[PATCH] elevator: move the backmerging logic into the elevator core

Right now, every IO scheduler implements its own backmerging (except for
noop, which does no merging). That results in duplicated code for
essentially the same operation, which is never a good thing. This patch
moves the backmerging out of the io schedulers and into the elevator
core. We save 1.6kb of text and as a bonus get backmerging for noop as
well. Win-win!

Signed-off-by: Jens Axboe <[e-mail]>

block/as-iosched.c

···
 #include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/compiler.h>
-#include <linux/hash.h>
 #include <linux/rbtree.h>
 #include <linux/interrupt.h>
 
···
 
 	struct as_rq *next_arq[2];	/* next in sort order */
 	sector_t last_sector[2];	/* last REQ_SYNC & REQ_ASYNC sectors */
-	struct hlist_head *hash;	/* request hash */
 
 	unsigned long exit_prob;	/* probability a task will exit while
 					   being waited on */
···
 	struct request *request;
 
 	struct io_context *io_context;	/* The submitting task */
-
-	/*
-	 * request hash, key is the ending offset (for back merge lookup)
-	 */
-	struct hlist_node hash;
 
 	/*
 	 * expire fifo
···
 	}
 
 	put_io_context(arq->io_context);
-}
-
-/*
- * the back merge hash support functions
- */
-static const int as_hash_shift = 6;
-#define AS_HASH_BLOCK(sec)	((sec) >> 3)
-#define AS_HASH_FN(sec)		(hash_long(AS_HASH_BLOCK((sec)), as_hash_shift))
-#define AS_HASH_ENTRIES		(1 << as_hash_shift)
-#define rq_hash_key(rq)		((rq)->sector + (rq)->nr_sectors)
-
-static inline void __as_del_arq_hash(struct as_rq *arq)
-{
-	hlist_del_init(&arq->hash);
-}
-
-static inline void as_del_arq_hash(struct as_rq *arq)
-{
-	if (!hlist_unhashed(&arq->hash))
-		__as_del_arq_hash(arq);
-}
-
-static void as_add_arq_hash(struct as_data *ad, struct as_rq *arq)
-{
-	struct request *rq = arq->request;
-
-	BUG_ON(!hlist_unhashed(&arq->hash));
-
-	hlist_add_head(&arq->hash, &ad->hash[AS_HASH_FN(rq_hash_key(rq))]);
-}
-
-/*
- * move hot entry to front of chain
- */
-static inline void as_hot_arq_hash(struct as_data *ad, struct as_rq *arq)
-{
-	struct request *rq = arq->request;
-	struct hlist_head *head = &ad->hash[AS_HASH_FN(rq_hash_key(rq))];
-
-	if (hlist_unhashed(&arq->hash)) {
-		WARN_ON(1);
-		return;
-	}
-
-	if (&arq->hash != head->first) {
-		hlist_del(&arq->hash);
-		hlist_add_head(&arq->hash, head);
-	}
-}
-
-static struct request *as_find_arq_hash(struct as_data *ad, sector_t offset)
-{
-	struct hlist_head *hash_list = &ad->hash[AS_HASH_FN(offset)];
-	struct hlist_node *entry, *next;
-	struct as_rq *arq;
-
-	hlist_for_each_entry_safe(arq, entry, next, hash_list, hash) {
-		struct request *__rq = arq->request;
-
-		BUG_ON(hlist_unhashed(&arq->hash));
-
-		if (!rq_mergeable(__rq)) {
-			as_del_arq_hash(arq);
-			continue;
-		}
-
-		if (rq_hash_key(__rq) == offset)
-			return __rq;
-	}
-
-	return NULL;
 }
 
 /*
···
 		ad->next_arq[data_dir] = as_find_next_arq(ad, arq);
 
 	list_del_init(&arq->fifo);
-	as_del_arq_hash(arq);
 	as_del_arq_rb(ad, arq);
 }
 
···
 	}
 
 	as_add_arq_rb(ad, arq);
-	if (rq_mergeable(arq->request))
-		as_add_arq_hash(ad, arq);
 
