dm: add bufio

The dm-bufio interface allows you to do cached I/O on devices,
holding recently-read blocks in memory and performing delayed writes.

We don't use buffer cache or page cache already present in the kernel, because:
* we need to handle block sizes larger than a page
* we can't allocate memory to perform reads or we'd have deadlocks

Currently, when a cache is required, we limit its size to a fraction of
available memory.  Usage can be viewed and changed in
/sys/module/dm_bufio/parameters/ .

The first user is thin provisioning, but more dm users are planned.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 1699 insertions, 0 deletions

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
new file mode 100644
index 000000000000..cb246667dd52
--- /dev/null
+++ b/drivers/md/dm-bufio.c
@@ -0,0 +1,1699 @@
+/*
+ * Copyright (C) 2009-2011 Red Hat, Inc.
+ *
+ * Author: Mikulas Patocka <mpatocka@redhat.com>
+ *
+ * This file is released under the GPL.
+ */
+
+#include "dm-bufio.h"
+
+#include <linux/device-mapper.h>
+#include <linux/dm-io.h>
+#include <linux/slab.h>
+#include <linux/vmalloc.h>
+#include <linux/version.h>
+#include <linux/shrinker.h>
+
+#define DM_MSG_PREFIX "bufio"
+
+/*
+ * Memory management policy:
+ *	Limit the number of buffers to DM_BUFIO_MEMORY_PERCENT of main memory
+ *	or DM_BUFIO_VMALLOC_PERCENT of vmalloc memory (whichever is lower).
+ *	Always allocate at least DM_BUFIO_MIN_BUFFERS buffers.
+ *	Start background writeback when there are DM_BUFIO_WRITEBACK_PERCENT
+ *	dirty buffers.
+ */
+#define DM_BUFIO_MIN_BUFFERS		8
+
+#define DM_BUFIO_MEMORY_PERCENT		2
+#define DM_BUFIO_VMALLOC_PERCENT	25
+#define DM_BUFIO_WRITEBACK_PERCENT	75
+
+/*
+ * Check buffer ages in this interval (seconds)
+ */
+#define DM_BUFIO_WORK_TIMER_SECS	10
+
+/*
+ * Free buffers when they are older than this (seconds)
+ */
+#define DM_BUFIO_DEFAULT_AGE_SECS	60
+
+/*
+ * The number of bvec entries that are embedded directly in the buffer.
+ * If the chunk size is larger, dm-io is used to do the io.
+ */
+#define DM_BUFIO_INLINE_VECS		16
+
+/*
+ * Buffer hash
+ */
+#define DM_BUFIO_HASH_BITS	20
+#define DM_BUFIO_HASH(block) \
+	((((block) >> DM_BUFIO_HASH_BITS) ^ (block)) & \
+	 ((1 << DM_BUFIO_HASH_BITS) - 1))
+
+/*
+ * Don't try to use kmem_cache_alloc for blocks larger than this.
+ * For explanation, see alloc_buffer_data below.
+ */
+#define DM_BUFIO_BLOCK_SIZE_SLAB_LIMIT	(PAGE_SIZE >> 1)
+#define DM_BUFIO_BLOCK_SIZE_GFP_LIMIT	(PAGE_SIZE << (MAX_ORDER - 1))
+
+/*
+ * dm_buffer->list_mode
+ */
+#define LIST_CLEAN	0
+#define LIST_DIRTY	1
+#define LIST_SIZE	2
+
+/*
+ * Linking of buffers:
+ *	All buffers are linked to cache_hash with their hash_list field.
+ *
+ *	Clean buffers that are not being written (B_WRITING not set)
+ *	are linked to lru[LIST_CLEAN] with their lru_list field.
+ *
+ *	Dirty and clean buffers that are being written are linked to
+ *	lru[LIST_DIRTY] with their lru_list field. When the write
+ *	finishes, the buffer cannot be relinked immediately (because we
+ *	are in an interrupt context and relinking requires process
+ *	context), so some clean-not-writing buffers can be held on
+ *	dirty_lru too.  They are later added to lru in the process
+ *	context.
+ */
+struct dm_bufio_client {
+	struct mutex lock;
+
+	struct list_head lru[LIST_SIZE];
+	unsigned long n_buffers[LIST_SIZE];
+
+	struct block_device *bdev;
+	unsigned block_size;
+	unsigned char sectors_per_block_bits;
+	unsigned char pages_per_block_bits;
+	unsigned char blocks_per_page_bits;
+	unsigned aux_size;
+	void (*alloc_callback)(struct dm_buffer *);
+	void (*write_callback)(struct dm_buffer *);
+
+	struct dm_io_client *dm_io;
+
+	struct list_head reserved_buffers;
+	unsigned need_reserved_buffers;
+
+	struct hlist_head *cache_hash;
+	wait_queue_head_t free_buffer_wait;
+
+	int async_write_error;
+
+	struct list_head client_list;
+	struct shrinker shrinker;
+};
+
+/*
+ * Buffer state bits.
+ */
+#define B_READING	0
+#define B_WRITING	1
+#define B_DIRTY		2
+
+/*
+ * Describes how the block was allocated:
+ * kmem_cache_alloc(), __get_free_pages() or vmalloc().
