filter at boot time. The debug code will only print errors for that 
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. These entries are preallocated at boot. The number

of preallocated entries is defined per architecture. If it is too low for you

boot with 'dma_debug_entries=<your_desired_number>' to overwrite the

architectural default.

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.



::

kernel/dma/debug.c

+41 −38
Original line number Diff line number Diff line
@@ -47,6 +47,8 @@ 
#ifndef PREALLOC_DMA_DEBUG_ENTRIES

#define PREALLOC_DMA_DEBUG_ENTRIES (1 << 16)

#endif

/* If the pool runs out, add this many new entries at once */

#define DMA_DEBUG_DYNAMIC_ENTRIES 256



enum {

	dma_debug_single,
@@ -646,6 +648,34 @@ static void add_dma_entry(struct dma_debug_entry *entry) 
	 */

}



static int dma_debug_create_entries(u32 num_entries, gfp_t gfp)

{

	struct dma_debug_entry *entry, *next_entry;

	int i;



	for (i = 0; i < num_entries; ++i) {

		entry = kzalloc(sizeof(*entry), gfp);

		if (!entry)

			goto out_err;



		list_add_tail(&entry->list, &free_entries);

	}



	num_free_entries += num_entries;

	nr_total_entries += num_entries;



	return 0;



out_err:



	list_for_each_entry_safe(entry, next_entry, &free_entries, list) {

		list_del(&entry->list);

		kfree(entry);

	}



	return -ENOMEM;

}



static struct dma_debug_entry *__dma_entry_alloc(void)

{

	struct dma_debug_entry *entry;
@@ -672,13 +702,15 @@ static struct dma_debug_entry *dma_entry_alloc(void) 
	unsigned long flags;



	spin_lock_irqsave(&free_entries_lock, flags);



	if (list_empty(&free_entries)) {

	if (num_free_entries == 0) {

		if (dma_debug_create_entries(DMA_DEBUG_DYNAMIC_ENTRIES,

					     GFP_ATOMIC)) {

			global_disable = true;

			spin_unlock_irqrestore(&free_entries_lock, flags);

			pr_err("debugging out of memory - disabling\n");

			return NULL;

		}

	}



	entry = __dma_entry_alloc();
@@ -764,36 +796,6 @@ int dma_debug_resize_entries(u32 num_entries) 
 *   2. Preallocate a given number of dma_debug_entry structs

 */



static int prealloc_memory(u32 num_entries)

{

	struct dma_debug_entry *entry, *next_entry;

	int i;



	for (i = 0; i < num_entries; ++i) {

		entry = kzalloc(sizeof(*entry), GFP_KERNEL);

		if (!entry)

			goto out_err;



		list_add_tail(&entry->list, &free_entries);

	}



	num_free_entries = num_entries;

	min_free_entries = num_entries;



	pr_info("preallocated %d debug entries\n", num_entries);



	return 0;



out_err:



	list_for_each_entry_safe(entry, next_entry, &free_entries, list) {

		list_del(&entry->list);

		kfree(entry);

	}



	return -ENOMEM;

}



static ssize_t filter_read(struct file *file, char __user *user_buf,

			   size_t count, loff_t *ppos)

{
@@ -1038,14 +1040,15 @@ static int dma_debug_init(void) 
		return 0;

	}



	if (prealloc_memory(nr_prealloc_entries) != 0) {

	if (dma_debug_create_entries(nr_prealloc_entries, GFP_KERNEL) != 0) {

		pr_err("debugging out of memory error - disabled\n");

		global_disable = true;



		return 0;

	}



	nr_total_entries = num_free_entries;

	min_free_entries = num_free_entries;

	pr_info("preallocated %d debug entries\n", nr_total_entries);



	dma_debug_initialized = true;

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