 	/*
 	 * set expire time (only used for reads) and add to fifo list
···
 	struct as_data *ad = q->elevator->elevator_data;
 	sector_t rb_key = bio->bi_sector + bio_sectors(bio);
 	struct request *__rq;
-	int ret;
-
-	/*
-	 * see if the merge hash can satisfy a back merge
-	 */
-	__rq = as_find_arq_hash(ad, bio->bi_sector);
-	if (__rq) {
-		BUG_ON(__rq->sector + __rq->nr_sectors != bio->bi_sector);
-
-		if (elv_rq_merge_ok(__rq, bio)) {
-			ret = ELEVATOR_BACK_MERGE;
-			goto out;
-		}
-	}
 
 	/*
 	 * check for front merge
 	 */
 	__rq = as_find_arq_rb(ad, rb_key, bio_data_dir(bio));
-	if (__rq) {
-		BUG_ON(rb_key != rq_rb_key(__rq));
-
-		if (elv_rq_merge_ok(__rq, bio)) {
-			ret = ELEVATOR_FRONT_MERGE;
-			goto out;
-		}
+	if (__rq && elv_rq_merge_ok(__rq, bio)) {
+		*req = __rq;
+		return ELEVATOR_FRONT_MERGE;
 	}
 
 	return ELEVATOR_NO_MERGE;
-out:
-	if (ret) {
-		if (rq_mergeable(__rq))
-			as_hot_arq_hash(ad, RQ_DATA(__rq));
-	}
-	*req = __rq;
-	return ret;
 }
 
 static void as_merged_request(request_queue_t *q, struct request *req)
 {
 	struct as_data *ad = q->elevator->elevator_data;
 	struct as_rq *arq = RQ_DATA(req);
-
-	/*
-	 * hash always needs to be repositioned, key is end sector
-	 */
-	as_del_arq_hash(arq);
-	as_add_arq_hash(ad, arq);
 
 	/*
 	 * if the merge was a front merge, we need to reposition request
···
 
 	BUG_ON(!arq);
 	BUG_ON(!anext);
-
-	/*
-	 * reposition arq (this is the merged request) in hash, and in rbtree
-	 * in case of a front merge
-	 */
-	as_del_arq_hash(arq);
-	as_add_arq_hash(ad, arq);
 
 	if (rq_rb_key(req) != arq->rb_key) {
 		as_del_arq_rb(ad, arq);
···
 		arq->request = rq;
 		arq->state = AS_RQ_PRESCHED;
 		arq->io_context = NULL;
-		INIT_HLIST_NODE(&arq->hash);
 		INIT_LIST_HEAD(&arq->fifo);
 		rq->elevator_private = arq;
 		return 0;
···
 
 	mempool_destroy(ad->arq_pool);
 	put_io_context(ad->io_context);
-	kfree(ad->hash);
 	kfree(ad);
 }
 
···
 static void *as_init_queue(request_queue_t *q, elevator_t *e)
 {
 	struct as_data *ad;
-	int i;
 
 	if (!arq_pool)
 		return NULL;
···
 
 	ad->q = q; /* Identify what queue the data belongs to */
 
-	ad->hash = kmalloc_node(sizeof(struct hlist_head)*AS_HASH_ENTRIES,
-				GFP_KERNEL, q->node);
-	if (!ad->hash) {
-		kfree(ad);
-		return NULL;
-	}
-
 	ad->arq_pool = mempool_create_node(BLKDEV_MIN_RQ, mempool_alloc_slab,
 				mempool_free_slab, arq_pool, q->node);
 	if (!ad->arq_pool) {
-		kfree(ad->hash);
 		kfree(ad);
 		return NULL;
 	}
···
 	ad->antic_timer.data = (unsigned long)q;
 	init_timer(&ad->antic_timer);
 	INIT_WORK(&ad->antic_work, as_work_handler, q);
-
-	for (i = 0; i < AS_HASH_ENTRIES; i++)
-		INIT_HLIST_HEAD(&ad->hash[i]);
 