+ * See the comment at alloc_buffer_data.
+ */
+enum data_mode {
+	DATA_MODE_SLAB = 0,
+	DATA_MODE_GET_FREE_PAGES = 1,
+	DATA_MODE_VMALLOC = 2,
+	DATA_MODE_LIMIT = 3
+};
+
+struct dm_buffer {
+	struct hlist_node hash_list;
+	struct list_head lru_list;
+	sector_t block;
+	void *data;
+	enum data_mode data_mode;
+	unsigned char list_mode;		/* LIST_* */
+	unsigned hold_count;
+	int read_error;
+	int write_error;
+	unsigned long state;
+	unsigned long last_accessed;
+	struct dm_bufio_client *c;
+	struct bio bio;
+	struct bio_vec bio_vec[DM_BUFIO_INLINE_VECS];
+};
+
+/*----------------------------------------------------------------*/
+
+static struct kmem_cache *dm_bufio_caches[PAGE_SHIFT - SECTOR_SHIFT];
+static char *dm_bufio_cache_names[PAGE_SHIFT - SECTOR_SHIFT];
+
+static inline int dm_bufio_cache_index(struct dm_bufio_client *c)
+{
+	unsigned ret = c->blocks_per_page_bits - 1;
+
+	BUG_ON(ret >= ARRAY_SIZE(dm_bufio_caches));
+
+	return ret;
+}
+
+#define DM_BUFIO_CACHE(c)	(dm_bufio_caches[dm_bufio_cache_index(c)])
+#define DM_BUFIO_CACHE_NAME(c)	(dm_bufio_cache_names[dm_bufio_cache_index(c)])
+
+#define dm_bufio_in_request()	(!!current->bio_list)
+
+static void dm_bufio_lock(struct dm_bufio_client *c)
+{
+	mutex_lock_nested(&c->lock, dm_bufio_in_request());
+}
+
+static int dm_bufio_trylock(struct dm_bufio_client *c)
+{
+	return mutex_trylock(&c->lock);
+}
+
+static void dm_bufio_unlock(struct dm_bufio_client *c)
+{
+	mutex_unlock(&c->lock);
+}
+
+/*
+ * FIXME Move to sched.h?
+ */
+#ifdef CONFIG_PREEMPT_VOLUNTARY
+#  define dm_bufio_cond_resched()		\
+do {						\
+	if (unlikely(need_resched()))		\
+		_cond_resched();		\
+} while (0)
+#else
+#  define dm_bufio_cond_resched()                do { } while (0)
+#endif
+
+/*----------------------------------------------------------------*/
+
+/*
+ * Default cache size: available memory divided by the ratio.
+ */
+static unsigned long dm_bufio_default_cache_size;
+
+/*
+ * Total cache size set by the user.
+ */
+static unsigned long dm_bufio_cache_size;
+
+/*
+ * A copy of dm_bufio_cache_size because dm_bufio_cache_size can change
+ * at any time.  If it disagrees, the user has changed cache size.
+ */
+static unsigned long dm_bufio_cache_size_latch;
+
+static DEFINE_SPINLOCK(param_spinlock);
+
+/*
+ * Buffers are freed after this timeout
+ */
+static unsigned dm_bufio_max_age = DM_BUFIO_DEFAULT_AGE_SECS;
+
+static unsigned long dm_bufio_peak_allocated;
+static unsigned long dm_bufio_allocated_kmem_cache;
+static unsigned long dm_bufio_allocated_get_free_pages;
+static unsigned long dm_bufio_allocated_vmalloc;
+static unsigned long dm_bufio_current_allocated;
+
+/*----------------------------------------------------------------*/
+
+/*
+ * Per-client cache: dm_bufio_cache_size / dm_bufio_client_count
+ */
+static unsigned long dm_bufio_cache_size_per_client;
+
+/*
+ * The current number of clients.
+ */
+static int dm_bufio_client_count;
+
+/*
+ * The list of all clients.
+ */
+static LIST_HEAD(dm_bufio_all_clients);
+
+/*
+ * This mutex protects dm_bufio_cache_size_latch,
+ * dm_bufio_cache_size_per_client and dm_bufio_client_count
+ */
+static DEFINE_MUTEX(dm_bufio_clients_lock);
+
+/*----------------------------------------------------------------*/
+
+static void adjust_total_allocated(enum data_mode data_mode, long diff)
+{
+	static unsigned long * const class_ptr[DATA_MODE_LIMIT] = {
+		&dm_bufio_allocated_kmem_cache,
+		&dm_bufio_allocated_get_free_pages,
+		&dm_bufio_allocated_vmalloc,
+	};
+
+	spin_lock(&param_spinlock);
+
+	*class_ptr[data_mode] += diff;
+
+	dm_bufio_current_allocated += diff;
+
+	if (dm_bufio_current_allocated > dm_bufio_peak_allocated)
+		dm_bufio_peak_allocated = dm_bufio_current_allocated;
+
+	spin_unlock(&param_spinlock);
+}
+
+/*
+ * Change the number of clients and recalculate per-client limit.
