[dm-devel] [PATCH] dm: eliminate some holes in data structures

[Mike Snitzer]

Eliminate 1 4-byte hole in 'struct dm_io_memory' by moving 'offset'
above the 'ptr' to which it applies (size reduced from 24 to 16 bytes).
And by association, 1 4 byte hole is eliminated in 'struct
dm_io_request' (size reduced from 56 to 48 bytes).

Eliminate all (3 4-byte) holes (and one cache-line) in 'struct
dm_snapshot' by moving 'pending_pool' after 'pending_exceptions_count'
and moving 'merge_failed' lower (size reduced from 648 to 632 bytes).

Signed-off-by: Mike Snitzer <snitzer at redhat.com>
---
 drivers/md/dm-snap.c  |   18 +++++++++---------
 include/linux/dm-io.h |    4 ++--
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index ee8eb28..0435442 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -83,10 +83,10 @@ struct dm_snapshot {
 	/* Whether or not owning mapped_device is suspended */
 	int suspended;
 
-	mempool_t *pending_pool;
-
 	atomic_t pending_exceptions_count;
 
+	mempool_t *pending_pool;
+
 	struct dm_exception_table pending;
 	struct dm_exception_table complete;
 
@@ -110,6 +110,13 @@ struct dm_snapshot {
 	spinlock_t tracked_chunk_lock;
 	struct hlist_head tracked_chunk_hash[DM_TRACKED_CHUNK_HASH_SIZE];
 
+	/* Wait for events based on state_bits */
+	unsigned long state_bits;
+
+	/* Range of chunks currently being merged. */
+	chunk_t first_merging_chunk;
+	int num_merging_chunks;
+
 	/*
 	 * The merge operation failed if this flag is set.
 	 * Failure modes are handled as follows:
@@ -125,13 +132,6 @@ struct dm_snapshot {
 	 */
 	int merge_failed;
 
-	/* Wait for events based on state_bits */
-	unsigned long state_bits;
-
-	/* Range of chunks currently being merged. */
-	chunk_t first_merging_chunk;
-	int num_merging_chunks;
-
 	/*
 	 * Incoming bios that overlap with chunks being merged must wait
 	 * for them to be committed.
diff --git a/include/linux/dm-io.h b/include/linux/dm-io.h
index b6bf17e..5c9186b 100644
--- a/include/linux/dm-io.h
+++ b/include/linux/dm-io.h
@@ -37,14 +37,14 @@ enum dm_io_mem_type {
 struct dm_io_memory {
 	enum dm_io_mem_type type;
 
+	unsigned offset;
+
 	union {
 		struct page_list *pl;
 		struct bio_vec *bvec;
 		void *vma;
 		void *addr;
 	} ptr;
-
-	unsigned offset;
 };
 
 struct dm_io_notify {