From first 2.6.17 release to first 2.6.18 release (haven't narrowed it down to exact versions), 3 PF_SYNCWRITE related lines have been removed from mm/msync.c.
And some PF_SYNCWRITE related stuff in block/cfq-iosched.c was added in 2.6.17 (diff between 2.6.16.62 and 2.6.17), and then removed in 2.6.18.
There's also fs/ sync related stuff between 2.6.16.62 and 2.6.17.
From first 2.6.17 release to first 2.6.18 release (haven't narrowed it down to exact versions), 3 PF_SYNCWRITE related lines have been removed from mm/msync.c.
And some PF_SYNCWRITE related stuff in block/cfq-iosched.c was added in 2.6.17 (diff between 2.6.16.62 and 2.6.17), and then removed in 2.6.18.
There's also fs/ sync related stuff between 2.6.16.62 and 2.6.17.
I hope I'm not spamming. :P