btrfs: remove reada infrastructure

Currently there is only one user for btrfs metadata readahead, and
that's scrub.

But even for the single user, it's not providing the correct
functionality it needs, as scrub needs reada for commit root, which
current readahead can't provide. (Although it's pretty easy to add such
feature).

Despite this, there are some extra problems related to metadata
readahead:

- Duplicated feature with btrfs_path::reada

- Partly duplicated feature of btrfs_fs_info::buffer_radix
  Btrfs already caches its metadata in buffer_radix, while readahead
  tries to read the tree block no matter if it's already cached.

- Poor layer separation
  Metadata readahead works kinda at device level.
  This is definitely not the correct layer it should be, since metadata
  is at btrfs logical address space, it should not bother device at all.

  This brings extra chance for bugs to sneak in, while brings
  unnecessary complexity.

- Dead code
  In the very beginning of scrub.c we have #undef DEBUG, rendering all
  the debug related code useless and unable to test.

Thus here I purpose to remove the metadata readahead mechanism
completely.

[BENCHMARK]
There is a full benchmark for the scrub performance difference using the
old btrfs_reada_add() and btrfs_path::reada.

For the worst case (no dirty metadata, slow HDD), there could be a 5%
performance drop for scrub.
For other cases (even SATA SSD), there is no distinguishable performance
difference.

The number is reported scrub speed, in MiB/s.
The resolution is limited by the reported duration, which only has a
resolution of 1 second.

	Old		New		Diff
SSD	455.3		466.332		+2.42%
HDD	103.927 	98.012		-5.69%

Comprehensive test methodology is in the cover letter of the patch.

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

1 files changed, 0 insertions, 36 deletions

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index ad10a5e70e73..679e881d7814 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -3188,10 +3188,7 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 	u64 physical_end;
 	u64 generation;
 	int mirror_num;
-	struct reada_control *reada1;
-	struct reada_control *reada2;
 	struct btrfs_key key;
-	struct btrfs_key key_end;
 	u64 increment = map->stripe_len;
 	u64 offset;
 	u64 extent_logical;
@@ -3242,11 +3239,6 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 	path->skip_locking = 1;
 	path->reada = READA_FORWARD;
 
-	/*
-	 * trigger the readahead for extent tree csum tree and wait for
-	 * completion. During readahead, the scrub is officially paused
-	 * to not hold off transaction commits
-	 */
 	logical = base + offset;
 	physical_end = physical + nstripes * map->stripe_len;
 	if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
@@ -3261,36 +3253,8 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 	scrub_blocked_if_needed(fs_info);
 
 	root = btrfs_extent_root(fs_info, logical);
-
-	/* FIXME it might be better to start readahead at commit root */
-	key.objectid = logical;
-	key.type = BTRFS_EXTENT_ITEM_KEY;
-	key.offset = (u64)0;
-	key_end.objectid = logic_end;
-	key_end.type = BTRFS_METADATA_ITEM_KEY;
-	key_end.offset = (u64)-1;
-	reada1 = btrfs_reada_add(root, &key, &key_end);
-
 	csum_root = btrfs_csum_root(fs_info, logical);
 
-	if (cache->flags & BTRFS_BLOCK_GROUP_DATA) {
-		key.objectid = BTRFS_EXTENT_CSUM_OBJECTID;
-		key.type = BTRFS_EXTENT_CSUM_KEY;
-		key.offset = logical;
-		key_end.objectid = BTRFS_EXTENT_CSUM_OBJECTID;
-		key_end.type = BTRFS_EXTENT_CSUM_KEY;
-		key_end.offset = logic_end;
-		reada2 = btrfs_reada_add(csum_root, &key, &key_end);
-	} else {
-		reada2 = NULL;
-	}
-
-	if (!IS_ERR(reada1))
-		btrfs_reada_wait(reada1);
-	if (!IS_ERR_OR_NULL(reada2))
-		btrfs_reada_wait(reada2);
-
-
 	/*
 	 * collect all data csums for the stripe to avoid seeking during
 	 * the scrub. This might currently (crc32) end up to be about 1MB