Re: [PATCH v5 0/6] Btrfs: implement swap file support

[Omar Sandoval <osandov@osandov.com>]

On Thu, Sep 06, 2018 at 01:59:54PM +0200, David Sterba wrote:
> On Fri, Aug 31, 2018 at 03:36:35PM -0700, Omar Sandoval wrote:
> > This series implements swap file support for Btrfs.
> > 
> > Changes since v4 [1]:
> > 
> > - Added a kernel doc for btrfs_get_chunk_map()
> > - Got rid of "Btrfs: push EXCL_OP set into btrfs_rm_device()"
> > - Made activate error messages more clear and consistent
> > - Changed clear vs unlock order in activate error case
> > - Added "mm: export add_swap_extent()" as a separate patch
> > - Added a btrfs_wait_ordered_range() at the beginning of
> >   btrfs_swap_activate() to catch newly created files
> > - Added some Reviewed-bys from Nikolay
> > 
> > I took a stab at adding support for balance when a swap file is active,
> > but it's a major pain: we need to mark block groups which contain swap
> > file extents, check the block group counter in relocate/scrub, then
> > unmark the block groups when the swap file is deactivated, which gets
> > really messy because the file can grow while it is an active swap file.
> > If this is a deal breaker, I can work something out, but I don't think
> > it's worth the trouble.
> 
> I'm afraid it is a deal breaker. Unlike dev-replace or resize, balance
> is used more often so switching off the swap file for the duration of
> the operation is administration pain.
> 
> If it's possible to constrain the swap file further, like no growing
> that you mention, or mandatory preallocation or similar, then I hope it
> would make it possible to implement in a sane way.

Alright, I'll have another go.