 	INIT_LIST_HEAD(&ad->fifo_list[REQ_SYNC]);
 	INIT_LIST_HEAD(&ad->fifo_list[REQ_ASYNC]);

block/cfq-iosched.c

···
 #define CFQ_QHASH_ENTRIES	(1 << CFQ_QHASH_SHIFT)
 #define list_entry_qhash(entry)	hlist_entry((entry), struct cfq_queue, cfq_hash)
 
-/*
- * for the hash of crq inside the cfqq
- */
-#define CFQ_MHASH_SHIFT		6
-#define CFQ_MHASH_BLOCK(sec)	((sec) >> 3)
-#define CFQ_MHASH_ENTRIES	(1 << CFQ_MHASH_SHIFT)
-#define CFQ_MHASH_FN(sec)	hash_long(CFQ_MHASH_BLOCK(sec), CFQ_MHASH_SHIFT)
-#define rq_hash_key(rq)		((rq)->sector + (rq)->nr_sectors)
-#define list_entry_hash(ptr)	hlist_entry((ptr), struct cfq_rq, hash)
-
 #define list_entry_cfqq(ptr)	list_entry((ptr), struct cfq_queue, cfq_list)
 #define list_entry_fifo(ptr)	list_entry((ptr), struct request, queuelist)
 
···
 	 * cfqq lookup hash
 	 */
 	struct hlist_head *cfq_hash;
-
-	/*
-	 * global crq hash for all queues
-	 */
-	struct hlist_head *crq_hash;
 
 	mempool_t *crq_pool;
 
···
 	struct rb_node rb_node;
 	sector_t rb_key;
 	struct request *request;
-	struct hlist_node hash;
 
 	struct cfq_queue *cfq_queue;
 	struct cfq_io_context *io_context;
···
 static struct cfq_queue *cfq_find_cfq_hash(struct cfq_data *, unsigned int, unsigned short);
 static void cfq_dispatch_insert(request_queue_t *, struct cfq_rq *);
 static struct cfq_queue *cfq_get_queue(struct cfq_data *cfqd, unsigned int key, struct task_struct *tsk, gfp_t gfp_mask);
-
-/*
- * lots of deadline iosched dupes, can be abstracted later...
- */
-static inline void cfq_del_crq_hash(struct cfq_rq *crq)
-{
-	hlist_del_init(&crq->hash);
-}
-
-static inline void cfq_add_crq_hash(struct cfq_data *cfqd, struct cfq_rq *crq)
-{
-	const int hash_idx = CFQ_MHASH_FN(rq_hash_key(crq->request));
-
-	hlist_add_head(&crq->hash, &cfqd->crq_hash[hash_idx]);
-}
-
-static struct request *cfq_find_rq_hash(struct cfq_data *cfqd, sector_t offset)
-{
-	struct hlist_head *hash_list = &cfqd->crq_hash[CFQ_MHASH_FN(offset)];
-	struct hlist_node *entry, *next;
-
-	hlist_for_each_safe(entry, next, hash_list) {
-		struct cfq_rq *crq = list_entry_hash(entry);
-		struct request *__rq = crq->request;
-
-		if (!rq_mergeable(__rq)) {
-			cfq_del_crq_hash(crq);
-			continue;
-		}
-
-		if (rq_hash_key(__rq) == offset)
-			return __rq;
-	}
-
-	return NULL;
-}
 
 /*
  * scheduler run of queue, if there are requests pending and no one in the
···
 
 	list_del_init(&rq->queuelist);
 	cfq_del_crq_rb(crq);
-	cfq_del_crq_hash(crq);
 }
 
 static int
···
 {
 	struct cfq_data *cfqd = q->elevator->elevator_data;
 	struct request *__rq;
-	int ret;
-
-	__rq = cfq_find_rq_hash(cfqd, bio->bi_sector);
-	if (__rq && elv_rq_merge_ok(__rq, bio)) {
-		ret = ELEVATOR_BACK_MERGE;
-		goto out;
-	}
 
