request.c

/*
 * Main bcache entry point - handle a read or a write request and decide what to
 * do with it; the make_request functions are called by the block layer.
 *
 * Copyright 2010, 2011 Kent Overstreet <kent.overstreet@gmail.com>
 * Copyright 2012 Google, Inc.
 */

#include "bcache.h"
#include "btree.h"
#include "debug.h"
#include "request.h"

#include <linux/cgroup.h>
#include <linux/module.h>
#include <linux/hash.h>
#include <linux/random.h>
#include "blk-cgroup.h"

#include <trace/events/bcache.h>

#define CUTOFF_CACHE_ADD	95
#define CUTOFF_CACHE_READA	90
#define CUTOFF_WRITEBACK	50
#define CUTOFF_WRITEBACK_SYNC	75

struct kmem_cache *bch_search_cache;

static void check_should_skip(struct cached_dev *, struct search *);

/* Cgroup interface */

#ifdef CONFIG_CGROUP_BCACHE
static struct bch_cgroup bcache_default_cgroup = { .cache_mode = -1 };

static struct bch_cgroup *cgroup_to_bcache(struct cgroup *cgroup)
{
	struct cgroup_subsys_state *css;
	return cgroup &&
		(css = cgroup_subsys_state(cgroup, bcache_subsys_id))
		? container_of(css, struct bch_cgroup, css)
		: &bcache_default_cgroup;
}

struct bch_cgroup *bch_bio_to_cgroup(struct bio *bio)
{
	struct cgroup_subsys_state *css = bio->bi_css
		? cgroup_subsys_state(bio->bi_css->cgroup, bcache_subsys_id)
		: task_subsys_state(current, bcache_subsys_id);

	return css
		? container_of(css, struct bch_cgroup, css)
		: &bcache_default_cgroup;
}

static ssize_t cache_mode_read(struct cgroup *cgrp, struct cftype *cft,
			struct file *file,
			char __user *buf, size_t nbytes, loff_t *ppos)
{
	char tmp[1024];
	int len = bch_snprint_string_list(tmp, PAGE_SIZE, bch_cache_modes,
					  cgroup_to_bcache(cgrp)->cache_mode + 1);

	if (len < 0)
		return len;

	return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
}

static int cache_mode_write(struct cgroup *cgrp, struct cftype *cft,
			    const char *buf)
{
	int v = bch_read_string_list(buf, bch_cache_modes);
	if (v < 0)
		return v;

	cgroup_to_bcache(cgrp)->cache_mode = v - 1;
	return 0;
}

static u64 bch_verify_read(struct cgroup *cgrp, struct cftype *cft)
{
	return cgroup_to_bcache(cgrp)->verify;
}

static int bch_verify_write(struct cgroup *cgrp, struct cftype *cft, u64 val)
{
	cgroup_to_bcache(cgrp)->verify = val;
	return 0;
}

static u64 bch_cache_hits_read(struct cgroup *cgrp, struct cftype *cft)
{
	struct bch_cgroup *bcachecg = cgroup_to_bcache(cgrp);
	return atomic_read(&bcachecg->stats.cache_hits);
}

static u64 bch_cache_misses_read(struct cgroup *cgrp, struct cftype *cft)
{
	struct bch_cgroup *bcachecg = cgroup_to_bcache(cgrp);
	return atomic_read(&bcachecg->stats.cache_misses);
}

static u64 bch_cache_bypass_hits_read(struct cgroup *cgrp,
					 struct cftype *cft)
{
	struct bch_cgroup *bcachecg = cgroup_to_bcache(cgrp);
	return atomic_read(&bcachecg->stats.cache_bypass_hits);
}

static u64 bch_cache_bypass_misses_read(struct cgroup *cgrp,
					   struct cftype *cft)
{
	struct bch_cgroup *bcachecg = cgroup_to_bcache(cgrp);
	return atomic_read(&bcachecg->stats.cache_bypass_misses);
}

