swiotlb: if swiotlb is full, fall back to a transient memory pool

Try to allocate a transient memory pool if no suitable slots can be found
and the respective SWIOTLB is allowed to grow. The transient pool is just
enough big for this one bounce buffer. It is inserted into a per-device
list of transient memory pools, and it is freed again when the bounce
buffer is unmapped.

Transient memory pools are kept in an RCU list. A memory barrier is
required after adding a new entry, because any address within a transient
buffer must be immediately recognized as belonging to the SWIOTLB, even if
it is passed to another CPU.

Deletion does not require any synchronization beyond RCU ordering
guarantees. After a buffer is unmapped, its physical addresses may no
longer be passed to the DMA API, so the memory range of the corresponding
stale entry in the RCU list never matches. If the memory range gets
allocated again, then it happens only after a RCU quiescent state.

Since bounce buffers can now be allocated from different pools, add a
parameter to swiotlb_alloc_pool() to let the caller know which memory pool
is used. Add swiotlb_find_pool() to find the memory pool corresponding to
an address. This function is now also used by is_swiotlb_buffer(), because
a simple boundary check is no longer sufficient.

The logic in swiotlb_alloc_tlb() is taken from __dma_direct_alloc_pages(),
simplified and enhanced to use coherent memory pools if needed.

Note that this is not the most efficient way to provide a bounce buffer,
but when a DMA buffer can't be mapped, something may (and will) actually
break. At that point it is better to make an allocation, even if it may be
an expensive operation.

Signed-off-by: Petr Tesarik <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>

include/linux/device.h

···
  * @dma_mem:	Internal for coherent mem override.
  * @cma_area:	Contiguous memory area for dma allocations
  * @dma_io_tlb_mem: Software IO TLB allocator.  Not for driver use.
+ * @dma_io_tlb_pools:	List of transient swiotlb memory pools.
+ * @dma_io_tlb_lock:	Protects changes to the list of active pools.
  * @archdata:	For arch-specific additions.
  * @of_node:	Associated device tree node.
  * @fwnode:	Associated device node supplied by platform firmware.
···
 #endif
 #ifdef CONFIG_SWIOTLB
 	struct io_tlb_mem *dma_io_tlb_mem;
+#endif
+#ifdef CONFIG_SWIOTLB_DYNAMIC
+	struct list_head dma_io_tlb_pools;
+	spinlock_t dma_io_tlb_lock;
 #endif
 	/* arch specific additions */
 	struct dev_archdata	archdata;

include/linux/dma-mapping.h

···
 #define dma_get_sgtable(d, t, v, h, s) dma_get_sgtable_attrs(d, t, v, h, s, 0)
 #define dma_mmap_coherent(d, v, c, h, s) dma_mmap_attrs(d, v, c, h, s, 0)
 
+bool dma_coherent_ok(struct device *dev, phys_addr_t phys, size_t size);
+
 static inline void *dma_alloc_coherent(struct device *dev, size_t size,
 		dma_addr_t *dma_handle, gfp_t gfp)
 {

include/linux/swiotlb.h

···
  * @area_nslabs: Number of slots in each area.
  * @areas:	Array of memory area descriptors.
  * @slots:	Array of slot descriptors.
+ * @node:	Member of the IO TLB memory pool list.
+ * @rcu:	RCU head for swiotlb_dyn_free().
+ * @transient:  %true if transient memory pool.
  */
 struct io_tlb_pool {
 	phys_addr_t start;
···
 	unsigned int area_nslabs;
 	struct io_tlb_area *areas;
 	struct io_tlb_slot *slots;
+#ifdef CONFIG_SWIOTLB_DYNAMIC
+	struct list_head node;
+	struct rcu_head rcu;
+	bool transient;
+#endif
 };
 
 /**
···
 #endif
 };
 
+#ifdef CONFIG_SWIOTLB_DYNAMIC
+
+struct io_tlb_pool *swiotlb_find_pool(struct device *dev, phys_addr_t paddr);
+
+#else
+
+static inline struct io_tlb_pool *swiotlb_find_pool(struct device *dev,
+						    phys_addr_t paddr)
+{
+	return &dev->dma_io_tlb_mem->defpool;
+}
+
+#endif
+
 /**
  * is_swiotlb_buffer() - check if a physical address belongs to a swiotlb
  * @dev:        Device which has mapped the buffer.
···
 {
 	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
 