 	__rq = cfq_find_rq_fmerge(cfqd, bio);
 	if (__rq && elv_rq_merge_ok(__rq, bio)) {
-		ret = ELEVATOR_FRONT_MERGE;
-		goto out;
+		*req = __rq;
+		return ELEVATOR_FRONT_MERGE;
 	}
 
 	return ELEVATOR_NO_MERGE;
-out:
-	*req = __rq;
-	return ret;
 }
 
 static void cfq_merged_request(request_queue_t *q, struct request *req)
 {
-	struct cfq_data *cfqd = q->elevator->elevator_data;
 	struct cfq_rq *crq = RQ_DATA(req);
-
-	cfq_del_crq_hash(crq);
-	cfq_add_crq_hash(cfqd, crq);
 
 	if (rq_rb_key(req) != crq->rb_key) {
 		struct cfq_queue *cfqq = crq->cfq_queue;
···
 
 	list_add_tail(&rq->queuelist, &cfqq->fifo);
 
-	if (rq_mergeable(rq))
-		cfq_add_crq_hash(cfqd, crq);
-
 	cfq_crq_enqueued(cfqd, cfqq, crq);
 }
 
···
 		RB_CLEAR_NODE(&crq->rb_node);
 		crq->rb_key = 0;
 		crq->request = rq;
-		INIT_HLIST_NODE(&crq->hash);
 		crq->cfq_queue = cfqq;
 		crq->io_context = cic;
 
···
 	cfq_shutdown_timer_wq(cfqd);
 
 	mempool_destroy(cfqd->crq_pool);
-	kfree(cfqd->crq_hash);
 	kfree(cfqd->cfq_hash);
 	kfree(cfqd);
 }
···
 	INIT_LIST_HEAD(&cfqd->empty_list);
 	INIT_LIST_HEAD(&cfqd->cic_list);
 
-	cfqd->crq_hash = kmalloc(sizeof(struct hlist_head) * CFQ_MHASH_ENTRIES, GFP_KERNEL);
-	if (!cfqd->crq_hash)
-		goto out_crqhash;
-
 	cfqd->cfq_hash = kmalloc(sizeof(struct hlist_head) * CFQ_QHASH_ENTRIES, GFP_KERNEL);
 	if (!cfqd->cfq_hash)
-		goto out_cfqhash;
+		goto out_crqhash;
 
 	cfqd->crq_pool = mempool_create_slab_pool(BLKDEV_MIN_RQ, crq_pool);
 	if (!cfqd->crq_pool)
 		goto out_crqpool;
 
-	for (i = 0; i < CFQ_MHASH_ENTRIES; i++)
-		INIT_HLIST_HEAD(&cfqd->crq_hash[i]);
 	for (i = 0; i < CFQ_QHASH_ENTRIES; i++)
 		INIT_HLIST_HEAD(&cfqd->cfq_hash[i]);
 
···
 	return cfqd;
 out_crqpool:
 	kfree(cfqd->cfq_hash);
-out_cfqhash:
-	kfree(cfqd->crq_hash);
 out_crqhash:
 	kfree(cfqd);
 	return NULL;

block/deadline-iosched.c

···
 #include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/compiler.h>
-#include <linux/hash.h>
 #include <linux/rbtree.h>
 
 /*
···
 static const int writes_starved = 2;    /* max times reads can starve a write */
 static const int fifo_batch = 16;       /* # of sequential requests treated as one
 				     by the above parameters. For throughput. */
-
-static const int deadline_hash_shift = 5;
-#define DL_HASH_BLOCK(sec)	((sec) >> 3)
-#define DL_HASH_FN(sec)		(hash_long(DL_HASH_BLOCK((sec)), deadline_hash_shift))
-#define DL_HASH_ENTRIES		(1 << deadline_hash_shift)
-#define rq_hash_key(rq)		((rq)->sector + (rq)->nr_sectors)
-#define ON_HASH(drq)		(!hlist_unhashed(&(drq)->hash))
 
 struct deadline_data {
 	/*
···
 	 * next in sort order. read, write or both are NULL
 	 */
 	struct deadline_rq *next_drq[2];
-	struct hlist_head *hash;	/* request hash */
 	unsigned int batching;		/* number of sequential requests made */
 	sector_t last_sector;		/* head position */
 	unsigned int starved;		/* times reads have starved writes */
···
 	struct request *request;
 