static struct cftype bch_files[] = {
	{
		.name		= "cache_mode",
		.read		= cache_mode_read,
		.write_string	= cache_mode_write,
	},
	{
		.name		= "verify",
		.read_u64	= bch_verify_read,
		.write_u64	= bch_verify_write,
	},
	{
		.name		= "cache_hits",
		.read_u64	= bch_cache_hits_read,
	},
	{
		.name		= "cache_misses",
		.read_u64	= bch_cache_misses_read,
	},
	{
		.name		= "cache_bypass_hits",
		.read_u64	= bch_cache_bypass_hits_read,
	},
	{
		.name		= "cache_bypass_misses",
		.read_u64	= bch_cache_bypass_misses_read,
	},
	{ }	/* terminate */
};

static void init_bch_cgroup(struct bch_cgroup *cg)
{
	cg->cache_mode = -1;
}

static struct cgroup_subsys_state *bcachecg_create(struct cgroup *cgroup)
{
	struct bch_cgroup *cg;

	cg = kzalloc(sizeof(*cg), GFP_KERNEL);
	if (!cg)
		return ERR_PTR(-ENOMEM);
	init_bch_cgroup(cg);
	return &cg->css;
}

static void bcachecg_destroy(struct cgroup *cgroup)
{
	struct bch_cgroup *cg = cgroup_to_bcache(cgroup);
	free_css_id(&bcache_subsys, &cg->css);
	kfree(cg);
}

struct cgroup_subsys bcache_subsys = {
	.create		= bcachecg_create,
	.destroy	= bcachecg_destroy,
	.subsys_id	= bcache_subsys_id,
	.name		= "bcache",
	.module		= THIS_MODULE,
};
EXPORT_SYMBOL_GPL(bcache_subsys);
#endif

static unsigned cache_mode(struct cached_dev *dc, struct bio *bio)
{
#ifdef CONFIG_CGROUP_BCACHE
	int r = bch_bio_to_cgroup(bio)->cache_mode;
	if (r >= 0)
		return r;
#endif
	return BDEV_CACHE_MODE(&dc->sb);
}

static bool verify(struct cached_dev *dc, struct bio *bio)
{
#ifdef CONFIG_CGROUP_BCACHE
	if (bch_bio_to_cgroup(bio)->verify)
		return true;
#endif
	return dc->verify;
}

static void bio_csum(struct bio *bio, struct bkey *k)
{
	struct bio_vec *bv;
	uint64_t csum = 0;
	int i;

	bio_for_each_segment(bv, bio, i) {
		void *d = kmap(bv->bv_page) + bv->bv_offset;
		csum = bch_crc64_update(csum, d, bv->bv_len);
		kunmap(bv->bv_page);
	}

	k->ptr[KEY_PTRS(k)] = csum & (~0ULL >> 1);
}

/* Insert data into cache */

static void bio_invalidate(struct closure *cl)
{
	struct btree_op *op = container_of(cl, struct btree_op, cl);
	struct bio *bio = op->cache_bio;

	pr_debug("invalidating %i sectors from %llu",
		 bio_sectors(bio), (uint64_t) bio->bi_sector);

	while (bio_sectors(bio)) {
		unsigned len = min(bio_sectors(bio), 1U << 14);

		if (bch_keylist_realloc(&op->keys, 0, op->c))
			goto out;

		bio->bi_sector	+= len;
		bio->bi_size	-= len << 9;

		bch_keylist_add(&op->keys,
				&KEY(op->inode, bio->bi_sector, len));
	}

	op->insert_data_done = true;
	bio_put(bio);
out:
	continue_at(cl, bch_journal, bcache_wq);
}

struct open_bucket {
	struct list_head	list;
	struct task_struct	*last;
	unsigned		sectors_free;
	BKEY_PADDED(key);
};

void bch_open_buckets_free(struct cache_set *c)
{
	struct open_bucket *b;

	while (!list_empty(&c->data_buckets)) {
		b = list_first_entry(&c->data_buckets,
				     struct open_bucket, list);
		list_del(&b->list);
		kfree(b);
	}
}