+ */
+static void __cache_size_refresh(void)
+{
+	BUG_ON(!mutex_is_locked(&dm_bufio_clients_lock));
+	BUG_ON(dm_bufio_client_count < 0);
+
+	dm_bufio_cache_size_latch = dm_bufio_cache_size;
+
+	barrier();
+
+	/*
+	 * Use default if set to 0 and report the actual cache size used.
+	 */
+	if (!dm_bufio_cache_size_latch) {
+		(void)cmpxchg(&dm_bufio_cache_size, 0,
+			      dm_bufio_default_cache_size);
+		dm_bufio_cache_size_latch = dm_bufio_default_cache_size;
+	}
+
+	dm_bufio_cache_size_per_client = dm_bufio_cache_size_latch /
+					 (dm_bufio_client_count ? : 1);
+}
+
+/*
+ * Allocating buffer data.
+ *
+ * Small buffers are allocated with kmem_cache, to use space optimally.
+ *
+ * For large buffers, we choose between get_free_pages and vmalloc.
+ * Each has advantages and disadvantages.
+ *
+ * __get_free_pages can randomly fail if the memory is fragmented.
+ * __vmalloc won't randomly fail, but vmalloc space is limited (it may be
+ * as low as 128M) so using it for caching is not appropriate.
+ *
+ * If the allocation may fail we use __get_free_pages. Memory fragmentation
+ * won't have a fatal effect here, but it just causes flushes of some other
+ * buffers and more I/O will be performed. Don't use __get_free_pages if it
+ * always fails (i.e. order >= MAX_ORDER).
+ *
+ * If the allocation shouldn't fail we use __vmalloc. This is only for the
+ * initial reserve allocation, so there's no risk of wasting all vmalloc
+ * space.
+ */
+static void *alloc_buffer_data(struct dm_bufio_client *c, gfp_t gfp_mask,
+			       enum data_mode *data_mode)
+{
+	if (c->block_size <= DM_BUFIO_BLOCK_SIZE_SLAB_LIMIT) {
+		*data_mode = DATA_MODE_SLAB;
+		return kmem_cache_alloc(DM_BUFIO_CACHE(c), gfp_mask);
+	}
+
+	if (c->block_size <= DM_BUFIO_BLOCK_SIZE_GFP_LIMIT &&
+	    gfp_mask & __GFP_NORETRY) {
+		*data_mode = DATA_MODE_GET_FREE_PAGES;
+		return (void *)__get_free_pages(gfp_mask,
+						c->pages_per_block_bits);
+	}
+
+	*data_mode = DATA_MODE_VMALLOC;
+	return __vmalloc(c->block_size, gfp_mask, PAGE_KERNEL);
+}
+
+/*
+ * Free buffer's data.
+ */
+static void free_buffer_data(struct dm_bufio_client *c,
+			     void *data, enum data_mode data_mode)
+{
+	switch (data_mode) {
+	case DATA_MODE_SLAB:
+		kmem_cache_free(DM_BUFIO_CACHE(c), data);
+		break;
+
+	case DATA_MODE_GET_FREE_PAGES:
+		free_pages((unsigned long)data, c->pages_per_block_bits);
+		break;
+
+	case DATA_MODE_VMALLOC:
+		vfree(data);
+		break;
+
+	default:
+		DMCRIT("dm_bufio_free_buffer_data: bad data mode: %d",
+		       data_mode);
+		BUG();
+	}
+}
+
+/*
+ * Allocate buffer and its data.
+ */
+static struct dm_buffer *alloc_buffer(struct dm_bufio_client *c, gfp_t gfp_mask)
+{
+	struct dm_buffer *b = kmalloc(sizeof(struct dm_buffer) + c->aux_size,
+				      gfp_mask);
+
+	if (!b)
+		return NULL;
+
+	b->c = c;
+
+	b->data = alloc_buffer_data(c, gfp_mask, &b->data_mode);
+	if (!b->data) {
+		kfree(b);
+		return NULL;
+	}
+
+	adjust_total_allocated(b->data_mode, (long)c->block_size);
+
+	return b;
+}
+
+/*
+ * Free buffer and its data.
+ */
+static void free_buffer(struct dm_buffer *b)
+{
+	struct dm_bufio_client *c = b->c;
+
+	adjust_total_allocated(b->data_mode, -(long)c->block_size);
+
+	free_buffer_data(c, b->data, b->data_mode);
+	kfree(b);
+}
+
+/*
+ * Link buffer to the hash list and clean or dirty queue.
+ */
+static void __link_buffer(struct dm_buffer *b, sector_t block, int dirty)
+{
+	struct dm_bufio_client *c = b->c;
+
+	c->n_buffers[dirty]++;
+	b->block = block;
+	b->list_mode = dirty;
+	list_add(&b->lru_list, &c->lru[dirty]);
+	hlist_add_head(&b->hash_list, &c->cache_hash[DM_BUFIO_HASH(block)]);
+	b->last_accessed = jiffies;
+}
+
+/*
+ * Unlink buffer from the hash list and dirty or clean queue.
