dma_pool_free() scales poorly when the pool contains many pages because
pool_find_page() does a linear scan of all allocated pages. Improve its
scalability by replacing the linear scan with virt_to_page() and storing
dmapool private data directly in 'struct page', thereby eliminating
'struct dma_page'. In big O notation, this improves the algorithm from
O(n^2) to O(n) while also reducing memory usage.
Thanks to Matthew Wilcox for the suggestion to use struct page.
Signed-off-by: Tony Battersby <to...@cybernetics.com>
---
Changes since v2:
Just a re-diff after the changes in prior patches.
--- linux/include/linux/mm_types.h.orig 2018-08-01 17:59:46.000000000 -0400
+++ linux/include/linux/mm_types.h 2018-08-01 17:59:56.000000000 -0400
@@ -153,6 +153,12 @@ struct page {
unsigned long _zd_pad_1; /* uses mapping */
};
+ struct { /* dma_pool pages */
+ struct list_head dma_list;
+ dma_addr_t dma;
+ unsigned int dma_free_off;
+ };
+
/** @rcu_head: You can use this to free a page by RCU. */
struct rcu_head rcu_head;
};
@@ -174,6 +180,8 @@ struct page {
unsigned int active; /* SLAB */
int units; /* SLOB */
+
+ unsigned int dma_in_use; /* dma_pool pages */
};
/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
--- linux/mm/dmapool.c.orig 2018-08-03 17:47:03.000000000 -0400
+++ linux/mm/dmapool.c 2018-08-03 17:47:22.000000000 -0400
@@ -25,6 +25,10 @@
* list tracks pages that have one or more free blocks, and the 'full' list
* tracks pages that have no free blocks. Pages are moved from one list to
* the other as their blocks are allocated and freed.
+ *
+ * When allocating DMA pages, we use some available space in 'struct page' to
+ * store data private to dmapool; search 'dma_pool' in the definition of
+ * 'struct page' for details.
*/
#include <linux/device.h>
@@ -61,14 +65,6 @@ struct dma_pool { /* the pool */
struct list_head pools;
};
-struct dma_page { /* cacheable header for 'allocation' bytes */
- struct list_head dma_list;
- void *vaddr;
- dma_addr_t dma;
- unsigned int dma_in_use;
- unsigned int dma_free_off;
-};
-
static DEFINE_MUTEX(pools_lock);
static DEFINE_MUTEX(pools_reg_lock);
@@ -95,7 +91,7 @@ show_pools(struct device *dev, struct de
spin_lock_irq(&pool->lock);
for (list_idx = 0; list_idx < POOL_MAX_IDX; list_idx++) {
- struct dma_page *page;
+ struct page *page;
list_for_each_entry(page,
&pool->page_list[list_idx],
@@ -218,7 +214,7 @@ struct dma_pool *dma_pool_create(const c
}
EXPORT_SYMBOL(dma_pool_create);
-static void pool_initialise_page(struct dma_pool *pool, struct dma_page *page)
+static void pool_initialize_free_block_list(struct dma_pool *pool, void *vaddr)
{
unsigned int offset = 0;
unsigned int next_boundary = pool->boundary;
@@ -229,47 +225,57 @@ static void pool_initialise_page(struct
next = next_boundary;
next_boundary += pool->boundary;
}
- *(int *)(page->vaddr + offset) = next;
+ *(int *)(vaddr + offset) = next;
offset = next;
} while (offset < pool->allocation);
}
-static struct dma_page *pool_alloc_page(struct dma_pool *pool, gfp_t mem_flags)
+static struct page *pool_alloc_page(struct dma_pool *pool, gfp_t mem_flags)
{
- struct dma_page *page;
+ struct page *page;
+ dma_addr_t dma;
+ void *vaddr;
- page = kmalloc(sizeof(*page), mem_flags);
- if (!page)
+ vaddr = dma_alloc_coherent(pool->dev, pool->allocation, &dma,
+ mem_flags);
+ if (!vaddr)
return NULL;
- page->vaddr = dma_alloc_coherent(pool->dev, pool->allocation,
- &page->dma, mem_flags);
- if (page->vaddr) {
+
#ifdef DMAPOOL_DEBUG
- memset(page->vaddr, POOL_POISON_FREED, pool->allocation);
+ memset(vaddr, POOL_POISON_FREED, pool->allocation);
#endif
- pool_initialise_page(pool, page);
- page->dma_in_use = 0;
- page->dma_free_off = 0;
- } else {
- kfree(page);
- page = NULL;
- }
+ pool_initialize_free_block_list(pool, vaddr);
+
+ page = virt_to_page(vaddr);
+ page->dma = dma;
+ page->dma_free_off = 0;
+ page->dma_in_use = 0;
+
return page;
}
-static inline bool is_page_busy(struct dma_page *page)
+static inline bool is_page_busy(struct page *page)
{
return page->dma_in_use != 0;
}
-static void pool_free_page(struct dma_pool *pool, struct dma_page *page)