 	/*
-	 * request hash, key is the ending offset (for back merge lookup)
-	 */
-	struct hlist_node hash;
-
-	/*
 	 * expire fifo
 	 */
 	struct list_head fifo;
···
 static kmem_cache_t *drq_pool;
 
 #define RQ_DATA(rq)	((struct deadline_rq *) (rq)->elevator_private)
-
-/*
- * the back merge hash support functions
- */
-static inline void __deadline_del_drq_hash(struct deadline_rq *drq)
-{
-	hlist_del_init(&drq->hash);
-}
-
-static inline void deadline_del_drq_hash(struct deadline_rq *drq)
-{
-	if (ON_HASH(drq))
-		__deadline_del_drq_hash(drq);
-}
-
-static inline void
-deadline_add_drq_hash(struct deadline_data *dd, struct deadline_rq *drq)
-{
-	struct request *rq = drq->request;
-
-	BUG_ON(ON_HASH(drq));
-
-	hlist_add_head(&drq->hash, &dd->hash[DL_HASH_FN(rq_hash_key(rq))]);
-}
-
-/*
- * move hot entry to front of chain
- */
-static inline void
-deadline_hot_drq_hash(struct deadline_data *dd, struct deadline_rq *drq)
-{
-	struct request *rq = drq->request;
-	struct hlist_head *head = &dd->hash[DL_HASH_FN(rq_hash_key(rq))];
-
-	if (ON_HASH(drq) && &drq->hash != head->first) {
-		hlist_del(&drq->hash);
-		hlist_add_head(&drq->hash, head);
-	}
-}
-
-static struct request *
-deadline_find_drq_hash(struct deadline_data *dd, sector_t offset)
-{
-	struct hlist_head *hash_list = &dd->hash[DL_HASH_FN(offset)];
-	struct hlist_node *entry, *next;
-	struct deadline_rq *drq;
-
-	hlist_for_each_entry_safe(drq, entry, next, hash_list, hash) {
-		struct request *__rq = drq->request;
-
-		BUG_ON(!ON_HASH(drq));
-
-		if (!rq_mergeable(__rq)) {
-			__deadline_del_drq_hash(drq);
-			continue;
-		}
-
-		if (rq_hash_key(__rq) == offset)
-			return __rq;
-	}
-
-	return NULL;
-}
 
 /*
  * rb tree support functions
···
 {
 	struct deadline_data *dd = q->elevator->elevator_data;
 	struct deadline_rq *drq = RQ_DATA(rq);
-
 	const int data_dir = rq_data_dir(drq->request);
 
 	deadline_add_drq_rb(dd, drq);
+
 	/*
 	 * set expire time (only used for reads) and add to fifo list
 	 */
 	drq->expires = jiffies + dd->fifo_expire[data_dir];
 	list_add_tail(&drq->fifo, &dd->fifo_list[data_dir]);
-
-	if (rq_mergeable(rq))
-		deadline_add_drq_hash(dd, drq);
 }
 
 /*
- * remove rq from rbtree, fifo, and hash
+ * remove rq from rbtree and fifo.
  */
 static void deadline_remove_request(request_queue_t *q, struct request *rq)
 {
···
 
 	list_del_init(&drq->fifo);
 	deadline_del_drq_rb(dd, drq);
-	deadline_del_drq_hash(drq);
 }
 
 static int
···
 	struct deadline_data *dd = q->elevator->elevator_data;
 	struct request *__rq;
 	int ret;
-
-	/*
-	 * see if the merge hash can satisfy a back merge
-	 */
-	__rq = deadline_find_drq_hash(dd, bio->bi_sector);
-	if (__rq) {
-		BUG_ON(__rq->sector + __rq->nr_sectors != bio->bi_sector);
-
-		if (elv_rq_merge_ok(__rq, bio)) {
-			ret = ELEVATOR_BACK_MERGE;
-			goto out;
-		}
-	}
 
