On Wed, Sep 19, 2018 at 02:09:09PM -0400, Johannes Weiner wrote:
> On Tue, Sep 11, 2018 at 03:34:45PM -0700, Omar Sandoval wrote:
> > From: Omar Sandoval <osandov@xxxxxx>
> >
> > Btrfs will need this for swap file support.
> >
> > Signed-off-by: Omar Sandoval <osandov@xxxxxx>
>
> That looks reasonable. After reading the last patch, it's somewhat
> understandable why you cannot simply implemnet ->bmap and use the
> generic activation code. But it would be good to explain the reason(s)
> for why you can't here briefly to justify this patch.
I'll rewrite it to:
Btrfs currently does not support swap files because swap's use of bmap
does not work with copy-on-write and multiple devices. See 35054394c4b3
("Btrfs: stop providing a bmap operation to avoid swapfile
corruptions"). However, the swap code has a mechanism for the filesystem
to manually add swap extents using add_swap_extent() from the
->swap_activate() aop. iomap has done this since 67482129cdab ("iomap:
add a swapfile activation function"). Btrfs will do the same in a later
patch, so export add_swap_extent().