+ */
+static void __unlink_buffer(struct dm_buffer *b)
+{
+	struct dm_bufio_client *c = b->c;
+
+	BUG_ON(!c->n_buffers[b->list_mode]);
+
+	c->n_buffers[b->list_mode]--;
+	hlist_del(&b->hash_list);
+	list_del(&b->lru_list);
+}
+
+/*
+ * Place the buffer to the head of dirty or clean LRU queue.
+ */
+static void __relink_lru(struct dm_buffer *b, int dirty)
+{
+	struct dm_bufio_client *c = b->c;
+
+	BUG_ON(!c->n_buffers[b->list_mode]);
+
+	c->n_buffers[b->list_mode]--;
+	c->n_buffers[dirty]++;
+	b->list_mode = dirty;
+	list_del(&b->lru_list);
+	list_add(&b->lru_list, &c->lru[dirty]);
+}
+
+/*----------------------------------------------------------------
+ * Submit I/O on the buffer.
+ *
+ * Bio interface is faster but it has some problems:
+ *	the vector list is limited (increasing this limit increases
+ *	memory-consumption per buffer, so it is not viable);
+ *
+ *	the memory must be direct-mapped, not vmalloced;
+ *
+ *	the I/O driver can reject requests spuriously if it thinks that
+ *	the requests are too big for the device or if they cross a
+ *	controller-defined memory boundary.
+ *
+ * If the buffer is small enough (up to DM_BUFIO_INLINE_VECS pages) and
+ * it is not vmalloced, try using the bio interface.
+ *
+ * If the buffer is big, if it is vmalloced or if the underlying device
+ * rejects the bio because it is too large, use dm-io layer to do the I/O.
+ * The dm-io layer splits the I/O into multiple requests, avoiding the above
+ * shortcomings.
+ *--------------------------------------------------------------*/
+
+/*
+ * dm-io completion routine. It just calls b->bio.bi_end_io, pretending
+ * that the request was handled directly with bio interface.
+ */
+static void dmio_complete(unsigned long error, void *context)
+{
+	struct dm_buffer *b = context;
+
+	b->bio.bi_end_io(&b->bio, error ? -EIO : 0);
+}
+
+static void use_dmio(struct dm_buffer *b, int rw, sector_t block,
+		     bio_end_io_t *end_io)
+{
+	int r;
+	struct dm_io_request io_req = {
+		.bi_rw = rw,
+		.notify.fn = dmio_complete,
+		.notify.context = b,
+		.client = b->c->dm_io,
+	};
+	struct dm_io_region region = {
+		.bdev = b->c->bdev,
+		.sector = block << b->c->sectors_per_block_bits,
+		.count = b->c->block_size >> SECTOR_SHIFT,
+	};
+
+	if (b->data_mode != DATA_MODE_VMALLOC) {
+		io_req.mem.type = DM_IO_KMEM;
+		io_req.mem.ptr.addr = b->data;
+	} else {
+		io_req.mem.type = DM_IO_VMA;
+		io_req.mem.ptr.vma = b->data;
+	}
+
+	b->bio.bi_end_io = end_io;
+
+	r = dm_io(&io_req, 1, &region, NULL);
+	if (r)
+		end_io(&b->bio, r);
+}
+
+static void use_inline_bio(struct dm_buffer *b, int rw, sector_t block,
+			   bio_end_io_t *end_io)
+{
+	char *ptr;
+	int len;
+
+	bio_init(&b->bio);
+	b->bio.bi_io_vec = b->bio_vec;
+	b->bio.bi_max_vecs = DM_BUFIO_INLINE_VECS;
+	b->bio.bi_sector = block << b->c->sectors_per_block_bits;
+	b->bio.bi_bdev = b->c->bdev;
+	b->bio.bi_end_io = end_io;
+
+	/*
+	 * We assume that if len >= PAGE_SIZE ptr is page-aligned.
+	 * If len < PAGE_SIZE the buffer doesn't cross page boundary.
+	 */
+	ptr = b->data;
+	len = b->c->block_size;
+
+	if (len >= PAGE_SIZE)
+		BUG_ON((unsigned long)ptr & (PAGE_SIZE - 1));
+	else
+		BUG_ON((unsigned long)ptr & (len - 1));
+
+	do {
+		if (!bio_add_page(&b->bio, virt_to_page(ptr),
+				  len < PAGE_SIZE ? len : PAGE_SIZE,
+				  virt_to_phys(ptr) & (PAGE_SIZE - 1))) {
+			BUG_ON(b->c->block_size <= PAGE_SIZE);
+			use_dmio(b, rw, block, end_io);
+			return;
+		}
+
+		len -= PAGE_SIZE;
+		ptr += PAGE_SIZE;
+	} while (len > 0);
+
+	submit_bio(rw, &b->bio);
+}
+
+static void submit_io(struct dm_buffer *b, int rw, sector_t block,
+		      bio_end_io_t *end_io)
+{
+	if (rw == WRITE && b->c->write_callback)
+		b->c->write_callback(b);
+
+	if (b->c->block_size <= DM_BUFIO_INLINE_VECS * PAGE_SIZE &&
+	    b->data_mode != DATA_MODE_VMALLOC)
+		use_inline_bio(b, rw, block, end_io);
+	else
+		use_dmio(b, rw, block, end_io);
+}
+
+/*----------------------------------------------------------------
+ * Writing dirty buffers
+ *--------------------------------------------------------------*/
+
+/*
+ * The endio routine for write.