 	/*
 	 * check for front merge
···
 
 	return ELEVATOR_NO_MERGE;
 out:
-	if (ret)
-		deadline_hot_drq_hash(dd, RQ_DATA(__rq));
 	*req = __rq;
 	return ret;
 }
···
 {
 	struct deadline_data *dd = q->elevator->elevator_data;
 	struct deadline_rq *drq = RQ_DATA(req);
-
-	/*
-	 * hash always needs to be repositioned, key is end sector
-	 */
-	deadline_del_drq_hash(drq);
-	deadline_add_drq_hash(dd, drq);
 
 	/*
 	 * if the merge was a front merge, we need to reposition request
···
 
 	BUG_ON(!drq);
 	BUG_ON(!dnext);
-
-	/*
-	 * reposition drq (this is the merged request) in hash, and in rbtree
-	 * in case of a front merge
-	 */
-	deadline_del_drq_hash(drq);
-	deadline_add_drq_hash(dd, drq);
 
 	if (rq_rb_key(req) != drq->rb_key) {
 		deadline_del_drq_rb(dd, drq);
···
 	BUG_ON(!list_empty(&dd->fifo_list[WRITE]));
 
 	mempool_destroy(dd->drq_pool);
-	kfree(dd->hash);
 	kfree(dd);
 }
 
···
 static void *deadline_init_queue(request_queue_t *q, elevator_t *e)
 {
 	struct deadline_data *dd;
-	int i;
 
 	if (!drq_pool)
 		return NULL;
···
 		return NULL;
 	memset(dd, 0, sizeof(*dd));
 
-	dd->hash = kmalloc_node(sizeof(struct hlist_head)*DL_HASH_ENTRIES,
-				GFP_KERNEL, q->node);
-	if (!dd->hash) {
-		kfree(dd);
-		return NULL;
-	}
-
 	dd->drq_pool = mempool_create_node(BLKDEV_MIN_RQ, mempool_alloc_slab,
 					mempool_free_slab, drq_pool, q->node);
 	if (!dd->drq_pool) {
-		kfree(dd->hash);
 		kfree(dd);
 		return NULL;
 	}
-
-	for (i = 0; i < DL_HASH_ENTRIES; i++)
-		INIT_HLIST_HEAD(&dd->hash[i]);
 
 	INIT_LIST_HEAD(&dd->fifo_list[READ]);
 	INIT_LIST_HEAD(&dd->fifo_list[WRITE]);
···
 		memset(drq, 0, sizeof(*drq));
 		RB_CLEAR_NODE(&drq->rb_node);
 		drq->request = rq;
-
-		INIT_HLIST_NODE(&drq->hash);
 
 		INIT_LIST_HEAD(&drq->fifo);
 

block/elevator.c

···
 #include <linux/compiler.h>
 #include <linux/delay.h>
 #include <linux/blktrace_api.h>
+#include <linux/hash.h>
 
 #include <asm/uaccess.h>
 
 static DEFINE_SPINLOCK(elv_list_lock);
 static LIST_HEAD(elv_list);
+
+/*
+ * Merge hash stuff.
+ */
+static const int elv_hash_shift = 6;
+#define ELV_HASH_BLOCK(sec)	((sec) >> 3)
+#define ELV_HASH_FN(sec)	(hash_long(ELV_HASH_BLOCK((sec)), elv_hash_shift))
+#define ELV_HASH_ENTRIES	(1 << elv_hash_shift)
+#define rq_hash_key(rq)		((rq)->sector + (rq)->nr_sectors)
+#define ELV_ON_HASH(rq)		(!hlist_unhashed(&(rq)->hash))
 
