Re: [PATCH 2/2] mm: Make swap_readpage() for SWP_FS_OPS use ->direct_IO() not ->readpage()

[Christoph Hellwig]

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