generic_file_buffered_write and O_SYNC

["Nate Diller" <nate.diller@gmail.com>]

So I'm trying to get an understanding of the interactions between the
various aio_read/aio_write paths, and I ran across this gem at the end
of generic_file_buffered_write:

       /*
        * For now, when the user asks for O_SYNC, we'll actually give O_DSYNC
        */
       if (likely(status >= 0)) {
               if (unlikely((file->f_flags & O_SYNC) || IS_SYNC(inode))) {
                       if (!a_ops->writepage || !is_sync_kiocb(iocb))
                               status = generic_osync_inode(inode, mapping,
                                               OSYNC_METADATA|OSYNC_DATA);
               }
       }

       /*
        * If we get here for O_DIRECT writes then we must have fallen through
        * to buffered writes (block instantiation inside i_size).  So we sync
        * the file data here, to try to honour O_DIRECT expectations.
        */
       if (unlikely(file->f_flags & O_DIRECT) && written)
               status = filemap_write_and_wait(mapping);

So if there's a writepage function AND we're doing async i/o, then
skip the writeout for O_SYNC files.  But always do writeout for dio,
synchronously.  Why do we check for the existence of ->writepage, but
not ->writepages?  Why do we ever skip writeback at all?

in addition to being poorly documented, this code looks to me like it
has a high likelyhood of being incorrect.  can anyone clarify?

thanks

NATE
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