ext4: add prefetch_block_bitmaps mount options

For file systems where we can afford to keep the buddy bitmaps cached,
we can speed up initial writes to large file systems by starting to
load the block allocation bitmaps as soon as the file system is
mounted.  This won't work well for _super_ large file systems, or
memory constrained systems, so we only enable this when it is
requested via a mount option.

Addresses-Google-Bug: 159488342
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iad3356da63f624f5dc6345874c4ec9dda6dff51d
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/3303
Reviewed-by: Theodore Tso <tytso@google.com>
(cherry picked from commit b910e70ff4c899093b4677ed91c17447057d79f8)

This commit hasn't been merged upstream yet but has been reviewed
and will be available upstream soon.
TEST=Verified that the FS can be mounted with this mount option

Change-Id: Iad3356da63f624f5dc6345874c4ec9dda6dff51d
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/4401
Reviewed-by: Theodore Tso <tytso@google.com>
Reviewed-by: Roy Yang <royyang@google.com>
4 files changed