-	return mem && paddr >= mem->defpool.start && paddr < mem->defpool.end;
+	if (!mem)
+		return false;
+
+	if (IS_ENABLED(CONFIG_SWIOTLB_DYNAMIC))
+		return swiotlb_find_pool(dev, paddr);
+	return paddr >= mem->defpool.start && paddr < mem->defpool.end;
 }
 
 static inline bool is_swiotlb_force_bounce(struct device *dev)

kernel/dma/direct.c

···
 	return 0;
 }
 
-static bool dma_coherent_ok(struct device *dev, phys_addr_t phys, size_t size)
+bool dma_coherent_ok(struct device *dev, phys_addr_t phys, size_t size)
 {
 	dma_addr_t dma_addr = phys_to_dma_direct(dev, phys);
 

kernel/dma/swiotlb.c

···
 #include <linux/memblock.h>
 #include <linux/mm.h>
 #include <linux/pfn.h>
+#include <linux/rculist.h>
 #include <linux/scatterlist.h>
 #include <linux/set_memory.h>
 #include <linux/spinlock.h>
···
 	memset(mem, 0, sizeof(*mem));
 }
 
+#ifdef CONFIG_SWIOTLB_DYNAMIC
+
+/**
+ * alloc_dma_pages() - allocate pages to be used for DMA
+ * @gfp:	GFP flags for the allocation.
+ * @bytes:	Size of the buffer.
+ *
+ * Allocate pages from the buddy allocator. If successful, make the allocated
+ * pages decrypted that they can be used for DMA.
+ *
+ * Return: Decrypted pages, or %NULL on failure.
+ */
+static struct page *alloc_dma_pages(gfp_t gfp, size_t bytes)
+{
+	unsigned int order = get_order(bytes);
+	struct page *page;
+	void *vaddr;
+
+	page = alloc_pages(gfp, order);
+	if (!page)
+		return NULL;
+
+	vaddr = page_address(page);
+	if (set_memory_decrypted((unsigned long)vaddr, PFN_UP(bytes)))
+		goto error;
+	return page;
+
+error:
+	__free_pages(page, order);
+	return NULL;
+}
+
+/**
+ * swiotlb_alloc_tlb() - allocate a dynamic IO TLB buffer
+ * @dev:	Device for which a memory pool is allocated.
+ * @bytes:	Size of the buffer.
+ * @phys_limit:	Maximum allowed physical address of the buffer.
+ * @gfp:	GFP flags for the allocation.
+ *
+ * Return: Allocated pages, or %NULL on allocation failure.
+ */
+static struct page *swiotlb_alloc_tlb(struct device *dev, size_t bytes,
+		u64 phys_limit, gfp_t gfp)
+{
+	struct page *page;
+
+	/*
+	 * Allocate from the atomic pools if memory is encrypted and
+	 * the allocation is atomic, because decrypting may block.
+	 */
+	if (!gfpflags_allow_blocking(gfp) && dev && force_dma_unencrypted(dev)) {
+		void *vaddr;
+
+		if (!IS_ENABLED(CONFIG_DMA_COHERENT_POOL))
+			return NULL;
+
+		return dma_alloc_from_pool(dev, bytes, &vaddr, gfp,
+					   dma_coherent_ok);
+	}
+
+	gfp &= ~GFP_ZONEMASK;
+	if (phys_limit <= DMA_BIT_MASK(zone_dma_bits))
+		gfp |= __GFP_DMA;
+	else if (phys_limit <= DMA_BIT_MASK(32))
+		gfp |= __GFP_DMA32;
+
+	while ((page = alloc_dma_pages(gfp, bytes)) &&
+	       page_to_phys(page) + bytes - 1 > phys_limit) {
+		/* allocated, but too high */
+		__free_pages(page, get_order(bytes));
+
+		if (IS_ENABLED(CONFIG_ZONE_DMA32) &&
+		    phys_limit < DMA_BIT_MASK(64) &&
+		    !(gfp & (__GFP_DMA32 | __GFP_DMA)))
+			gfp |= __GFP_DMA32;
+		else if (IS_ENABLED(CONFIG_ZONE_DMA) &&
+			 !(gfp & __GFP_DMA))
+			gfp = (gfp & ~__GFP_DMA32) | __GFP_DMA;
+		else
+			return NULL;
+	}
+
+	return page;
+}
+
+/**
+ * swiotlb_free_tlb() - free a dynamically allocated IO TLB buffer
+ * @vaddr:	Virtual address of the buffer.
+ * @bytes:	Size of the buffer.
+ */
+static void swiotlb_free_tlb(void *vaddr, size_t bytes)
+{
+	if (IS_ENABLED(CONFIG_DMA_COHERENT_POOL) &&
+	    dma_free_from_pool(NULL, vaddr, bytes))
+		return;
+
