From: Joe Thornber <ejt@redhat.com>
Subject: dm cache: promotion optimisation for writes

If a write triggers promotion and covers a whole block we can avoid a
copy.

FIXME:
  CC [M]  drivers/md/dm-cache-target.o
drivers/md/dm-cache-target.c: In function ‘issue_copy’:
drivers/md/dm-cache-target.c:974: warning: unused variable ‘bio’
drivers/md/dm-cache-target.c: At top level:
drivers/md/dm-cache-target.c:943: warning: ‘issue_overwrite’ defined but not used
drivers/md/dm-cache-target.c:953: warning: ‘bio_writes_complete_block’ defined but not used

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm-cache-target.c |   58 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 56 insertions(+), 2 deletions(-)

Index: linux/drivers/md/dm-cache-target.c
===================================================================
--- linux.orig/drivers/md/dm-cache-target.c
+++ linux/drivers/md/dm-cache-target.c
@@ -256,6 +256,7 @@ struct dm_cache_migration {
 	bool writeback:1;
 	bool demote:1;
 	bool promote:1;
+	bool requeue_holder:1;
 
 	struct dm_bio_prison_cell *old_ocell;
 	struct dm_bio_prison_cell *new_ocell;
@@ -865,7 +866,12 @@ static void migration_success_post_commi
 			cleanup_migration(mg);
 
 	} else {
-		cell_defer(cache, mg->new_ocell, true);
+		if (mg->requeue_holder)
+			cell_defer(cache, mg->new_ocell, true);
+		else {
+			bio_endio(mg->new_ocell->holder, 0);
+			cell_defer(cache, mg->new_ocell, false);
+		}
 		clear_dirty(cache, mg->new_oblock, mg->cblock);
 		cleanup_migration(mg);
 	}
@@ -914,6 +920,42 @@ static void issue_copy_real(struct dm_ca
 		migration_failure(mg);
 }
 
+static void overwrite_endio(struct bio *bio, int err)
+{
+	struct dm_cache_migration *mg = bio->bi_private;
+	struct cache *cache = mg->cache;
+	size_t pb_data_size = get_per_bio_data_size(cache);
+	struct per_bio_data *pb = get_per_bio_data(bio, pb_data_size);
+	unsigned long flags;
+
+	if (err)
+		mg->err = true;
+
+	spin_lock_irqsave(&cache->lock, flags);
+	list_add_tail(&mg->list, &cache->completed_migrations);
+	unhook_bio(&pb->hook_info, bio);
+	mg->requeue_holder = false;
+	spin_unlock_irqrestore(&cache->lock, flags);
+
+	wake_worker(cache);
+}
+
+static void issue_overwrite(struct dm_cache_migration *mg, struct bio *bio)
+{
+	size_t pb_data_size = get_per_bio_data_size(mg->cache);
+	struct per_bio_data *pb = get_per_bio_data(bio, pb_data_size);
+
+	hook_bio(&pb->hook_info, bio, overwrite_endio, mg);
+	remap_to_cache_dirty(mg->cache, bio, mg->new_oblock, mg->cblock);
+	generic_make_request(bio);
+}
+
+static bool bio_writes_complete_block(struct cache *cache, struct bio *bio)
+{
+	return (bio_data_dir(bio) == WRITE) &&
+		(bio->bi_size == (cache->sectors_per_block << SECTOR_SHIFT));
+}
+
 static void avoid_copy(struct dm_cache_migration *mg)
 {
 	atomic_inc(&mg->cache->stats.copies_avoided);
@@ -928,8 +970,17 @@ static void issue_copy(struct dm_cache_m
 	if (mg->writeback || mg->demote)
 		avoid = !is_dirty(cache, mg->cblock) ||
 			is_discarded_oblock(cache, mg->old_oblock);
-	else
+	else {
+		struct bio *bio = mg->new_ocell->holder;
+
 		avoid = is_discarded_oblock(cache, mg->new_oblock);
+#if 0
+		if (!avoid && bio_writes_complete_block(cache, bio)) {
+			issue_overwrite(mg, bio);
+			return;
+		}
+#endif
+	}
 
 	avoid ? avoid_copy(mg) : issue_copy_real(mg);
 }
@@ -1020,6 +1071,7 @@ static void promote(struct cache *cache,
 	mg->writeback = false;
 	mg->demote = false;
 	mg->promote = true;
+	mg->requeue_holder = true;
 	mg->cache = cache;
 	mg->new_oblock = oblock;
 	mg->cblock = cblock;
@@ -1041,6 +1093,7 @@ static void writeback(struct cache *cach
 	mg->writeback = true;
 	mg->demote = false;
 	mg->promote = false;
+	mg->requeue_holder = true;
 	mg->cache = cache;
 	mg->old_oblock = oblock;
 	mg->cblock = cblock;
@@ -1064,6 +1117,7 @@ static void demote_then_promote(struct c
 	mg->writeback = false;
 	mg->demote = true;
 	mg->promote = true;
+	mg->requeue_holder = true;
 	mg->cache = cache;
 	mg->old_oblock = old_oblock;
 	mg->new_oblock = new_oblock;