+ *
+ * Set the error, clear B_WRITING bit and wake anyone who was waiting on
+ * it.
+ */
+static void write_endio(struct bio *bio, int error)
+{
+	struct dm_buffer *b = container_of(bio, struct dm_buffer, bio);
+
+	b->write_error = error;
+	if (error) {
+		struct dm_bufio_client *c = b->c;
+		(void)cmpxchg(&c->async_write_error, 0, error);
+	}
+
+	BUG_ON(!test_bit(B_WRITING, &b->state));
+
+	smp_mb__before_clear_bit();
+	clear_bit(B_WRITING, &b->state);
+	smp_mb__after_clear_bit();
+
+	wake_up_bit(&b->state, B_WRITING);
+}
+
+/*
+ * This function is called when wait_on_bit is actually waiting.
+ */
+static int do_io_schedule(void *word)
+{
+	io_schedule();
+
+	return 0;
+}
+
+/*
+ * Initiate a write on a dirty buffer, but don't wait for it.
+ *
+ * - If the buffer is not dirty, exit.
+ * - If there some previous write going on, wait for it to finish (we can't
+ *   have two writes on the same buffer simultaneously).
+ * - Submit our write and don't wait on it. We set B_WRITING indicating
+ *   that there is a write in progress.
+ */
+static void __write_dirty_buffer(struct dm_buffer *b)
+{
+	if (!test_bit(B_DIRTY, &b->state))
+		return;
+
+	clear_bit(B_DIRTY, &b->state);
+	wait_on_bit_lock(&b->state, B_WRITING,
+			 do_io_schedule, TASK_UNINTERRUPTIBLE);
+
+	submit_io(b, WRITE, b->block, write_endio);
+}
+
+/*
+ * Wait until any activity on the buffer finishes.  Possibly write the
+ * buffer if it is dirty.  When this function finishes, there is no I/O
+ * running on the buffer and the buffer is not dirty.
+ */
+static void __make_buffer_clean(struct dm_buffer *b)
+{
+	BUG_ON(b->hold_count);
+
+	if (!b->state)	/* fast case */
+		return;
+
+	wait_on_bit(&b->state, B_READING, do_io_schedule, TASK_UNINTERRUPTIBLE);
+	__write_dirty_buffer(b);
+	wait_on_bit(&b->state, B_WRITING, do_io_schedule, TASK_UNINTERRUPTIBLE);
+}
+
+/*
+ * Find some buffer that is not held by anybody, clean it, unlink it and
+ * return it.
+ */
+static struct dm_buffer *__get_unclaimed_buffer(struct dm_bufio_client *c)
+{
+	struct dm_buffer *b;
+
+	list_for_each_entry_reverse(b, &c->lru[LIST_CLEAN], lru_list) {
+		BUG_ON(test_bit(B_WRITING, &b->state));
+		BUG_ON(test_bit(B_DIRTY, &b->state));
+
+		if (!b->hold_count) {
+			__make_buffer_clean(b);
+			__unlink_buffer(b);
+			return b;
+		}
+		dm_bufio_cond_resched();
+	}
+
+	list_for_each_entry_reverse(b, &c->lru[LIST_DIRTY], lru_list) {
+		BUG_ON(test_bit(B_READING, &b->state));
+
+		if (!b->hold_count) {
+			__make_buffer_clean(b);
+			__unlink_buffer(b);
+			return b;
+		}
+		dm_bufio_cond_resched();
+	}
+
+	return NULL;
+}
+
+/*
+ * Wait until some other threads free some buffer or release hold count on
+ * some buffer.
+ *
+ * This function is entered with c->lock held, drops it and regains it
+ * before exiting.
+ */
+static void __wait_for_free_buffer(struct dm_bufio_client *c)
+{
+	DECLARE_WAITQUEUE(wait, current);
+
+	add_wait_queue(&c->free_buffer_wait, &wait);
+	set_task_state(current, TASK_UNINTERRUPTIBLE);
+	dm_bufio_unlock(c);
+
+	io_schedule();
+
+	set_task_state(current, TASK_RUNNING);
+	remove_wait_queue(&c->free_buffer_wait, &wait);
+
+	dm_bufio_lock(c);
+}
+
+/*
+ * Allocate a new buffer. If the allocation is not possible, wait until
+ * some other thread frees a buffer.
+ *
+ * May drop the lock and regain it.
+ */
+static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client *c)
+{
+	struct dm_buffer *b;
+
+	/*
+	 * dm-bufio is resistant to allocation failures (it just keeps
+	 * one buffer reserved in cases all the allocations fail).