int bch_open_buckets_alloc(struct cache_set *c)
{
	int i;

	spin_lock_init(&c->data_bucket_lock);

	for (i = 0; i < 6; i++) {
		struct open_bucket *b = kzalloc(sizeof(*b), GFP_KERNEL);
		if (!b)
			return -ENOMEM;

		list_add(&b->list, &c->data_buckets);
	}

	return 0;
}

/*
 * We keep multiple buckets open for writes, and try to segregate different
 * write streams for better cache utilization: first we look for a bucket where
 * the last write to it was sequential with the current write, and failing that
 * we look for a bucket that was last used by the same task.
 *
 * The ideas is if you've got multiple tasks pulling data into the cache at the
 * same time, you'll get better cache utilization if you try to segregate their
 * data and preserve locality.
 *
 * For example, say you've starting Firefox at the same time you're copying a
 * bunch of files. Firefox will likely end up being fairly hot and stay in the
 * cache awhile, but the data you copied might not be; if you wrote all that
 * data to the same buckets it'd get invalidated at the same time.
 *
 * Both of those tasks will be doing fairly random IO so we can't rely on
 * detecting sequential IO to segregate their data, but going off of the task
 * should be a sane heuristic.
 */
static struct open_bucket *pick_data_bucket(struct cache_set *c,
					    const struct bkey *search,
					    struct task_struct *task,
					    struct bkey *alloc)
{
	struct open_bucket *ret, *ret_task = NULL;

	list_for_each_entry_reverse(ret, &c->data_buckets, list)
		if (!bkey_cmp(&ret->key, search))
			goto found;
		else if (ret->last == task)
			ret_task = ret;

	ret = ret_task ?: list_first_entry(&c->data_buckets,
					   struct open_bucket, list);
found:
	if (!ret->sectors_free && KEY_PTRS(alloc)) {
		ret->sectors_free = c->sb.bucket_size;
		bkey_copy(&ret->key, alloc);
		bkey_init(alloc);
	}

	if (!ret->sectors_free)
		ret = NULL;

	return ret;
}

/*
 * Allocates some space in the cache to write to, and k to point to the newly
 * allocated space, and updates KEY_SIZE(k) and KEY_OFFSET(k) (to point to the
 * end of the newly allocated space).
 *
 * May allocate fewer sectors than @sectors, KEY_SIZE(k) indicates how many
 * sectors were actually allocated.
 *
 * If s->writeback is true, will not fail.
 */
static bool bch_alloc_sectors(struct bkey *k, unsigned sectors,
			      struct search *s)
{
	struct cache_set *c = s->op.c;
	struct open_bucket *b;
	BKEY_PADDED(key) alloc;
	struct closure cl, *w = NULL;
	unsigned i;

	if (s->writeback) {
		closure_init_stack(&cl);
		w = &cl;
	}

	/*
	 * We might have to allocate a new bucket, which we can't do with a
	 * spinlock held. So if we have to allocate, we drop the lock, allocate
	 * and then retry. KEY_PTRS() indicates whether alloc points to
	 * allocated bucket(s).
	 */

	bkey_init(&alloc.key);
	spin_lock(&c->data_bucket_lock);

	while (!(b = pick_data_bucket(c, k, s->task, &alloc.key))) {
		unsigned watermark = s->op.write_prio
			? WATERMARK_MOVINGGC
			: WATERMARK_NONE;

		spin_unlock(&c->data_bucket_lock);

		if (bch_bucket_alloc_set(c, watermark, &alloc.key, 1, w))
			return false;

		spin_lock(&c->data_bucket_lock);
	}

	/*
	 * If we had to allocate, we might race and not need to allocate the
	 * second time we call find_data_bucket(). If we allocated a bucket but
	 * didn't use it, drop the refcount bch_bucket_alloc_set() took:
	 */
	if (KEY_PTRS(&alloc.key))
		__bkey_put(c, &alloc.key);

	for (i = 0; i < KEY_PTRS(&b->key); i++)
		EBUG_ON(ptr_stale(c, &b->key, i));

