ext4: improve write performance with disabled delalloc

ext4: improve write performance with disabled delalloc

When delayed allocation is disabled (either through mount option or
because we are running low on free space), ext4_write_begin() allocates
blocks with EXT4_GET_BLOCKS_IO_CREATE_EXT flag. With this flag extent
merging is disabled and since ext4_write_begin() is called for each page
separately, we end up with a *lot* of 1 block extents in the extent tree
and following writeback is writing 1 block at a time which results in
very poor write throughput (4 MB/s instead of 200 MB/s). These days when
ext4_get_block_unwritten() is used only by ext4_write_begin(),
ext4_page_mkwrite() and inline data conversion, we can safely allow
extent merging to happen from these paths since following writeback will
happen on different boundaries anyway. So use
EXT4_GET_BLOCKS_CREATE_UNRIT_EXT instead which restores the performance.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220520111402.4252-1-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>