On Thu, Aug 12, 2021 at 07:02:33PM +0200, Christoph Hellwig wrote:

On Thu, Aug 12, 2021 at 04:39:40PM +0100, Matthew Wilcox wrote:

quoted

I agree with David; we want something lower-level for swap to call into.
I'd suggest aops->swap_rw and an implementation might well look
something like:

static ssize_t ext4_swap_rw(struct kiocb *iocb, struct iov_iter *iter)
{
	return iomap_dio_rw(iocb, iter, &ext4_iomap_ops, NULL, 0);
}

Yes, that might make sense and would also replace the awkward IOCB_SWAP
flag for the write side.

For file systems like ext4 and xfs that have an in-memory block mapping
tree this would be way better than the current version and also support
swap on say multi-device file systems properly.  We'd just need to be
careful to read the extent information in at extent_activate time,
by doing xfs_iread_extents for XFS or the equivalents in other file
systems.

You'd still want to walk the extent map at activation time to reject
swapfiles with holes, shared extents, etc., right?

--D