	/* Set up the pointer to the space we're allocating: */

	for (i = 0; i < KEY_PTRS(&b->key); i++)
		k->ptr[i] = b->key.ptr[i];

	sectors = min(sectors, b->sectors_free);

	SET_KEY_OFFSET(k, KEY_OFFSET(k) + sectors);
	SET_KEY_SIZE(k, sectors);
	SET_KEY_PTRS(k, KEY_PTRS(&b->key));

	/*
	 * Move b to the end of the lru, and keep track of what this bucket was
	 * last used for:
	 */
	list_move_tail(&b->list, &c->data_buckets);
	bkey_copy_key(&b->key, k);
	b->last = s->task;

	b->sectors_free	-= sectors;

	for (i = 0; i < KEY_PTRS(&b->key); i++) {
		SET_PTR_OFFSET(&b->key, i, PTR_OFFSET(&b->key, i) + sectors);

		atomic_long_add(sectors,
				&PTR_CACHE(c, &b->key, i)->sectors_written);
	}

	if (b->sectors_free < c->sb.block_size)
		b->sectors_free = 0;

	/*
	 * k takes refcounts on the buckets it points to until it's inserted
	 * into the btree, but if we're done with this bucket we just transfer
	 * get_data_bucket()'s refcount.
	 */
	if (b->sectors_free)
		for (i = 0; i < KEY_PTRS(&b->key); i++)
			atomic_inc(&PTR_BUCKET(c, &b->key, i)->pin);

	spin_unlock(&c->data_bucket_lock);
	return true;
}

static void bch_insert_data_error(struct closure *cl)
{
	struct btree_op *op = container_of(cl, struct btree_op, cl);

	/*
	 * Our data write just errored, which means we've got a bunch of keys to
	 * insert that point to data that wasn't succesfully written.
	 *
	 * We don't have to insert those keys but we still have to invalidate
	 * that region of the cache - so, if we just strip off all the pointers
	 * from the keys we'll accomplish just that.
	 */

	struct bkey *src = op->keys.bottom, *dst = op->keys.bottom;

	while (src != op->keys.top) {
		struct bkey *n = bkey_next(src);

		SET_KEY_PTRS(src, 0);
		bkey_copy(dst, src);

		dst = bkey_next(dst);
		src = n;
	}

	op->keys.top = dst;

	bch_journal(cl);
}

static void bch_insert_data_endio(struct bio *bio, int error)
{
	struct closure *cl = bio->bi_private;
	struct btree_op *op = container_of(cl, struct btree_op, cl);
	struct search *s = container_of(op, struct search, op);

	if (error) {
		/* TODO: We could try to recover from this. */
		if (s->writeback)
			s->error = error;
		else if (s->write)
			set_closure_fn(cl, bch_insert_data_error, bcache_wq);
		else
			set_closure_fn(cl, NULL, NULL);
	}

	bch_bbio_endio(op->c, bio, error, "writing data to cache");
}

static void bch_insert_data_loop(struct closure *cl)
{
	struct btree_op *op = container_of(cl, struct btree_op, cl);
	struct search *s = container_of(op, struct search, op);
	struct bio *bio = op->cache_bio, *n;

	if (op->skip)
		return bio_invalidate(cl);

	if (atomic_sub_return(bio_sectors(bio), &op->c->sectors_to_gc) < 0) {
		set_gc_sectors(op->c);
		bch_queue_gc(op->c);
	}

	do {
		unsigned i;
		struct bkey *k;
		struct bio_set *split = s->d
			? s->d->bio_split : op->c->bio_split;