+	/* Intentional leak if pages cannot be encrypted again. */
+	if (!set_memory_encrypted((unsigned long)vaddr, PFN_UP(bytes)))
+		__free_pages(virt_to_page(vaddr), get_order(bytes));
+}
+
+/**
+ * swiotlb_alloc_pool() - allocate a new IO TLB memory pool
+ * @dev:	Device for which a memory pool is allocated.
+ * @nslabs:	Desired number of slabs.
+ * @phys_limit:	Maximum DMA buffer physical address.
+ * @gfp:	GFP flags for the allocations.
+ *
+ * Allocate and initialize a new IO TLB memory pool.
+ *
+ * Return: New memory pool, or %NULL on allocation failure.
+ */
+static struct io_tlb_pool *swiotlb_alloc_pool(struct device *dev,
+		unsigned int nslabs, u64 phys_limit, gfp_t gfp)
+{
+	struct io_tlb_pool *pool;
+	struct page *tlb;
+	size_t pool_size;
+	size_t tlb_size;
+
+	pool_size = sizeof(*pool) + array_size(sizeof(*pool->areas), 1) +
+		array_size(sizeof(*pool->slots), nslabs);
+	pool = kzalloc(pool_size, gfp);
+	if (!pool)
+		goto error;
+	pool->areas = (void *)pool + sizeof(*pool);
+	pool->slots = (void *)pool->areas + sizeof(*pool->areas);
+
+	tlb_size = nslabs << IO_TLB_SHIFT;
+	tlb = swiotlb_alloc_tlb(dev, tlb_size, phys_limit, gfp);
+	if (!tlb)
+		goto error_tlb;
+
+	swiotlb_init_io_tlb_pool(pool, page_to_phys(tlb), nslabs, true, 1);
+	return pool;
+
+error_tlb:
+	kfree(pool);
+error:
+	return NULL;
+}
+
+/**
+ * swiotlb_dyn_free() - RCU callback to free a memory pool
+ * @rcu:	RCU head in the corresponding struct io_tlb_pool.
+ */
+static void swiotlb_dyn_free(struct rcu_head *rcu)
+{
+	struct io_tlb_pool *pool = container_of(rcu, struct io_tlb_pool, rcu);
+	size_t tlb_size = pool->end - pool->start;
+
+	swiotlb_free_tlb(pool->vaddr, tlb_size);
+	kfree(pool);
+}
+
+/**
+ * swiotlb_find_pool() - find the IO TLB pool for a physical address
+ * @dev:        Device which has mapped the DMA buffer.
+ * @paddr:      Physical address within the DMA buffer.
+ *
+ * Find the IO TLB memory pool descriptor which contains the given physical
+ * address, if any.
+ *
+ * Return: Memory pool which contains @paddr, or %NULL if none.
+ */
+struct io_tlb_pool *swiotlb_find_pool(struct device *dev, phys_addr_t paddr)
+{
+	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
+	struct io_tlb_pool *pool = &mem->defpool;
+
+	if (paddr >= pool->start && paddr < pool->end)
+		return pool;
+
+	/* Pairs with smp_wmb() in swiotlb_find_slots(). */
+	smp_rmb();
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(pool, &dev->dma_io_tlb_pools, node) {
+		if (paddr >= pool->start && paddr < pool->end)
+			goto out;
+	}
+	pool = NULL;
+out:
+	rcu_read_unlock();
+	return pool;
+}
+
+/**
+ * swiotlb_del_pool() - remove an IO TLB pool from a device
+ * @dev:	Owning device.
+ * @pool:	Memory pool to be removed.
+ */
+static void swiotlb_del_pool(struct device *dev, struct io_tlb_pool *pool)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&dev->dma_io_tlb_lock, flags);
+	list_del_rcu(&pool->node);
+	spin_unlock_irqrestore(&dev->dma_io_tlb_lock, flags);
+
+	call_rcu(&pool->rcu, swiotlb_dyn_free);
+}
+
+#endif	/* CONFIG_SWIOTLB_DYNAMIC */
+
 /**
  * swiotlb_dev_init() - initialize swiotlb fields in &struct device
  * @dev:	Device to be initialized.
···
 void swiotlb_dev_init(struct device *dev)
 {
 	dev->dma_io_tlb_mem = &io_tlb_default_mem;
+#ifdef CONFIG_SWIOTLB_DYNAMIC
+	INIT_LIST_HEAD(&dev->dma_io_tlb_pools);
+	spin_lock_init(&dev->dma_io_tlb_lock);
+#endif
 }
 
