Re: SEEK_DATA/SEEK_HOLE support

[Dave Chinner <david@fromorbit.com>]

On Mon, Oct 03, 2011 at 12:04:11PM +0800, Jeff Liu wrote:
> On 10/03/2011 01:59 AM, Christoph Hellwig wrote:
> 
> > On Mon, Oct 03, 2011 at 12:06:37AM +0800, Jeff Liu wrote:
> >> IMHO, to avoid data loss in some user application like cp(1), for
> >> unwritten extents, we always need to check the pages status.  Just as
> >> you mentioned above, return the map offset if pages are dirty for
> >> SEEK_DATA, or a hole found.
> > 
> > I'd suggest to first implement the simple versions I schemed below,
> > which would treat unwritten extents as data.  That is sub-optimal,
> > but a) safe and b) easy to implement.  The second step would be to
> > add probing for unwritten extents, which is even something we could
> > do as a common helper routine shared by filesystems.
> 
> So I'll wait for Dave's patch become ready, and then continue to improve
> it if necessary.
> In the meantime, I'll try to figure out how to add a helper which can be
> shared by all file systems for UNWRITTEN extents.

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...

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com

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