+static void pool_free_page(struct dma_pool *pool, struct page *page)
{
- void *vaddr = page->vaddr;
+ /* Save local copies of some page fields. */
+ void *vaddr = page_to_virt(page);
+ bool busy = is_page_busy(page);
dma_addr_t dma = page->dma;
list_del(&page->dma_list);
- if (is_page_busy(page)) {
+ /* Clear all the page fields we use. */
+ page->dma_list.next = NULL;
+ page->dma_list.prev = NULL;
+ page->dma = 0;
+ page->dma_free_off = 0;
+ page_mapcount_reset(page); /* clear dma_in_use */
+
+ if (busy) {
dev_err(pool->dev,
"dma_pool_destroy %s, %p busy\n",
pool->name, vaddr);
@@ -280,7 +286,6 @@ static void pool_free_page(struct dma_po
#endif
dma_free_coherent(pool->dev, pool->allocation, vaddr, dma);
}
- kfree(page);
}
/**
@@ -310,11 +315,11 @@ void dma_pool_destroy(struct dma_pool *p
mutex_unlock(&pools_reg_lock);
for (list_idx = 0; list_idx < POOL_MAX_IDX; list_idx++) {
- struct dma_page *page;
+ struct page *page;
while ((page = list_first_entry_or_null(
&pool->page_list[list_idx],
- struct dma_page,
+ struct page,
dma_list))) {
pool_free_page(pool, page);
}
@@ -338,15 +343,16 @@ void *dma_pool_alloc(struct dma_pool *po
dma_addr_t *handle)
{
unsigned long flags;
- struct dma_page *page;
+ struct page *page;
size_t offset;
void *retval;
+ void *vaddr;
might_sleep_if(gfpflags_allow_blocking(mem_flags));
spin_lock_irqsave(&pool->lock, flags);
page = list_first_entry_or_null(&pool->page_list[POOL_AVAIL_IDX],
- struct dma_page,
+ struct page,
dma_list);
if (page)
goto ready;
@@ -362,14 +368,15 @@ void *dma_pool_alloc(struct dma_pool *po
list_add(&page->dma_list, &pool->page_list[POOL_AVAIL_IDX]);
ready:
+ vaddr = page_to_virt(page);
page->dma_in_use++;
offset = page->dma_free_off;
- page->dma_free_off = *(int *)(page->vaddr + offset);
+ page->dma_free_off = *(int *)(vaddr + offset);
if (page->dma_free_off >= pool->allocation)
/* Move page from the "available" list to the "full" list. */
list_move_tail(&page->dma_list,
&pool->page_list[POOL_FULL_IDX]);
- retval = offset + page->vaddr;
+ retval = offset + vaddr;
*handle = offset + page->dma;
#ifdef DMAPOOL_DEBUG
{
@@ -404,25 +411,6 @@ void *dma_pool_alloc(struct dma_pool *po
}
EXPORT_SYMBOL(dma_pool_alloc);
-static struct dma_page *pool_find_page(struct dma_pool *pool, dma_addr_t dma)
-{
- int list_idx;
-
- for (list_idx = 0; list_idx < POOL_MAX_IDX; list_idx++) {
- struct dma_page *page;
-
- list_for_each_entry(page,
- &pool->page_list[list_idx],
- dma_list) {
- if (dma < page->dma)
- continue;
- if ((dma - page->dma) < pool->allocation)
- return page;
- }
- }
- return NULL;
-}
-
/**
* dma_pool_free - put block back into dma pool
* @pool: the dma pool holding the block
@@ -434,34 +422,35 @@ static struct dma_page *pool_find_page(s
*/
void dma_pool_free(struct dma_pool *pool, void *vaddr, dma_addr_t dma)
{
- struct dma_page *page;
+ struct page *page;
unsigned long flags;
unsigned int offset;
- spin_lock_irqsave(&pool->lock, flags);
- page = pool_find_page(pool, dma);
- if (!page) {
- spin_unlock_irqrestore(&pool->lock, flags);
+ if (unlikely(!virt_addr_valid(vaddr))) {
dev_err(pool->dev,
- "dma_pool_free %s, %p/%lx (bad dma)\n",
- pool->name, vaddr, (unsigned long)dma);
+ "dma_pool_free %s, %p (bad vaddr)/%pad\n",
+ pool->name, vaddr, &dma);
return;
}
- offset = vaddr - page->vaddr;
-#ifdef DMAPOOL_DEBUG
- if ((dma - page->dma) != offset) {
- spin_unlock_irqrestore(&pool->lock, flags);
+ page = virt_to_page(vaddr);
+ offset = offset_in_page(vaddr);
+
+ if (unlikely((dma - page->dma) != offset)) {
dev_err(pool->dev,
- "dma_pool_free %s, %p (bad vaddr)/%pad\n",
+ "dma_pool_free %s, %p (bad vaddr)/%pad (or bad dma)\n",
pool->name, vaddr, &dma);
return;
}
+
+ spin_lock_irqsave(&pool->lock, flags);
+#ifdef DMAPOOL_DEBUG
{
+ void *page_vaddr = vaddr - offset;
unsigned int chain = page->dma_free_off;
while (chain < pool->allocation) {
if (chain != offset) {
- chain = *(int *)(page->vaddr + chain);
+ chain = *(int *)(page_vaddr + chain);
continue;
}
spin_unlock_irqrestore(&pool->lock, flags);
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