		/* 1 for the device pointer and 1 for the chksum */
		if (bch_keylist_realloc(&op->keys,
					1 + (op->csum ? 1 : 0),
					op->c))
			continue_at(cl, bch_journal, bcache_wq);

		k = op->keys.top;
		bkey_init(k);
		SET_KEY_INODE(k, op->inode);
		SET_KEY_OFFSET(k, bio->bi_sector);

		if (!bch_alloc_sectors(k, bio_sectors(bio), s))
			goto err;

		n = bch_bio_split(bio, KEY_SIZE(k), GFP_NOIO, split);
		if (!n) {
			__bkey_put(op->c, k);
			continue_at(cl, bch_insert_data_loop, bcache_wq);
		}

		n->bi_end_io	= bch_insert_data_endio;
		n->bi_private	= cl;

		if (s->writeback) {
			SET_KEY_DIRTY(k, true);

			for (i = 0; i < KEY_PTRS(k); i++)
				SET_GC_MARK(PTR_BUCKET(op->c, k, i),
					    GC_MARK_DIRTY);
		}

		SET_KEY_CSUM(k, op->csum);
		if (KEY_CSUM(k))
			bio_csum(n, k);

		pr_debug("%s", pkey(k));
		bch_keylist_push(&op->keys);

		trace_bcache_cache_insert(n, n->bi_sector, n->bi_bdev);
		n->bi_rw |= REQ_WRITE;
		bch_submit_bbio(n, op->c, k, 0);
	} while (n != bio);

	op->insert_data_done = true;
	continue_at(cl, bch_journal, bcache_wq);
err:
	/* bch_alloc_sectors() blocks if s->writeback = true */
	BUG_ON(s->writeback);

	/*
	 * But if it's not a writeback write we'd rather just bail out if
	 * there aren't any buckets ready to write to - it might take awhile and
	 * we might be starving btree writes for gc or something.
	 */

	if (s->write) {
		/*
		 * Writethrough write: We can't complete the write until we've
		 * updated the index. But we don't want to delay the write while
		 * we wait for buckets to be freed up, so just invalidate the
		 * rest of the write.
		 */
		op->skip = true;
		return bio_invalidate(cl);
	} else {
		/*
		 * From a cache miss, we can just insert the keys for the data
		 * we have written or bail out if we didn't do anything.
		 */
		op->insert_data_done = true;
		bio_put(bio);

		if (!bch_keylist_empty(&op->keys))
			continue_at(cl, bch_journal, bcache_wq);
		else
			closure_return(cl);
	}
}

/**
 * bch_insert_data - stick some data in the cache
 *
 * This is the starting point for any data to end up in a cache device; it could
 * be from a normal write, or a writeback write, or a write to a flash only
 * volume - it's also used by the moving garbage collector to compact data in
 * mostly empty buckets.
 *
 * It first writes the data to the cache, creating a list of keys to be inserted
 * (if the data had to be fragmented there will be multiple keys); after the
 * data is written it calls bch_journal, and after the keys have been added to
 * the next journal write they're inserted into the btree.
 *
 * It inserts the data in op->cache_bio; bi_sector is used for the key offset,
 * and op->inode is used for the key inode.
 *
 * If op->skip is true, instead of inserting the data it invalidates the region
 * of the cache represented by op->cache_bio and op->inode.
 */
void bch_insert_data(struct closure *cl)
{
	struct btree_op *op = container_of(cl, struct btree_op, cl);

	bch_keylist_init(&op->keys);
	bio_get(op->cache_bio);
	bch_insert_data_loop(cl);
}

void bch_btree_insert_async(struct closure *cl)
{
	struct btree_op *op = container_of(cl, struct btree_op, cl);
	struct search *s = container_of(op, struct search, op);

	if (bch_btree_insert(op, op->c)) {
		s->error		= -ENOMEM;
		op->insert_data_done	= true;
	}

	if (op->insert_data_done) {
		bch_keylist_free(&op->keys);
		closure_return(cl);
	} else
		continue_at(cl, bch_insert_data_loop, bcache_wq);
}

/* Common code for the make_request functions */

static void request_endio(struct bio *bio, int error)
{
	struct closure *cl = bio->bi_private;

	if (error) {
		struct search *s = container_of(cl, struct search, cl);
		s->error = error;
		/* Only cache read errors are recoverable */
		s->recoverable = false;
	}

	bio_put(bio);
	closure_put(cl);
}

void bch_cache_read_endio(struct bio *bio, int error)
{
	struct bbio *b = container_of(bio, struct bbio, bio);
	struct closure *cl = bio->bi_private;
	struct search *s = container_of(cl, struct search, cl);