 /*
  * can we safely merge with this request?
···
 
 static elevator_t *elevator_alloc(struct elevator_type *e)
 {
-	elevator_t *eq = kmalloc(sizeof(elevator_t), GFP_KERNEL);
-	if (eq) {
-		memset(eq, 0, sizeof(*eq));
-		eq->ops = &e->ops;
-		eq->elevator_type = e;
-		kobject_init(&eq->kobj);
-		snprintf(eq->kobj.name, KOBJ_NAME_LEN, "%s", "iosched");
-		eq->kobj.ktype = &elv_ktype;
-		mutex_init(&eq->sysfs_lock);
-	} else {
-		elevator_put(e);
-	}
+	elevator_t *eq;
+	int i;
+
+	eq = kmalloc(sizeof(elevator_t), GFP_KERNEL);
+	if (unlikely(!eq))
+		goto err;
+
+	memset(eq, 0, sizeof(*eq));
+	eq->ops = &e->ops;
+	eq->elevator_type = e;
+	kobject_init(&eq->kobj);
+	snprintf(eq->kobj.name, KOBJ_NAME_LEN, "%s", "iosched");
+	eq->kobj.ktype = &elv_ktype;
+	mutex_init(&eq->sysfs_lock);
+
+	eq->hash = kmalloc(sizeof(struct hlist_head) * ELV_HASH_ENTRIES, GFP_KERNEL);
+	if (!eq->hash)
+		goto err;
+
+	for (i = 0; i < ELV_HASH_ENTRIES; i++)
+		INIT_HLIST_HEAD(&eq->hash[i]);
+
 	return eq;
+err:
+	kfree(eq);
+	elevator_put(e);
+	return NULL;
 }
 
 static void elevator_release(struct kobject *kobj)
 {
 	elevator_t *e = container_of(kobj, elevator_t, kobj);
+
 	elevator_put(e->elevator_type);
+	kfree(e->hash);
 	kfree(e);
 }
 
···
 	kobject_put(&e->kobj);
 }
 
+static inline void __elv_rqhash_del(struct request *rq)
+{
+	hlist_del_init(&rq->hash);
+}
+
+static void elv_rqhash_del(request_queue_t *q, struct request *rq)
+{
+	if (ELV_ON_HASH(rq))
+		__elv_rqhash_del(rq);
+}
+
+static void elv_rqhash_add(request_queue_t *q, struct request *rq)
+{
+	elevator_t *e = q->elevator;
+
+	BUG_ON(ELV_ON_HASH(rq));
+	hlist_add_head(&rq->hash, &e->hash[ELV_HASH_FN(rq_hash_key(rq))]);
+}
+
+static void elv_rqhash_reposition(request_queue_t *q, struct request *rq)
+{
+	__elv_rqhash_del(rq);
+	elv_rqhash_add(q, rq);
+}
+
+static struct request *elv_rqhash_find(request_queue_t *q, sector_t offset)
+{
+	elevator_t *e = q->elevator;
+	struct hlist_head *hash_list = &e->hash[ELV_HASH_FN(offset)];
+	struct hlist_node *entry, *next;
+	struct request *rq;
+
+	hlist_for_each_entry_safe(rq, entry, next, hash_list, hash) {
+		BUG_ON(!ELV_ON_HASH(rq));
+
+		if (unlikely(!rq_mergeable(rq))) {
+			__elv_rqhash_del(rq);
+			continue;
+		}
+
+		if (rq_hash_key(rq) == offset)
+			return rq;
+	}
+
+	return NULL;
+}
+
 /*
  * Insert rq into dispatch queue of q.  Queue lock must be held on
  * entry.  If sort != 0, rq is sort-inserted; otherwise, rq will be
···
 
 	if (q->last_merge == rq)
 		q->last_merge = NULL;
+
+	elv_rqhash_del(q, rq);
+
 	q->nr_sorted--;
 
 	boundary = q->end_sector;
···
 	list_add(&rq->queuelist, entry);
 }
 
+/*
+ * This should be in elevator.h, but that requires pulling in rq and q
+ */
+void elv_dispatch_add_tail(struct request_queue *q, struct request *rq)
+{
+	if (q->last_merge == rq)
+		q->last_merge = NULL;
+
+	elv_rqhash_del(q, rq);
+
+	q->nr_sorted--;
+
+	q->end_sector = rq_end_sector(rq);
+	q->boundary_rq = rq;
+	list_add_tail(&rq->queuelist, &q->queue_head);
+}
+
 int elv_merge(request_queue_t *q, struct request **req, struct bio *bio)
 {
 	elevator_t *e = q->elevator;
+	struct request *__rq;
 	int ret;
 
