Re: [PATCH RFC] mm: ghost swapfile support for zswap

[Barry Song <21cnbao@gmail.com>]

On Sat, Nov 22, 2025 at 6:00 PM Kairui Song <ryncsn@gmail.com> wrote:
>
> On Fri, Nov 21, 2025 at 5:52 PM Chris Li <chrisl@kernel.org> wrote:
> >
> > The current zswap requires a backing swapfile. The swap slot used
> > by zswap is not able to be used by the swapfile. That waste swapfile
> > space.
> >
> > The ghost swapfile is a swapfile that only contains the swapfile header
> > for zswap. The swapfile header indicate the size of the swapfile. There
> > is no swap data section in the ghost swapfile, therefore, no waste of
> > swapfile space.  As such, any write to a ghost swapfile will fail. To
> > prevents accidental read or write of ghost swapfile, bdev of
> > swap_info_struct is set to NULL. Ghost swapfile will also set the SSD
> > flag because there is no rotation disk access when using zswap.
> >
> > The zswap write back has been disabled if all swapfiles in the system
> > are ghost swap files.
>
> Thanks for sharing this, I've been hearing about the ghost swapfile
> design for a long time, glad to see it finally got posted.
>
> >
> > Signed-off-by: Chris Li <chrisl@kernel.org>
> > ---
> >  include/linux/swap.h |  2 ++
> >  mm/page_io.c         | 18 +++++++++++++++---
> >  mm/swap.h            |  2 +-
> >  mm/swap_state.c      |  7 +++++++
> >  mm/swapfile.c        | 42 +++++++++++++++++++++++++++++++++++++-----
> >  mm/zswap.c           | 17 +++++++++++------
> >  6 files changed, 73 insertions(+), 15 deletions(-)
>
> In general I think this aligns quite well with what I had in mind and
> an idea that was mention during LSFMM this year (the 3rd one in the
> "Issues" part, it wasn't clearly described in the cover letter, more
> details in the slides):
> https://lore.kernel.org/all/CAMgjq7BvQ0ZXvyLGp2YP96+i+6COCBBJCYmjXHGBnfisCAb8VA@mail.gmail.com/
>
> The good part is that we will reuse everything we have with the
> current swap stack, and stay optional. Everything is a swap device, no
> special layers required. All other features will be available in a
> cleaner way.
>
> And /etc/fstab just works the same way for the ghost swapfile.

Apologies — let me raise a question that may be annoying.
I understand that people may already be feeling tense and sensitive.

Despite the benefit of compatibility with /etc/fstab, we still need to provide
a physical file on disk (or elsewhere), even if it contains only a header.
Personally, this feels a bit odd to me. Is it possible to avoid having a
“ghost” swap file altogether and instead implement all "ghost" functionality
entirely within the kernel? Ideally, we wouldn’t need to introduce a new
“ghost” concept to users at all.

In short, we provide the functionality of a ghost swap file without actually
having any file or “ghost” at all.

Thanks
Barry