[PATCH v2 2/2] btrfs-progs: Avoid unnecessary block group item COW if the content hasn't changed

[Qu Wenruo]

In write_one_cache_group() we always do COW to update BLOCK_GROUP_ITEM.
However under a lot of cases, the cache->item is not changed at all.

E.g:
Transaction 123, block group [1M, 1M + 16M)

tree block 1M + 0 get freed
tree block 1M + 16K get allocated.

Transaction 123 get committed.

In this case, used space of block group [1M, 1M + 16M) doesn't changed
at all, thus we don't need to do COW to update block group item.

This patch will make write_one_cache_group() to do a read-only search
first, then do COW if we really need to update block group item.

This should reduce the btrfs_write_dirty_block_groups() and
btrfs_run_delayed_refs() loop introduced in previous commit.

Signed-off-by: Qu Wenruo <w...@suse.com>
---
 extent-tree.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/extent-tree.c b/extent-tree.c
index 932af2c644bd..24d3a1ab3f25 100644
--- a/extent-tree.c
+++ b/extent-tree.c
@@ -1533,10 +1533,34 @@ static int write_one_cache_group(struct 
btrfs_trans_handle *trans,
        unsigned long bi;
        struct extent_buffer *leaf;
 
+       /* Do a read only check to see if we need to update BLOCK_GROUP_ITEM */
+       ret = btrfs_search_slot(NULL, extent_root, &cache->key, path, 0, 0);
+       if (ret < 0)
+               goto fail;
+       if (ret > 0) {
+               ret = -ENOENT;
+               error("failed to find block group %llu in extent tree",
+                       cache->key.objectid);
+               goto fail;
+       }
+       leaf = path->nodes[0];
+       bi = btrfs_item_ptr_offset(leaf, path->slots[0]);
+       ret = memcmp_extent_buffer(leaf, &cache->item, bi, sizeof(cache->item));
+       btrfs_release_path(path);
+       /* No need to update */
+       if (ret == 0)
+               return ret;
+
+       /* Do the COW to update BLOCK_GROUP_ITEM */
        ret = btrfs_search_slot(trans, extent_root, &cache->key, path, 0, 1);
        if (ret < 0)
                goto fail;
-       BUG_ON(ret);
+       if (ret > 0) {
+               ret = -ENOENT;
+               error("failed to find block group %llu in extent tree",
+                       cache->key.objectid);
+               goto fail;
+       }
 
        leaf = path->nodes[0];
        bi = btrfs_item_ptr_offset(leaf, path->slots[0]);
-- 
2.22.0