The situation being that I want to invoke
some code in do_writepages path for a
particular filesystem ( identified by the
super_block, say ). In that case, when I
issue writes on the filesystem, I do see the
commit/prepare write path activated ASAP as
it it should be. But on a explicit sync on
the filesystem, I don't see writepages for
that filesystem activated.

sys_sync->sync_fdev->sync_inodes>__sync_one->filemap_fdatasync->writepage if
"drty pages are avilable"