btrfs: open code key_search (995e9a16) · Commits · 戴 / test

This function wraps the optimisation implemented by ("Btrfs: optimize key searches in btrfs_search_slot") however this optimisation is really used in only one place - btrfs_search_slot. Just open code the optimisation and also add a comment explaining how it works since it's not clear just by looking at the code - the key point here is it depends on an internal invariant that BTRFS' btree provides, namely intermediate pointers always contain the key at slot0 at the child node. So in the case of exact match we can safely assume that the given key will always be in slot 0 on lower levels. Furthermore this results in a reduction of btrfs_search_slot's size: ./scripts/bloat-o-meter ctree.orig fs/btrfs/ctree.o add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-75 (-75) Function old new delta btrfs_search_slot 2783 2708 -75 Total: Before=50423, After=50348, chg -0.15% Reviewed-by:

Johannes Thumshirn <johannes.thumshirn@wdc.com> Signed-off-by:

Nikolay Borisov <nborisov@suse.com> Reviewed-by:

David Sterba <dsterba@suse.com> Signed-off-by:

David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

+18 −23

Original line number	Diff line number	Diff line
		@@ -2488,19 +2488,6 @@ done:
		return ret;
		}

		static int key_search(struct extent_buffer b, const struct btrfs_key key,
		int prev_cmp, int slot)
		{
		if (*prev_cmp != 0) {
		*prev_cmp = btrfs_bin_search(b, key, slot);
		return *prev_cmp;
		}

		*slot = 0;

		return 0;
		}

		int btrfs_find_item(struct btrfs_root fs_root, struct btrfs_path path,
		u64 iobjectid, u64 ioff, u8 key_type,
		struct btrfs_key *found_key)
		@@ -2770,9 +2757,23 @@ cow_done:
		}
		}

		ret = key_search(b, key, &prev_cmp, &slot);
		/*
		* If btrfs_bin_search returns an exact match (prev_cmp == 0)
		* we can safely assume the target key will always be in slot 0
		* on lower levels due to the invariants BTRFS' btree provides,
		* namely that a btrfs_key_ptr entry always points to the
		* lowest key in the child node, thus we can skip searching
		* lower levels
		*/
		if (prev_cmp == 0) {
		slot = 0;
		ret = 0;
		} else {
		ret = btrfs_bin_search(b, key, &slot);
		prev_cmp = ret;
		if (ret < 0)
		goto done;
		}

		if (level == 0) {
		p->slots[level] = slot;
		@@ -2896,7 +2897,6 @@ int btrfs_search_old_slot(struct btrfs_root root, const struct btrfs_key key,
		int level;
		int lowest_unlock = 1;
		u8 lowest_level = 0;
		int prev_cmp = -1;

		lowest_level = p->lowest_level;
		WARN_ON(p->nodes[0] != NULL);
		@@ -2929,12 +2929,7 @@ again:
		*/
		btrfs_unlock_up_safe(p, level + 1);

		/*
		* Since we can unwind ebs we want to do a real search every
		* time.
		*/
		prev_cmp = -1;
		ret = key_search(b, key, &prev_cmp, &slot);
		ret = btrfs_bin_search(b, key, &slot);
		if (ret < 0)
		goto done;

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