 /*
···
 static void swiotlb_bounce(struct device *dev, phys_addr_t tlb_addr, size_t size,
 			   enum dma_data_direction dir)
 {
-	struct io_tlb_pool *mem = &dev->dma_io_tlb_mem->defpool;
+	struct io_tlb_pool *mem = swiotlb_find_pool(dev, tlb_addr);
 	int index = (tlb_addr - mem->start) >> IO_TLB_SHIFT;
 	phys_addr_t orig_addr = mem->slots[index].orig_addr;
 	size_t alloc_size = mem->slots[index].alloc_size;
···
 	return -1;
 }
 
+#ifdef CONFIG_SWIOTLB_DYNAMIC
+
 /**
  * swiotlb_find_slots() - search for slots in the whole swiotlb
  * @dev:	Device which maps the buffer.
···
  * @alloc_size: Total requested size of the bounce buffer,
  *		including initial alignment padding.
  * @alloc_align_mask:	Required alignment of the allocated buffer.
+ * @retpool:	Used memory pool, updated on return.
  *
  * Search through the whole software IO TLB to find a sequence of slots that
  * match the allocation constraints.
···
  * Return: Index of the first allocated slot, or -1 on error.
  */
 static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
-		size_t alloc_size, unsigned int alloc_align_mask)
+		size_t alloc_size, unsigned int alloc_align_mask,
+		struct io_tlb_pool **retpool)
 {
-	return swiotlb_pool_find_slots(dev, &dev->dma_io_tlb_mem->defpool,
+	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
+	struct io_tlb_pool *pool;
+	unsigned long nslabs;
+	unsigned long flags;
+	u64 phys_limit;
+	int index;
+
+	pool = &mem->defpool;
+	index = swiotlb_pool_find_slots(dev, pool, orig_addr,
+					alloc_size, alloc_align_mask);
+	if (index >= 0)
+		goto found;
+
+	if (!mem->can_grow)
+		return -1;
+
+	nslabs = nr_slots(alloc_size);
+	phys_limit = min_not_zero(*dev->dma_mask, dev->bus_dma_limit);
+	pool = swiotlb_alloc_pool(dev, nslabs, phys_limit,
+				  GFP_NOWAIT | __GFP_NOWARN);
+	if (!pool)
+		return -1;
+
+	index = swiotlb_pool_find_slots(dev, pool, orig_addr,
+					alloc_size, alloc_align_mask);
+	if (index < 0) {
+		swiotlb_dyn_free(&pool->rcu);
+		return -1;
+	}
+
+	pool->transient = true;
+	spin_lock_irqsave(&dev->dma_io_tlb_lock, flags);
+	list_add_rcu(&pool->node, &dev->dma_io_tlb_pools);
+	spin_unlock_irqrestore(&dev->dma_io_tlb_lock, flags);
+
+	/* Pairs with smp_rmb() in swiotlb_find_pool(). */
+	smp_wmb();
+found:
+	*retpool = pool;
+	return index;
+}
+
+#else  /* !CONFIG_SWIOTLB_DYNAMIC */
+
+static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
+		size_t alloc_size, unsigned int alloc_align_mask,
+		struct io_tlb_pool **retpool)
+{
+	*retpool = &dev->dma_io_tlb_mem->defpool;
+	return swiotlb_pool_find_slots(dev, *retpool,
 				       orig_addr, alloc_size, alloc_align_mask);
 }
+
+#endif /* CONFIG_SWIOTLB_DYNAMIC */
 
