Btrfs: readahead checksums during btrfs_finish_ordered_io

This reads in blocks in the checksum btree before starting the
transaction in btrfs_finish_ordered_io.  It makes it much more likely
we'll be able to do operations inside the transaction without
needing any btree reads, which limits transaction latencies overall.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 33 insertions, 2 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index b83a45dc717..9b4faac50c1 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1497,6 +1497,30 @@ static int insert_reserved_file_extent(struct btrfs_trans_handle *trans,
         return 0;
 }
 
+/*
+ * helper function for btrfs_finish_ordered_io, this
+ * just reads in some of the csum leaves to prime them into ram
+ * before we start the transaction.  It limits the amount of btree
+ * reads required while inside the transaction.
+ */
+static noinline void reada_csum(struct btrfs_root *root,
+                                struct btrfs_path *path,
+                                struct btrfs_ordered_extent *ordered_extent)
+{
+        struct btrfs_ordered_sum *sum;
+        u64 bytenr;
+
+        sum = list_entry(ordered_extent->list.next, struct btrfs_ordered_sum,
+                         list);
+        bytenr = sum->sums[0].bytenr;
+
+        /*
+         * we don't care about the results, the point of this search is
+         * just to get the btree leaves into ram
+         */
+        btrfs_lookup_csum(NULL, root->fs_info->csum_root, path, bytenr, 0);
+}
+
 /* as ordered data IO finishes, this gets called so we can finish
  * an ordered extent if the range of bytes in the file it covers are
  * fully written.
@@ -1505,7 +1529,7 @@ static int btrfs_finish_ordered_io(struct inode *inode, u64 start, u64 end)
 {
         struct btrfs_root *root = BTRFS_I(inode)->root;
         struct btrfs_trans_handle *trans;
-        struct btrfs_ordered_extent *ordered_extent;
+        struct btrfs_ordered_extent *ordered_extent = NULL;
         struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
         struct btrfs_path *path;
         int compressed = 0;
@@ -1528,13 +1552,20 @@ static int btrfs_finish_ordered_io(struct inode *inode, u64 start, u64 end)
                         ret = btrfs_lookup_file_extent(NULL, root, path,
                                                        inode->i_ino,
                                                        start, 0);
+                        ordered_extent = btrfs_lookup_ordered_extent(inode,
+                                                                     start);
+                        if (!list_empty(&ordered_extent->list)) {
+                                btrfs_release_path(root, path);
+                                reada_csum(root, path, ordered_extent);
+                        }
                         btrfs_free_path(path);
                 }
         }
 
         trans = btrfs_join_transaction(root, 1);
 
-        ordered_extent = btrfs_lookup_ordered_extent(inode, start);
+        if (!ordered_extent)
+                ordered_extent = btrfs_lookup_ordered_extent(inode, start);
         BUG_ON(!ordered_extent);
         if (test_bit(BTRFS_ORDERED_NOCOW, &ordered_extent->flags))
                 goto nocow;