	/*
	 * If the bucket was reused while our bio was in flight, we might have
	 * read the wrong data. Set s->error but not error so it doesn't get
	 * counted against the cache device, but we'll still reread the data
	 * from the backing device.
	 */

	if (error)
		s->error = error;
	else if (ptr_stale(s->op.c, &b->key, 0)) {
		atomic_long_inc(&s->op.c->cache_read_races);
		s->error = -EINTR;
	}

	bch_bbio_endio(s->op.c, bio, error, "reading from cache");
}

static void bio_complete(struct search *s)
{
	if (s->orig_bio) {
		int cpu, rw = bio_data_dir(s->orig_bio);
		unsigned long duration = jiffies - s->start_time;

		cpu = part_stat_lock();
		part_round_stats(cpu, &s->d->disk->part0);
		part_stat_add(cpu, &s->d->disk->part0, ticks[rw], duration);
		part_stat_unlock();

		trace_bcache_request_end(s, s->orig_bio);
		bio_endio(s->orig_bio, s->error);
		s->orig_bio = NULL;
	}
}

static void do_bio_hook(struct search *s)
{
	struct bio *bio = &s->bio.bio;
	memcpy(bio, s->orig_bio, sizeof(struct bio));

	bio->bi_end_io		= request_endio;
	bio->bi_private		= &s->cl;
	atomic_set(&bio->bi_cnt, 3);
}

static void search_free(struct closure *cl)
{
	struct search *s = container_of(cl, struct search, cl);
	bio_complete(s);

	if (s->op.cache_bio)
		bio_put(s->op.cache_bio);

	if (s->unaligned_bvec)
		mempool_free(s->bio.bio.bi_io_vec, s->d->unaligned_bvec);

	closure_debug_destroy(cl);
	mempool_free(s, s->d->c->search);
}

static struct search *search_alloc(struct bio *bio, struct bcache_device *d)
{
	struct bio_vec *bv;
	struct search *s = mempool_alloc(d->c->search, GFP_NOIO);
	memset(s, 0, offsetof(struct search, op.keys));

	__closure_init(&s->cl, NULL);

	s->op.inode		= d->id;
	s->op.c			= d->c;
	s->d			= d;
	s->op.lock		= -1;
	s->task			= current;
	s->orig_bio		= bio;
	s->write		= (bio->bi_rw & REQ_WRITE) != 0;
	s->op.flush_journal	= (bio->bi_rw & REQ_FLUSH) != 0;
	s->op.skip		= (bio->bi_rw & REQ_DISCARD) != 0;
	s->recoverable		= 1;
	s->start_time		= jiffies;
	do_bio_hook(s);

	if (bio->bi_size != bio_segments(bio) * PAGE_SIZE) {
		bv = mempool_alloc(d->unaligned_bvec, GFP_NOIO);
		memcpy(bv, bio_iovec(bio),
		       sizeof(struct bio_vec) * bio_segments(bio));

		s->bio.bio.bi_io_vec	= bv;
		s->unaligned_bvec	= 1;
	}

	return s;
}

static void btree_read_async(struct closure *cl)
{
	struct btree_op *op = container_of(cl, struct btree_op, cl);

	int ret = btree_root(search_recurse, op->c, op);

	if (ret == -EAGAIN)
		continue_at(cl, btree_read_async, bcache_wq);

	closure_return(cl);
}

/* Cached devices */

static void cached_dev_bio_complete(struct closure *cl)
{
	struct search *s = container_of(cl, struct search, cl);
	struct cached_dev *dc = container_of(s->d, struct cached_dev, disk);

	search_free(cl);
	cached_dev_put(dc);
}

/* Process reads */

static void cached_dev_read_complete(struct closure *cl)
{
	struct search *s = container_of(cl, struct search, cl);

	if (s->op.insert_collision)
		bch_mark_cache_miss_collision(s);

	if (s->op.cache_bio) {
		int i;
		struct bio_vec *bv;

