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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: Robin Murphy <robin.murphy@arm.com>
Tested-by: Qian Cai <cai@lca.pw>
Signed-off-by: Christoph Hellwig <hch@lst.de>
This commit is contained in:
Robin Murphy 2018-12-10 14:00:29 +00:00 committed by Christoph Hellwig
parent f737b095c6
commit 2b9d9ac02b
2 changed files with 46 additions and 44 deletions
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11
Documentation/DMA-API.txt
View File

@ -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.
::

79
kernel/dma/debug.c
View File

@ -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;
@ -672,12 +702,14 @@ static struct dma_debug_entry *dma_entry_alloc(void)
unsigned long flags;
spin_lock_irqsave(&free_entries_lock, flags);
if (list_empty(&free_entries)) {
global_disable = true;
spin_unlock_irqrestore(&free_entries_lock, flags);
pr_err("debugging out of memory - disabling\n");
return NULL;
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;