+	 * So set flags to not try too hard:
+	 *	GFP_NOIO: don't recurse into the I/O layer
+	 *	__GFP_NORETRY: don't retry and rather return failure
+	 *	__GFP_NOMEMALLOC: don't use emergency reserves
+	 *	__GFP_NOWARN: don't print a warning in case of failure
+	 *
+	 * For debugging, if we set the cache size to 1, no new buffers will
+	 * be allocated.
+	 */
+	while (1) {
+		if (dm_bufio_cache_size_latch != 1) {
+			b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
+			if (b)
+				return b;
+		}
+
+		if (!list_empty(&c->reserved_buffers)) {
+			b = list_entry(c->reserved_buffers.next,
+				       struct dm_buffer, lru_list);
+			list_del(&b->lru_list);
+			c->need_reserved_buffers++;
+
+			return b;
+		}
+
+		b = __get_unclaimed_buffer(c);
+		if (b)
+			return b;
+
+		__wait_for_free_buffer(c);
+	}
+}
+
+static struct dm_buffer *__alloc_buffer_wait(struct dm_bufio_client *c)
+{
+	struct dm_buffer *b = __alloc_buffer_wait_no_callback(c);
+
+	if (c->alloc_callback)
+		c->alloc_callback(b);
+
+	return b;
+}
+
+/*
+ * Free a buffer and wake other threads waiting for free buffers.
+ */
+static void __free_buffer_wake(struct dm_buffer *b)
+{
+	struct dm_bufio_client *c = b->c;
+
+	if (!c->need_reserved_buffers)
+		free_buffer(b);
+	else {
+		list_add(&b->lru_list, &c->reserved_buffers);
+		c->need_reserved_buffers--;
+	}
+
+	wake_up(&c->free_buffer_wait);
+}
+
+static void __write_dirty_buffers_async(struct dm_bufio_client *c, int no_wait)
+{
+	struct dm_buffer *b, *tmp;
+
+	list_for_each_entry_safe_reverse(b, tmp, &c->lru[LIST_DIRTY], lru_list) {
+		BUG_ON(test_bit(B_READING, &b->state));
+
+		if (!test_bit(B_DIRTY, &b->state) &&
+		    !test_bit(B_WRITING, &b->state)) {
+			__relink_lru(b, LIST_CLEAN);
+			continue;
+		}
+
+		if (no_wait && test_bit(B_WRITING, &b->state))
+			return;
+
+		__write_dirty_buffer(b);
+		dm_bufio_cond_resched();
+	}
+}
+
+/*
+ * Get writeback threshold and buffer limit for a given client.
+ */
+static void __get_memory_limit(struct dm_bufio_client *c,
+			       unsigned long *threshold_buffers,
+			       unsigned long *limit_buffers)
+{
+	unsigned long buffers;
+
+	if (dm_bufio_cache_size != dm_bufio_cache_size_latch) {
+		mutex_lock(&dm_bufio_clients_lock);
+		__cache_size_refresh();
+		mutex_unlock(&dm_bufio_clients_lock);
+	}
+
+	buffers = dm_bufio_cache_size_per_client >>
+		  (c->sectors_per_block_bits + SECTOR_SHIFT);
+
+	if (buffers < DM_BUFIO_MIN_BUFFERS)
+		buffers = DM_BUFIO_MIN_BUFFERS;
+
+	*limit_buffers = buffers;
+	*threshold_buffers = buffers * DM_BUFIO_WRITEBACK_PERCENT / 100;
+}
+
+/*
+ * Check if we're over watermark.
+ * If we are over threshold_buffers, start freeing buffers.
+ * If we're over "limit_buffers", block until we get under the limit.
+ */
+static void __check_watermark(struct dm_bufio_client *c)
+{
+	unsigned long threshold_buffers, limit_buffers;
+
+	__get_memory_limit(c, &threshold_buffers, &limit_buffers);
+
+	while (c->n_buffers[LIST_CLEAN] + c->n_buffers[LIST_DIRTY] >
+	       limit_buffers) {
+
+		struct dm_buffer *b = __get_unclaimed_buffer(c);
+
+		if (!b)
+			return;
+
+		__free_buffer_wake(b);
+		dm_bufio_cond_resched();
+	}
+
+	if (c->n_buffers[LIST_DIRTY] > threshold_buffers)
+		__write_dirty_buffers_async(c, 1);
+}
+
+/*
+ * Find a buffer in the hash.
+ */
+static struct dm_buffer *__find(struct dm_bufio_client *c, sector_t block)
+{
+	struct dm_buffer *b;
+	struct hlist_node *hn;
+
+	hlist_for_each_entry(b, hn, &c->cache_hash[DM_BUFIO_HASH(block)],
+			     hash_list) {
+		dm_bufio_cond_resched();
+		if (b->block == block)
+			return b;
+	}
+
+	return NULL;
+}
+
+/*----------------------------------------------------------------
+ * Getting a buffer
+ *--------------------------------------------------------------*/
+
+enum new_flag {
+	NF_FRESH = 0,
+	NF_READ = 1,
+	NF_GET = 2
+};
+
+static struct dm_buffer *__bufio_new(struct dm_bufio_client *c, sector_t block,
+				     enum new_flag nf, struct dm_buffer **bp,
+				     int *need_submit)
+{
+	struct dm_buffer *b, *new_b = NULL;
+
+	*need_submit = 0;
+
+	b = __find(c, block);
+	if (b) {
+		b->hold_count++;
+		__relink_lru(b, test_bit(B_DIRTY, &b->state) ||
+			     test_bit(B_WRITING, &b->state));
+		return b;
+	}
+
+	if (nf == NF_GET)
+		return NULL;
+
+	new_b = __alloc_buffer_wait(c);
+
+	/*
+	 * We've had a period where the mutex was unlocked, so need to
+	 * recheck the hash table.