		__bio_for_each_segment(bv, s->op.cache_bio, i, 0)
			__free_page(bv->bv_page);
	}

	cached_dev_bio_complete(cl);
}

static void request_read_error(struct closure *cl)
{
	struct search *s = container_of(cl, struct search, cl);
	struct bio_vec *bv;
	int i;

	if (s->recoverable) {
		/* The cache read failed, but we can retry from the backing
		 * device.
		 */
		pr_debug("recovering at sector %llu",
			 (uint64_t) s->orig_bio->bi_sector);

		s->error = 0;
		bv = s->bio.bio.bi_io_vec;
		do_bio_hook(s);
		s->bio.bio.bi_io_vec = bv;

		if (!s->unaligned_bvec)
			bio_for_each_segment(bv, s->orig_bio, i)
				bv->bv_offset = 0, bv->bv_len = PAGE_SIZE;
		else
			memcpy(s->bio.bio.bi_io_vec,
			       bio_iovec(s->orig_bio),
			       sizeof(struct bio_vec) *
			       bio_segments(s->orig_bio));

		/* XXX: invalidate cache */

		trace_bcache_read_retry(&s->bio.bio);
		closure_bio_submit(&s->bio.bio, &s->cl, s->d);
	}

	continue_at(cl, cached_dev_read_complete, NULL);
}

static void request_read_done(struct closure *cl)
{
	struct search *s = container_of(cl, struct search, cl);
	struct cached_dev *dc = container_of(s->d, struct cached_dev, disk);

	/*
	 * s->cache_bio != NULL implies that we had a cache miss; cache_bio now
	 * contains data ready to be inserted into the cache.
	 *
	 * First, we copy the data we just read from cache_bio's bounce buffers
	 * to the buffers the original bio pointed to:
	 */

	if (s->op.cache_bio) {
		struct bio_vec *src, *dst;
		unsigned src_offset, dst_offset, bytes;
		void *dst_ptr;

		bio_reset(s->op.cache_bio);
		s->op.cache_bio->bi_sector	= s->cache_miss->bi_sector;
		s->op.cache_bio->bi_bdev	= s->cache_miss->bi_bdev;
		s->op.cache_bio->bi_size	= s->cache_bio_sectors << 9;
		bch_bio_map(s->op.cache_bio, NULL);

		src = bio_iovec(s->op.cache_bio);
		dst = bio_iovec(s->cache_miss);
		src_offset = src->bv_offset;
		dst_offset = dst->bv_offset;
		dst_ptr = kmap(dst->bv_page);

		while (1) {
			if (dst_offset == dst->bv_offset + dst->bv_len) {
				kunmap(dst->bv_page);
				dst++;
				if (dst == bio_iovec_idx(s->cache_miss,
						s->cache_miss->bi_vcnt))
					break;

				dst_offset = dst->bv_offset;
				dst_ptr = kmap(dst->bv_page);
			}

			if (src_offset == src->bv_offset + src->bv_len) {
				src++;
				if (src == bio_iovec_idx(s->op.cache_bio,
						 s->op.cache_bio->bi_vcnt))
					BUG();

				src_offset = src->bv_offset;
			}

			bytes = min(dst->bv_offset + dst->bv_len - dst_offset,
				    src->bv_offset + src->bv_len - src_offset);

			memcpy(dst_ptr + dst_offset,
			       page_address(src->bv_page) + src_offset,
			       bytes);

			src_offset	+= bytes;
			dst_offset	+= bytes;
		}

		bio_put(s->cache_miss);
		s->cache_miss = NULL;
	}

	if (verify(dc, &s->bio.bio) && s->recoverable)
		bch_data_verify(s);

	bio_complete(s);

