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

[Baoquan He <bhe@redhat.com>]

On 12/02/25 at 10:56am, Barry Song wrote:
> 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.

That's actually what I would like to see. Just to make that we may need
change syscall swapon, to specify the flag to mark it and initial size.
People may complain about adjustment in syscall swapon.