ext4 not currently doing (much) multi-block allocation?

[Theodore Tso <tytso@mit.edu>]

So I was looking at the ext4 code to see how hard it would be to add a
function that would take a struct inode *, and make sure that all of
the pages in the page cache had been allocated a physical block on
disk (but not necessarily writing the I/O to disk).  The idea would be
to do this on close if the file had been truncated or opened with
O_TRUNC, and to also call this function if the inode had been renamed
and in the process a destination inode was freed.  That way if we have
data=ordered, the blocks would be allocated, and at the next commit,
we would force the data blocks to disk.

While I was looking at the code, it looks to me like we are currently
only allocating a page at a time; ext4_da_writepages() may end up
allocating a number of pages, but it's doing it one page at a time,
not an extent at a time.  So if the filesystem blocksize is 4k (and
the page size is 4k), the only time we will ever call the mballoc with
an allocation request greater than 1 is in the fallocate() system call
handler.   This seems... non-optimal.   Am I missing something?

	   		 		     - Ted