	if (s->op.cache_bio &&
	    !test_bit(CACHE_SET_STOPPING, &s->op.c->flags)) {
		s->op.type = BTREE_REPLACE;
		closure_call(&s->op.cl, bch_insert_data, NULL, cl);
	}

	continue_at(cl, cached_dev_read_complete, NULL);
}

static void request_read_done_bh(struct closure *cl)
{
	struct search *s = container_of(cl, struct search, cl);
	struct cached_dev *dc = container_of(s->d, struct cached_dev, disk);

	bch_mark_cache_accounting(s, !s->cache_miss, s->op.skip);

	if (s->error)
		continue_at_nobarrier(cl, request_read_error, bcache_wq);
	else if (s->op.cache_bio || verify(dc, &s->bio.bio))
		continue_at_nobarrier(cl, request_read_done, bcache_wq);
	else
		continue_at_nobarrier(cl, cached_dev_read_complete, NULL);
}

static int cached_dev_cache_miss(struct btree *b, struct search *s,
				 struct bio *bio, unsigned sectors)
{
	int ret = 0;
	unsigned reada;
	struct cached_dev *dc = container_of(s->d, struct cached_dev, disk);
	struct bio *miss;

	miss = bch_bio_split(bio, sectors, GFP_NOIO, s->d->bio_split);
	if (!miss)
		return -EAGAIN;

	if (miss == bio)
		s->op.lookup_done = true;

	miss->bi_end_io		= request_endio;
	miss->bi_private	= &s->cl;

	if (s->cache_miss || s->op.skip)
		goto out_submit;

	if (miss != bio ||
	    (bio->bi_rw & REQ_RAHEAD) ||
	    (bio->bi_rw & REQ_META) ||
	    s->op.c->gc_stats.in_use >= CUTOFF_CACHE_READA)
		reada = 0;
	else {
		reada = min(dc->readahead >> 9,
			    sectors - bio_sectors(miss));

		if (bio_end(miss) + reada > bdev_sectors(miss->bi_bdev))
			reada = bdev_sectors(miss->bi_bdev) - bio_end(miss);
	}

	s->cache_bio_sectors = bio_sectors(miss) + reada;
	s->op.cache_bio = bio_alloc_bioset(GFP_NOWAIT,
			DIV_ROUND_UP(s->cache_bio_sectors, PAGE_SECTORS),
			dc->disk.bio_split);

	if (!s->op.cache_bio)
		goto out_submit;

	s->op.cache_bio->bi_sector	= miss->bi_sector;
	s->op.cache_bio->bi_bdev	= miss->bi_bdev;
	s->op.cache_bio->bi_size	= s->cache_bio_sectors << 9;

	s->op.cache_bio->bi_end_io	= request_endio;
	s->op.cache_bio->bi_private	= &s->cl;

	/* btree_search_recurse()'s btree iterator is no good anymore */
	ret = -EINTR;
	if (!bch_btree_insert_check_key(b, &s->op, s->op.cache_bio))
		goto out_put;

	bch_bio_map(s->op.cache_bio, NULL);
	if (bch_bio_alloc_pages(s->op.cache_bio, __GFP_NOWARN|GFP_NOIO))
		goto out_put;

	s->cache_miss = miss;
	bio_get(s->op.cache_bio);

	trace_bcache_cache_miss(s->orig_bio);
	closure_bio_submit(s->op.cache_bio, &s->cl, s->d);

	return ret;
out_put:
	bio_put(s->op.cache_bio);
	s->op.cache_bio = NULL;
out_submit:
	closure_bio_submit(miss, &s->cl, s->d);
	return ret;
}

static void request_read(struct cached_dev *dc, struct search *s)
{
	struct closure *cl = &s->cl;

	check_should_skip(dc, s);
	closure_call(&s->op.cl, btree_read_async, NULL, cl);

	continue_at(cl, request_read_done_bh, NULL);
}

/* Process writes */

static void cached_dev_write_complete(struct closure *cl)
{
	struct search *s = container_of(cl, struct search, cl);
	struct cached_dev *dc = container_of(s->d, struct cached_dev, disk);