+	/*
+	 * First try one-hit cache.
+	 */
 	if (q->last_merge) {
 		ret = elv_try_merge(q->last_merge, bio);
 		if (ret != ELEVATOR_NO_MERGE) {
 			*req = q->last_merge;
 			return ret;
 		}
+	}
+
+	/*
+	 * See if our hash lookup can find a potential backmerge.
+	 */
+	__rq = elv_rqhash_find(q, bio->bi_sector);
+	if (__rq && elv_rq_merge_ok(__rq, bio)) {
+		*req = __rq;
+		return ELEVATOR_BACK_MERGE;
 	}
 
 	if (e->ops->elevator_merge_fn)
···
 	if (e->ops->elevator_merged_fn)
 		e->ops->elevator_merged_fn(q, rq);
 
+	elv_rqhash_reposition(q, rq);
+
 	q->last_merge = rq;
 }
 
···
 
 	if (e->ops->elevator_merge_req_fn)
 		e->ops->elevator_merge_req_fn(q, rq, next);
-	q->nr_sorted--;
 
+	elv_rqhash_reposition(q, rq);
+	elv_rqhash_del(q, next);
+
+	q->nr_sorted--;
 	q->last_merge = rq;
 }
 
···
 		BUG_ON(!blk_fs_request(rq));
 		rq->cmd_flags |= REQ_SORTED;
 		q->nr_sorted++;
-		if (q->last_merge == NULL && rq_mergeable(rq))
-			q->last_merge = rq;
+		if (rq_mergeable(rq)) {
+			elv_rqhash_add(q, rq);
+			if (!q->last_merge)
+				q->last_merge = rq;
+		}
+
 		/*
 		 * Some ioscheds (cfq) run q->request_fn directly, so
 		 * rq cannot be accessed after calling
···
 void elv_dequeue_request(request_queue_t *q, struct request *rq)
 {
 	BUG_ON(list_empty(&rq->queuelist));
+	BUG_ON(ELV_ON_HASH(rq));
 
 	list_del_init(&rq->queuelist);
 

block/ll_rw_blk.c

···
 {
 	INIT_LIST_HEAD(&rq->queuelist);
 	INIT_LIST_HEAD(&rq->donelist);
+	INIT_HLIST_NODE(&rq->hash);
 
 	rq->errors = 0;
 	rq->rq_status = RQ_ACTIVE;
···
 		int priv = req->cmd_flags & REQ_ELVPRIV;
 
 		BUG_ON(!list_empty(&req->queuelist));
+		BUG_ON(!hlist_unhashed(&req->hash));
 
 		blk_free_request(q, req);
 		freed_request(q, rw, priv);

include/linux/blkdev.h

···
 	struct bio *bio;
 	struct bio *biotail;
 
+	struct hlist_node hash;	/* merge hash */
+
 	void *elevator_private;
 	void *completion_data;
 
···
 static inline void blkdev_dequeue_request(struct request *req)
 {
 	elv_dequeue_request(req->q, req);
-}
-
-/*
- * This should be in elevator.h, but that requires pulling in rq and q
- */
-static inline void elv_dispatch_add_tail(struct request_queue *q,
-					 struct request *rq)
-{
-	if (q->last_merge == rq)
-		q->last_merge = NULL;
-	q->nr_sorted--;
-
-	q->end_sector = rq_end_sector(rq);
-	q->boundary_rq = rq;
-	list_add_tail(&rq->queuelist, &q->queue_head);
 }
 
 /*

include/linux/elevator.h

···
 	struct kobject kobj;
 	struct elevator_type *elevator_type;
 	struct mutex sysfs_lock;
+	struct hlist_head *hash;
 };
 
 /*
  * block elevator interface
  */
 extern void elv_dispatch_sort(request_queue_t *, struct request *);
+extern void elv_dispatch_add_tail(request_queue_t *, struct request *);
 extern void elv_add_request(request_queue_t *, struct request *, int, int);
 extern void __elv_add_request(request_queue_t *, struct request *, int, int);
 extern void elv_insert(request_queue_t *, struct request *, int);