btrfs: trimming some start_transaction() code away

Just call kmem_cache_zalloc() instead of calling kmem_cache_alloc(). We're just initializing most fields to 0, false and NULL later on _anyway_, so to make the code mode readable and potentially gain a bit of performance (completely untested claim), we should fill our btrfs_trans_handle with zeros on allocation then just initialize those five remaining fields (not counting the list_heads) as normal.

---
 fs/btrfs/transaction.c | 15 +--------------
 1 file changed, 1 insertion(+), 14 deletions(-)

index f5021fc..d874ce25 100644
@@ -468,7 +468,7 @@ start_transaction(struct btrfs_root *root, u64 num_items, unsigned int type,
 			goto reserve_fail;
 	}
 again:
-	h = kmem_cache_alloc(btrfs_trans_handle_cachep, GFP_NOFS);
+	h = kmem_cache_zalloc(btrfs_trans_handle_cachep, GFP_NOFS);
 	if (!h) {
 		ret = -ENOMEM;
 		goto alloc_fail;

@@ -509,22 +509,9 @@ again:
 
 	h->transid = cur_trans->transid;
 	h->transaction = cur_trans;
-	h->blocks_used = 0;
-	h->bytes_reserved = 0;
-	h->chunk_bytes_reserved = 0;
 	h->root = root;
-	h->delayed_ref_updates = 0;
 	h->use_count = 1;
-	h->adding_csums = 0;
-	h->block_rsv = NULL;
-	h->orig_rsv = NULL;
-	h->aborted = 0;
-	h->qgroup_reserved = 0;
-	h->delayed_ref_elem.seq = 0;
 	h->type = type;
-	h->allocating_chunk = false;
-	h->reloc_reserved = false;
-	h->sync = false;
 	INIT_LIST_HEAD(&h->qgroup_ref_list);
 	INIT_LIST_HEAD(&h->new_bgs);
 	INIT_LIST_HEAD(&h->ordered);

2.5.0

The performance gain is arguable but the generated code should be smaller, which counts.

Yeah, I ran a few iozone benchmarks on a Samsung 850 PRO SSD on 3 kernels, the latest archlinux kernel, my custom kernel which has:
CONFIG_BTRFS_ASSERT=y
CONFIG_BTRFS_DEBUG=y
CONFIG_BTRFS_FS_RUN_SANITY_TESTS=y CONFIG_BTRFS_FS_POSIX_ACL=y
with the patch, and my custom kernel without the patch.
I ran iozone 5 times on each kernel, There were huge differences between my custom kernels and arch's kernel, but nothing conclusive between my custom kernel with or without the patch. So it's safe to say that it has not much of a visible effect on performance.

Thank you for your time!