Re: [PATCH v3 1/2] iomap: add a swapfile activation function

[Omar Sandoval <osandov@osandov.com>]

On Thu, May 03, 2018 at 10:46:59AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Add a new iomap_swapfile_activate function so that filesystems can
> activate swap files without having to use the obsolete and slow bmap
> function.  This enables XFS to support fallocate'd swap files and
> swap files on realtime devices.
> 

Shouldn't we also prevent the extents of an active swapfile from
becoming shared? If I swapon(a) and reflink(a, b), swapout to a now has
to break the reflink or corrupt b! In my old Btrfs swapfile series [1] I
just forbid all reflink operations on active swapfiles. 

One thing to note is that then this will need a matching
->swap_deactivate(), which currently isn't called if ->swap_activate()
returned > 0.

1: https://github.com/osandov/linux/tree/btrfs-swap

> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> v3: catch null iomap addr, fix too-short extent detection
> v2: document the swap file layout requirements, combine adjacent
>     real/unwritten extents, align reported swap extents to physical page
>     size boundaries, fix compiler errors when swap disabled