Re: [RFC PATCH 00/10] mm/swap: ghost swapfile with backend switching via Redirect entries

[Baoquan He <baoquan.he@linux.dev>]

On 07/07/26 at 04:26pm, Baoquan He wrote:
> This series introduces a virtual "ghost" swapfile that can be backed
> by multiple physical swap devices, switching between them at runtime
> using a new "Redirect" swap table entry type.

Forgot mentioning this patchset is based on below patchset:

[RFC PATCH 0/7] mm: store zswap entries in swap table (Pointer entry)
https://lore.kernel.org/all/20260707073215.72183-1-baoquan.he@linux.dev/T/#u

and they sit on top of akpm/mm-unstale commit cfb8731f5396
("mm: fix CONFIG_STACK_GROWSUP typo in tools/testing/vma/include/dup.h")

> 
> Motivation
> ----------
> Physical swap devices have fixed, large sizes but a machine may only
> need a fraction at any given time.  Ghost swap decouples the logical
> swap capacity from physical swap allocation: it presents a large
> virtual swap device to userspace while lazily mapping slots to
> physical devices only when data must leave zswap (writeback).  This
> enables overcommit of swap capacity without wasting physical disk
> space, and allows the ghost device to grow dynamically at runtime.
> 
> Ghost swapfile characteristics
> --------------------------------
> Ghost swap decouples logical swap capacity from physical swap allocation.
> Key properties:
> 
> - No backing store required.  A ghost swap device can operate purely
>   as a zswap container ??? compressed pages stay in the zswap pool without
>   ever touching a physical swap device.  No physical swap device needs
>   to be attached.
> 
> - Optional physical backing.  One or more physical swap devices can be
>   swapon'd alongside the ghost device, acting as writeback targets when
>   the zswap pool needs to evict cold pages.  The Redirect entry in the
>   ghost swap table links each evicted slot to its physical destination.
> 
> - Dynamic size growth.  The ghost device can be extended at runtime via
>   sysfs, up to a current maximum of 1TB.  This is implemented through
>   lazy vmalloc: a sparse 64MB virtual address space is reserved at
>   swapon time to cover the full 1TB range, but physical pages for
>   cluster_info[] structures are allocated only on demand as the device
>   grows.  The cluster_info pointer never changes, so all existing
>   accessors (__swap_offset_to_cluster, cluster_index) work unchanged.
> 
> - sysfs control.  Each ghost device exposes attributes under
>   /sys/kernel/mm/ghost_swap/ghost_<N>/:
>     path (RO) ??? backing device name
>     max  (RW) ??? current max pages; writing a larger value triggers
>                 swap_ghost_extend_max()
> 
> Design overview
> ---------------
> A swap table slot can now hold a Redirect entry:
> 
>   Redirect: |----- physical swp_entry_t -----|101|
> 
> This stores a plain swp_entry_t pointing to a slot on a real (physical)
> swap device.  The low 3-bit mark (0b101) distinguishes it from Pointer
> (0b100), PFN (0b10), Shadow (0b1), and NULL (0) entries.
> 
> The lifecycle is:
> 
> 1. swapon a ghost device ??? reserves sparse vmalloc area (64MB virtual)
>    and physically backs only the initial range.  The device starts with
>    no physical backing store: pages swapped out go to zswap, and zswap
>    pool evictions go to any concurrently attached physical device.
> 
> 2. zswap writeback from ghost: when the zswap pool needs to evict,
>    zswap_writeback_ghost() allocates a physical slot from any real
>    swap device, writes the data there, and plants a Redirect entry
>    in the ghost's swap table pointing to the physical slot.
> 
> 3. Swap-in from ghost: swap_read_folio() detects the ghost device
>    (SWP_GHOST), resolves the Redirect to the physical swp_entry_t,
>    and forwards the read I/O to the physical device.
> 
> 4. When the swap cache folio is removed, the Redirect entry is restored
>    from an xarray (redirect_xa) that persists the mapping, so
>    subsequent reads still find the physical backing.
> 
> 5. When the physical slot is freed, the ghost slot is also freed,
>    cascading through swap_range_free().
> 
> Note:
> ------
> Ghost swapfile is a proof of concept. Compared with the other patchset:
> [RFC PATCH v2 0/7] mm, swap:  Virtual Swap Space (Swap Table Edition),
> it explores a different approach to implementing virtual swap on top of
> the existing swap table infrastructure, with a simpler implementation.
> It builds on Chris Li's earlier ghost swapfile work and is currently
> tentatively named "ghost swapfile" ??? alternative naming suggestions are
> welcome. Ghost swapfile setup can also be integrated through modifications
> to the swapon API and the swapon command in util-linux, for example:
> swapon --ghost ghost_1:2G -p 5.
> 
> Baoquan He (9):
>   mm/swap_table: add Redirect entry encoding for ghost swap backend
>     switching
>   mm/swap: add redirect_xa field and ghost redirect helper declarations
>   mm/swapfile: implement ghost redirect helpers and free-path cascade
>   mm/swap_state: restore Redirect entry when swap cache folio is removed
>   mm/zswap: implement ghost-to-physical writeback for backend switching
>   mm/page_io: forward ghost swap reads to physical device via Redirect
>   mm/swapfile: manage ghost cluster_info via lazy vmalloc
>   mm/swapfile: implement swap_ghost_extend_max() for dynamic growth
>   mm/swapfile: add sysfs interface for ghost swap extension
> 
> Chris Li (1):
>   mm: ghost swapfile support for zswap
> 
>  include/linux/swap.h |  10 +
>  mm/page_io.c         |  51 +++-
>  mm/swap.h            |  16 +-
>  mm/swap_state.c      |  28 +++
>  mm/swap_table.h      |  38 +++
>  mm/swapfile.c        | 579 ++++++++++++++++++++++++++++++++++++++++---
>  mm/zswap.c           |  98 +++++++-
>  7 files changed, 772 insertions(+), 48 deletions(-)
> 
> -- 
> 2.54.0
>