+	 */
+	b = __find(c, block);
+	if (b) {
+		__free_buffer_wake(new_b);
+		b->hold_count++;
+		__relink_lru(b, test_bit(B_DIRTY, &b->state) ||
+			     test_bit(B_WRITING, &b->state));
+		return b;
+	}
+
+	__check_watermark(c);
+
+	b = new_b;
+	b->hold_count = 1;
+	b->read_error = 0;
+	b->write_error = 0;
+	__link_buffer(b, block, LIST_CLEAN);
+
+	if (nf == NF_FRESH) {
+		b->state = 0;
+		return b;
+	}
+
+	b->state = 1 << B_READING;
+	*need_submit = 1;
+
+	return b;
+}
+
+/*
+ * The endio routine for reading: set the error, clear the bit and wake up
+ * anyone waiting on the buffer.
+ */
+static void read_endio(struct bio *bio, int error)
+{
+	struct dm_buffer *b = container_of(bio, struct dm_buffer, bio);
+
+	b->read_error = error;
+
+	BUG_ON(!test_bit(B_READING, &b->state));
+
+	smp_mb__before_clear_bit();
+	clear_bit(B_READING, &b->state);
+	smp_mb__after_clear_bit();
+
+	wake_up_bit(&b->state, B_READING);
+}
+
+/*
+ * A common routine for dm_bufio_new and dm_bufio_read.  Operation of these
+ * functions is similar except that dm_bufio_new doesn't read the
+ * buffer from the disk (assuming that the caller overwrites all the data
+ * and uses dm_bufio_mark_buffer_dirty to write new data back).
+ */
+static void *new_read(struct dm_bufio_client *c, sector_t block,
+		      enum new_flag nf, struct dm_buffer **bp)
+{
+	int need_submit;
+	struct dm_buffer *b;
+
+	dm_bufio_lock(c);
+	b = __bufio_new(c, block, nf, bp, &need_submit);
+	dm_bufio_unlock(c);
+
+	if (!b || IS_ERR(b))
+		return b;
+
+	if (need_submit)
+		submit_io(b, READ, b->block, read_endio);
+
+	wait_on_bit(&b->state, B_READING, do_io_schedule, TASK_UNINTERRUPTIBLE);
+
+	if (b->read_error) {
+		int error = b->read_error;
+
+		dm_bufio_release(b);
+
+		return ERR_PTR(error);
+	}
+
+	*bp = b;
+
+	return b->data;
+}
+
+void *dm_bufio_get(struct dm_bufio_client *c, sector_t block,
+		   struct dm_buffer **bp)
+{
+	return new_read(c, block, NF_GET, bp);
+}
+EXPORT_SYMBOL_GPL(dm_bufio_get);
+
+void *dm_bufio_read(struct dm_bufio_client *c, sector_t block,
+		    struct dm_buffer **bp)
+{
+	BUG_ON(dm_bufio_in_request());
+
+	return new_read(c, block, NF_READ, bp);
+}
+EXPORT_SYMBOL_GPL(dm_bufio_read);
+
+void *dm_bufio_new(struct dm_bufio_client *c, sector_t block,
+		   struct dm_buffer **bp)
+{
+	BUG_ON(dm_bufio_in_request());
+
+	return new_read(c, block, NF_FRESH, bp);
+}
+EXPORT_SYMBOL_GPL(dm_bufio_new);
+
+void dm_bufio_release(struct dm_buffer *b)
+{
+	struct dm_bufio_client *c = b->c;
+
+	dm_bufio_lock(c);
+
+	BUG_ON(test_bit(B_READING, &b->state));
+	BUG_ON(!b->hold_count);
+
+	b->hold_count--;
+	if (!b->hold_count) {
+		wake_up(&c->free_buffer_wait);
+
+		/*
+		 * If there were errors on the buffer, and the buffer is not
+		 * to be written, free the buffer. There is no point in caching
+		 * invalid buffer.
