Re: fallocate on XFS for swap

[Dave Chinner <david@fromorbit.com>]

On Sat, Mar 10, 2018 at 01:38:44AM -0800, Christoph Hellwig wrote:
> On Sat, Mar 10, 2018 at 11:58:50AM +1100, Dave Chinner wrote:
> > > > 3. Add an XFS-specific implementation of swapfile_activate.
> > > 
> > > Ugh no.
> > 
> > What we want is an iomap-based re-implementation of
> > generic_swap_activate(). One of the ways to plumb that in is to
> > use ->swapfile_activate() like so:
> 
> Hmm.  Fundamentally swap is the same problem as the pNFS block layout
> or get_user_pages on DAX mappings - we want to get a 'lease' on the
> current block mapping, and make sure it stays that way as the external
> user (the swap code in this case) uses it.  The twist for the swap code
> is mostly that it never wants to break the least but instead disallow
> any external operation, but that's not really such a big difference.

True.

> So maybe we want a layout based swap code instead of reinventing it,
> with the slight twist to the layout break code to never try a lease
> break and just return an error for the IS_SWAPFILE case.

Hmmm - won't that change user visible behaviour on swapfiles? Not
that it would be a bad thing to reject read/write from root on swap
files, but it would make XFS different to everything else.

Speaking of which - we probably need to spend some time at LSFMM in
the fs track talking about the iomap infrastructure and long term
plans to migrate the major filesystems to it....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com