do_generic_mapping_read()
{
	isize1 = i_size_read();
	...
	readpage
	copy_to_user up to isize1;
}

readpage()
{
	isize2 = i_size_read();
	...
	read blocks
	...
	zero-fill all blocks past isize2
}

If a second thread runs truncate and shrinks i_size, so isize1 and isize2 are
different, the read can return up to a page of zero-fill that shouldn't really
exist.

The trick is to read isize1 after doing the readpage.  I realised this is the
right way to do it without having to change the readpage API.

The patch should not cost any cycles when reading from pagecache.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>