SEEK_DATA/SEEK_HOLE support

[Christoph Hellwig]

On Tue, Oct 04, 2011 at 10:43:05AM +1100, Dave Chinner wrote:
> The lookup is pretty simple - if there's cached data over the
> unwritten range, then I'm considering it a data range. If there's no
> cached data over the unwritten extent, it's a hole. That makes the
> lookup simply a case of finding the first cached page in the
> unwritten extent.
> 
> It'll end up reading something like this:
> 
> 	iomap = offset_to_extent(offset);
> 	first_index = extent_to_page_index(iomap);
> 
> 	nr_found = pagevec_lookup(&pvec, inode->i_mapping, first_index, 1);
> 	if (!nr_found)
> 		break;
> 
> 	offset = page->index << PAGECACHE_SHIFT;
> 	pagevec_release(&pvec);
> 
> 	/* If we fell off the end of the extent lookup next extent */
> 	if (offset >= end_of_extent(iomap)) {
> 		offset = end_of_extent(iomap);
> 		goto next_extent;
> 	}
> 
> All the extent manipulations are pretty filesystem specific, so
> there's not much that can be extracted into generic helper, I
> think...

Actually pretty similar code will work just fine if you passt the
start + len of the extents in (which we got from looking it up
fs-specificly):

Note that we have to look for both dirty and writeback pages to
make it safe.