btrfs: defrag with regular and preallocated extents

btrfs: defrag with regular and preallocated extents

Recent v5.16 has some regression around btrfs autodefrag mount
option, and the extra scrutiny around defrag code exposes some
questionable behavior from the old code.

One behavior is to defrag extents along with the next preallocated
extent.

This behavior will cause extra IO and convert all the preallocated
extent to regular zero filled extents, rendering the preallocated
extent useless.

The kernel fix is titled:

  btrfs: defrag: don't try to merge regular extents with preallocated extents

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>