+		 */
+		if ((b->read_error || b->write_error) &&
+		    !test_bit(B_WRITING, &b->state) &&
+		    !test_bit(B_DIRTY, &b->state)) {
+			__unlink_buffer(b);
+			__free_buffer_wake(b);
+		}
+	}
+
+	dm_bufio_unlock(c);
+}
+EXPORT_SYMBOL_GPL(dm_bufio_release);
+
+void dm_bufio_mark_buffer_dirty(struct dm_buffer *b)
+{
+	struct dm_bufio_client *c = b->c;
+
+	dm_bufio_lock(c);
+
+	if (!test_and_set_bit(B_DIRTY, &b->state))
+		__relink_lru(b, LIST_DIRTY);
+
+	dm_bufio_unlock(c);
+}
+EXPORT_SYMBOL_GPL(dm_bufio_mark_buffer_dirty);
+
+void dm_bufio_write_dirty_buffers_async(struct dm_bufio_client *c)
+{
+	BUG_ON(dm_bufio_in_request());
+
+	dm_bufio_lock(c);
+	__write_dirty_buffers_async(c, 0);
+	dm_bufio_unlock(c);
+}
+EXPORT_SYMBOL_GPL(dm_bufio_write_dirty_buffers_async);
+
+/*
+ * For performance, it is essential that the buffers are written asynchronously
+ * and simultaneously (so that the block layer can merge the writes) and then
+ * waited upon.
+ *
+ * Finally, we flush hardware disk cache.
+ */
+int dm_bufio_write_dirty_buffers(struct dm_bufio_client *c)
+{
+	int a, f;
+	unsigned long buffers_processed = 0;
+	struct dm_buffer *b, *tmp;
+
+	dm_bufio_lock(c);
+	__write_dirty_buffers_async(c, 0);
+
+again:
+	list_for_each_entry_safe_reverse(b, tmp, &c->lru[LIST_DIRTY], lru_list) {
+		int dropped_lock = 0;
+
+		if (buffers_processed < c->n_buffers[LIST_DIRTY])
+			buffers_processed++;
+
+		BUG_ON(test_bit(B_READING, &b->state));
+
+		if (test_bit(B_WRITING, &b->state)) {
+			if (buffers_processed < c->n_buffers[LIST_DIRTY]) {
+				dropped_lock = 1;
+				b->hold_count++;
+				dm_bufio_unlock(c);
+				wait_on_bit(&b->state, B_WRITING,
+					    do_io_schedule,
+					    TASK_UNINTERRUPTIBLE);
+				dm_bufio_lock(c);
+				b->hold_count--;
+			} else
+				wait_on_bit(&b->state, B_WRITING,
+					    do_io_schedule,
+					    TASK_UNINTERRUPTIBLE);
+		}
+
+		if (!test_bit(B_DIRTY, &b->state) &&
+		    !test_bit(B_WRITING, &b->state))
+			__relink_lru(b, LIST_CLEAN);
+
+		dm_bufio_cond_resched();
+
+		/*
+		 * If we dropped the lock, the list is no longer consistent,
+		 * so we must restart the search.
+		 *
+		 * In the most common case, the buffer just processed is
+		 * relinked to the clean list, so we won't loop scanning the
+		 * same buffer again and again.
+		 *
+		 * This may livelock if there is another thread simultaneously
+		 * dirtying buffers, so we count the number of buffers walked
+		 * and if it exceeds the total number of buffers, it means that
+		 * someone is doing some writes simultaneously with us.  In
+		 * this case, stop, dropping the lock.
+		 */
+		if (dropped_lock)
+			goto again;
+	}
+	wake_up(&c->free_buffer_wait);
+	dm_bufio_unlock(c);
+
+	a = xchg(&c->async_write_error, 0);
+	f = dm_bufio_issue_flush(c);
+	if (a)
+		return a;
+
+	return f;
+}
+EXPORT_SYMBOL_GPL(dm_bufio_write_dirty_buffers);
+
+/*
+ * Use dm-io to send and empty barrier flush the device.
+ */
+int dm_bufio_issue_flush(struct dm_bufio_client *c)
+{
+	struct dm_io_request io_req = {
+		.bi_rw = REQ_FLUSH,
+		.mem.type = DM_IO_KMEM,
+		.mem.ptr.addr = NULL,
+		.client = c->dm_io,
+	};
+	struct dm_io_region io_reg = {
+		.bdev = c->bdev,
+		.sector = 0,
+		.count = 0,
+	};
+
+	BUG_ON(dm_bufio_in_request());
+
+	return dm_io(&io_req, 1, &io_reg, NULL);
+}
+EXPORT_SYMBOL_GPL(dm_bufio_issue_flush);
+
+/*
+ * We first delete any other buffer that may be at that new location.
+ *
+ * Then, we write the buffer to the original location if it was dirty.
+ *
+ * Then, if we are the only one who is holding the buffer, relink the buffer
+ * in the hash queue for the new location.
+ *
+ * If there was someone else holding the buffer, we write it to the new
+ * location but not relink it, because that other user needs to have the buffer
+ * at the same place.
+ */
+void dm_bufio_release_move(struct dm_buffer *b, sector_t new_block)
+{
+	struct dm_bufio_client *c = b->c;
+	struct dm_buffer *new;
+
+	BUG_ON(dm_bufio_in_request());
+
+	dm_bufio_lock(c);
+
+retry:
+	new = __find(c, new_block);
+	if (new) {
+		if (new->hold_count) {
+			__wait_for_free_buffer(c);
+			goto retry;
+		}
+
+		/*
+		 * FIXME: Is there any point waiting for a write that's going
+		 * to be overwritten in a bit?
+		 */
+		__make_buffer_clean(new);
+		__unlink_buffer(new);
+		__free