e2fsck: read-ahead metadata during passes 1, 2, and 4

e2fsck: read-ahead metadata during passes 1, 2, and 4

e2fsck pass1 is modified to use the block group data prefetch function
to try to fetch the inode tables into the pagecache before it is
needed.  We iterate through the blockgroups until we have enough inode
tables that need reading such that we can issue readahead; then we sit
and wait until the last inode table block read of the last group to
start fetching the next bunch.

pass2 is modified to use the dirblock prefetching function to prefetch
the list of directory blocks that are assembled in pass1.  We use the
"iterate a subset of a dblist" and avoid copying the dblist.  Directory
blocks are fetched incrementally as we walk through the directory
block list.  In previous iterations of this patch we would free the
directory blocks after processing, but the performance hit to e2fsck
itself wasn't worth it.  Furthermore, it is anticipated that most
users will then mount the FS and start using the directories, so they
may as well remain in the page cache.

pass4 is modified to prefetch the block and inode bitmaps in
anticipation of pass 5, because pass4 is entirely CPU bound.

In general, these mechanisms can decrease fsck time by 10-40%, if the
host system has sufficient memory and the storage system can provide a
lot of IOPs.  Pretty much any storage system capable of handling
multiple IOs in-flight at any time will see a fairly large performance
boost.  (Single-issue USB mass storage disks seem to suffer badly.)

By default, the readahead buffer size will be set to the size of a block
group's inode table (which is 2MiB for a regular ext4 FS).  The -E
readahead_kb= option can be given to specify the amount of memory to
use for readahead or zero to disable it entirely; or an option can be
given in e2fsck.conf.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>