 #ifdef CONFIG_DEBUG_FS
 
···
 	}
 
 	index = swiotlb_find_slots(dev, orig_addr,
-				   alloc_size + offset, alloc_align_mask);
+				   alloc_size + offset, alloc_align_mask, &pool);
 	if (index == -1) {
 		if (!(attrs & DMA_ATTR_NO_WARN))
 			dev_warn_ratelimited(dev,
···
 	 * This is needed when we sync the memory.  Then we sync the buffer if
 	 * needed.
 	 */
-	pool = &mem->defpool;
 	for (i = 0; i < nr_slots(alloc_size + offset); i++)
 		pool->slots[index + i].orig_addr = slot_addr(orig_addr, i);
 	tlb_addr = slot_addr(pool->start, index) + offset;
···
 
 static void swiotlb_release_slots(struct device *dev, phys_addr_t tlb_addr)
 {
-	struct io_tlb_pool *mem = &dev->dma_io_tlb_mem->defpool;
+	struct io_tlb_pool *mem = swiotlb_find_pool(dev, tlb_addr);
 	unsigned long flags;
 	unsigned int offset = swiotlb_align_offset(dev, tlb_addr);
 	int index = (tlb_addr - offset - mem->start) >> IO_TLB_SHIFT;
···
 	dec_used(dev->dma_io_tlb_mem, nslots);
 }
 
+#ifdef CONFIG_SWIOTLB_DYNAMIC
+
+/**
+ * swiotlb_del_transient() - delete a transient memory pool
+ * @dev:	Device which mapped the buffer.
+ * @tlb_addr:	Physical address within a bounce buffer.
+ *
+ * Check whether the address belongs to a transient SWIOTLB memory pool.
+ * If yes, then delete the pool.
+ *
+ * Return: %true if @tlb_addr belonged to a transient pool that was released.
+ */
+static bool swiotlb_del_transient(struct device *dev, phys_addr_t tlb_addr)
+{
+	struct io_tlb_pool *pool;
+
+	pool = swiotlb_find_pool(dev, tlb_addr);
+	if (!pool->transient)
+		return false;
+
+	dec_used(dev->dma_io_tlb_mem, pool->nslabs);
+	swiotlb_del_pool(dev, pool);
+	return true;
+}
+
+#else  /* !CONFIG_SWIOTLB_DYNAMIC */
+
+static inline bool swiotlb_del_transient(struct device *dev,
+					 phys_addr_t tlb_addr)
+{
+	return false;
+}
+
+#endif	/* CONFIG_SWIOTLB_DYNAMIC */
+
 /*
  * tlb_addr is the physical address of the bounce buffer to unmap.
  */
···
 	    (dir == DMA_FROM_DEVICE || dir == DMA_BIDIRECTIONAL))
 		swiotlb_bounce(dev, tlb_addr, mapping_size, DMA_FROM_DEVICE);
 
+	if (swiotlb_del_transient(dev, tlb_addr))
+		return;
 	swiotlb_release_slots(dev, tlb_addr);
 }
 
···
 	if (!mem)
 		return NULL;
 
-	index = swiotlb_find_slots(dev, 0, size, 0);
+	index = swiotlb_find_slots(dev, 0, size, 0, &pool);
 	if (index == -1)
 		return NULL;
 
-	pool = &mem->defpool;
 	tlb_addr = slot_addr(pool->start, index);
 
 	return pfn_to_page(PFN_DOWN(tlb_addr));