Re: [PATCH 01/18] Structural cleanup for filesystem-based swap

[Christoph Hellwig]

On Fri, Dec 17, 2021 at 10:48:22AM +1100, NeilBrown wrote:
> Linux primarily uses IO to block devices for swap, but can send the IO
> requests to a filesystem.  This has only ever worked for NFS, and that
> hasn't worked for a while due to a lack of testing.  This seems like a
> good time for some tidy-up before restoring swap-over-NFS functionality.

The changes look good to me, but I think this needs to be split into
separate, self-contained patches.

> 
> This patch:

Patch 1:

>  - updates the documentation (both copies!) for swap_activate which
>    is woefully out-of-date

Patch 2:

>  - drops the call to the filesystem for ->set_page_dirty().  These
>    pages do not belong to the filesystem, and it has no interest
>    in the dirty status.

Patch 3:


>  - move the responsibility for setting SWP_FS_OPS to ->swap_activate()
>    and also requires it to always call add_swap_extent().  This makes
>    it much easier to find filesystems that require SWP_FS_OPS.

Patch 4:

>  - introduces a new address_space operation "swap_rw" for swap IO.
>    The code currently used ->readpage for reads and ->direct_IO for
>    writes.  The former imposes a limit of one-page-at-a-time, the
>    later means that direct writes and swap writes are encouraged to
>    use the same path.  While similar, swap can often be simpler as
>    it can assume that no allocation is needed, and coherence with the
>    page cache is irrelevant.